Solar Power Made Easy?

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tictac1's picture
tictac1
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Solar Power Made Easy?

OK, so I've done the requisite thread surfing looking for an answer to this, but came up dry.  Perhaps one of you would be good enough to answer this, or point me to where the answer might reside.

The question is:  have the members of this board come to a conclusion on the "best" solar set up?  What I'm looking for is more or less a recipe, as in "buy Brand X panels, buy Brand Z inverter, etc."

I realize the answer will depend on my actual situation, so here it is in a nutshell-

- I live in CA, lots of insolation where I'm at

- I'd like about 3-4kW worth of power

- Not (currently) interested in battery back up, but would like the ability to use solar power during the day without grid.

The goal is to eliminate my 2nd and 3rd tier electric costs (0.28 to 0.33 cents/kWh), and back up my deep well pump in the event of a prolonged outage.  Current electric bill ranges from $200-350/month.

Many thanks to those who can help!

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Easy solar power

 I've been looking at solar photovoltaic options for my own house and have had extensive discussions with an installer.  

Sounds like you want a simple grid-tied solar system.  You just have to size it to meet your average electricity demand for the year.  When your system produces more electricity than you need during the day, it feeds into the grid.  When you need electricity at night, you draw it off the grid.  The utility under net metering rules carries credits forward if you generate more than you need on average in a month, and you draw down that credit in the winter months with less insolation.

Batteries are expensive and higher maintenance and not cost effective as you have probably figured out.  Off-grid systems with batteries are more for cabins and such that are no where near the grid and are more complicated and you have to size the system to meet your peak load on a day with less sun.

For emergency power to run your well pump, a generator is most cost effective.  Grid-tied system inverters automatically disconnect when utility power is lost so not to put power on the grid when lineworkers might be working on it.  

My emergency power solution is reduce my critical loads, basically refrigeration only, to under 1KW; I'll run a 1KW inverter off my Prius if we ever get another week outage like has happened here in New England due to ice storms.  Then I don't have another engine (generator) sitting around doing nothing 99.9% of the time.  

My installer recommended Canadian solar modules with Enphase microinverters based on performance, reliablity, and cost.  There's lots of options out there I'm sure.

You may be better off getting a professional to select and size a systme for you, unless you have the time to really invest learning all this stuff.  You'll still need an electrician to legally install it anyway, at least in my state. 

good luck

Tom

 

 

 

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earthwise
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First, conservation.

 

 Excuse me for stating the obvious.

You may already know this, but before you size your system, it's well advised to reduce your power consumption as much as possible i.e. compact flourescents lights etc. You should be able to get by with a smaller system  that way.

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Check out the "What should I do series"
tictac wrote:

- Not (currently) interested in battery back up, but would like the ability to use solar power during the day without grid.

The goal is to eliminate my 2nd and 3rd tier electric costs (0.28 to 0.33 cents/kWh), and back up my deep well pump in the event of a prolonged outage.  Current electric bill ranges from $200-350/month.

Grid tied - cost mitigation only.

Off-Grid or grid tied w/battery backup - cost mitigation and resiliency.

But as Woodman points out, and I'm sure you've discovered batteries add to the cost of the system considerably.  However, you don't have to size the batteries to meet your entire loads.  You can separate off critical loads (well, refrigeration, etc) and size batteries to meet those needs. 

I'm still happy with our system, but it was not cheap.  You can read about it the "What should I do series".    If you haven't looked through the articles I strongly recommend them, lot's of great ideas.  On that note, we built our system as an AC coupled battery backup which is different than most off-grid systems.  You can always put in the grid tied system and then add the battery backup system later since it is essentially a large UPS.  If you choose your inverters properly, you can create a micro-grid that allows your grid-tied inverters to be used when the normal grid is down - but again, none of this is cheap.

I would recommend finding an installer that does off-grid systems and finding out what is available.  When we built our system we had to do some stuff from scratch but SMA had a "battery backup" solution (basically a pre-built version of what is describe in the article) that was not marketed in the US yet.  At CES this year, Panasonic had some interesting home rack mount modular battery backup systems based on Lithium Ion batteries - not sure how long until they might be on the market, but apparently the smaller system they were showing is available in Japan.

As soon as I get some time I need to write a follow up to the article since we've now had the system for a couple of years.

 

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Damnthematrix
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First, conservation.
earthwise wrote:

 

 Excuse me for stating the obvious.

You may already know this, but before you size your system, it's well advised to reduce your power consumption as much as possible i.e. compact flourescents lights etc. You should be able to get by with a smaller system  that way.

 

 

http://damnthematrix.wordpress.com/2011/09/04/the-power-of-energy-effici...

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Thanks

Thanks for all the suggestions.

We have gone the conservation route, it's been gradual for the past 5 years.  However, I don't use many CFLs.

Why?!?

Well, I've bought several brands, and put the meter on them.  Simply put, I have yet to find a brand that actually uses the rated watts.  For example, the last batch I bought from Lowe's were rated at 17 watts each, and actually consumed 40 watts, which ironically was the incandescent equivalent.

If anyone out there has a meter and time, I'd love to hear what readings you're getting.  We had ALL CFLs until the failure rate on the CFLs made it obvious they were not the hot deal they were cracked up to be, then I checked the actually power usage and that was the deal breaker.

Bear in mind, I did all this several years ago.  We had multiple bulbs fail prematurely, some actually melted (I've still got some laying around, maybe I can post some pictures), and not one used the rated watts, all were at least 50% more.  Maybe the name-brand, current models are better?

I have done some rough sizing, and it seems like 3k would be about right, 4k would provide a little overhead.

On the grid separation issue, couldn't you simply install a disconnect between the PG&E drop and the inverter, to be opened when the power goes out, then reset the inverter?  Or does the inverter have to "see" the grid on the other side to function? 

DTM, have you put a meter on your LED bulbs?  If they are legit I would be willing to go that route.  Your efforts are impressive, lots of info to glean there.  Do you have a good write-up on your solar system you can send me?

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Grid tied inverters need grid...
tictac wrote:

On the grid separation issue, couldn't you simply install a disconnect between the PG&E drop and the inverter, to be opened when the power goes out, then reset the inverter?  Or does the inverter have to "see" the grid on the other side to function? 

The grid tied inverters require synchronization with a power grid so when there is no grid they will not start up.   In our setup, when the grid goes down, the SMA Sunny Island inverters disconnect the section of our system that has the PV grid tied inverters from the grid and form a micro-grid.  Then the Sunny Islands produce a "grid" signal from the battery that allows the grid-tied inverters to start-up and feed power to the house and charge the batteries.  The SMA inverters (Sunny Boy (grid) and Sunny Island (off-grid)) also have a feature where the Sunny Islands can monitor the power required by the house/battery charging and can throttle the Sunny Boy inverters.  Without this feature, the grid tied inverters would be either on full or off and the batteries would have to handle a larger fluctuation during the day when more PV power was available than needed.  I'm not sure if other manufacturers have that feature. NOTE: This only applies with an AC coupled system.

One way to think about why you need batteries is they offer a buffer.  They allow power to fluctuate coming from the PVs (think clouds) and still keep producing steady AC power to your appliances which will not be happy with a fluctuating power source.

Time to pull out the watt meter - I never considered checking that the CFLs were actually using more power than rated...   We haven't changed out many of ours since we have lots of dimmers.

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tictac1
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thank you

...making much more sense now!

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On-Grid Inverters with Battery Backup
rhare wrote:

The grid tied inverters require synchronization with a power grid so when there is no grid they will not start up.   In our setup, when the grid goes down, the SMA Sunny Island inverters disconnect the section of our system that has the PV grid tied inverters from the grid and form a micro-grid.  Then the Sunny Islands produce a "grid" signal from the battery that allows the grid-tied inverters to start-up and feed power to the house and charge the batteries.  The SMA inverters (Sunny Boy (grid) and Sunny Island (off-grid)) also have a feature where the Sunny Islands can monitor the power required by the house/battery charging and can throttle the Sunny Boy inverters.  Without this feature, the grid tied inverters would be either on full or off and the batteries would have to handle a larger fluctuation during the day when more PV power was available than needed.  I'm not sure if other manufacturers have that feature.

We have a german SunProfi 1500W inverter that does all this in one box, sadly no longer  available...

BUT there is an extremely well made and designed Australian inverter made by Selectronic which, if we do make the move to Tasmania will be my first choice... don't know if they make a US version though:

On-Grid Inverters with Battery Backup

Inverters   |    Solutions

 

Image of SP PRO grid interactive inverter charger

SP PRO Inverter Series

This versatile inverter offers a grid feed ability with the added security of a battery backup.

Feed-In-Tariffs and Grid Feed

The SP PRO Grid Feed function allows the user to export any excess generated renewable power to the mains electricity grid and take advantage of Feed-In-Tariffs.

Security of Backup Power

During a power outage, the SP PRO sources backup power from a pre-charged battery, offering the user mains quality power and peace of mind.

Once the grid electricity is restored, the SP PRO automatically reverts to providing electricity from the grid, simultaneously recharging the battery and maintaining them in optimum condition.

The SP PRO is an innovation unlike any other. Designed from the ground up, the SP PRO features advanced thermal power management and super high speed digital signal processing; setting new standards for reliability, features, surge capability and power density.

Each model of the SP PRO Series inverter is suitable for use in On Grid power systems for residential, commercial and industrial applications.

Find out more about this product at the SP PRO page.

Xantrex also make something similar, but not for Australia!  Obviously OK for the states:

 

865-1010
Xantrex XW - hybrid inverter / charger XW4024-120/240-60 - input: 178A DC
  Characteristics  

Main
Range of product Xantrex XW
Device short name XW4024-120/240-60
Product or component type Hybrid inverter / charger
Network number of phases Single phase
Type of signal True sine wave
Continuous power 4000 W AC - 120 V)
4000 W AC - 240 V)
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Grid tied battery backup

I just started up our grid tied battery backup PV system mid December.  Running a 5.16kW array feeding the new Outback Radian GS8048 inverter/charger.  The Radian puts out 8000 watts continuous and can be stacked up to 10 providing 80,000 watts.  Still early, but so far very satisfied.  Battery bank of 8 Trojan L-16 RE-B's with 17.6kWhr of rated storage power provide my backup.  I am hoping by being very frugal in off grid situations that I can get by.  Off grid, winter and several very cloudy days may require a fossil fuel generator, but will see.  If necessary, the generator can be integrated into the Radian and will automatically  fire off of a low battery voltage mark.  Again, as others have mentioned not necessarily cheap, but does provide good resiliency.

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Looks like some nice additional options have become available.

Mike & cnbbaldwin,

Thanks for posting those other options.  I suspect as more people put in solar and want the resiliency, additional manufacturers will provide solutions.  When I was looking the Xantrax XW was just coming to market.  I had a really hard time finding anyone that knew about these type of solutions and I had to do a lot of research myself.  Most installers will tell you that you are crazy for wanting a battery backup, particularly if you are in an area with very stable power.

Mike on that SP Pro, it may not quite do what the SMA inverters do, this wording seems odd:

DamnTheMatrix from SP Pro literature wrote:

During a power outage, the SP PRO sources backup power from a pre-charged battery, offering the user mains quality power and peace of mind.

This makes me think it can't use the grid tied inverters to charge the batteries when the grid is down.  I found this forum on it that makes me think that might be the case - although I didn't read the many many entries, however, tic-tac, you might find the discussions informative.  There is a lot of talk about the Sunny-Backup, which was not available in the US when we built our system so we used Sunny Islands and Sunny Boys to do the same thing.   I would definitely look at the Outback solution also, as I know they are very highly regarded in the off-grid community.

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Grid/Battery

I used Enphase Energy grid tied Micro inverters and Magnum Energy battery inveters.

This has worked well for several years so far.

I am able to charge my batteries using the micro inverters while the grid is down.

 

John

 

 

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LED Power Consumption
tictac1 wrote:

Thanks for all the suggestions.

We have gone the conservation route, it's been gradual for the past 5 years.  However, I don't use many CFLs.

Why?!?

DTM, have you put a meter on your LED bulbs?  If they are legit I would be willing to go that route.  Your efforts are impressive, lots of info to glean there.  Do you have a good write-up on your solar system you can send me?

 

I have quite a few LED bulbs.  They seem to be far closer to advertised power draw than CFLs.

 

 

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Selectronic inverter
rhare wrote:

.

Mike on that SP Pro, it may not quite do what the SMA inverters do, this wording seems odd:

DamnTheMatrix from SP Pro literature wrote:

During a power outage, the SP PRO sources backup power from a pre-charged battery, offering the user mains quality power and peace of mind.

This makes me think it can't use the grid tied inverters to charge the batteries when the grid is down.  I found this forum on it that makes me think that might be the case - although I didn't read the many many entries, however, tic-tac, you might find the discussions informative.  There is a lot of talk about the Sunny-Backup, which was not available in the US when we built our system so we used Sunny Islands and Sunny Boys to do the same thing.   I would definitely look at the Outback solution also, as I know they are very highly regarded in the off-grid community.

I am aware of that, but all you need is a battery charger hooked up to the inverter.  Why they don't build it into the inverter is puzzling (our SunProfi does the lot).  The owner of the solar shop I worked for in 2010 swears by them reckons they're really well built...

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Chasing the Elusive Power Factor

LEDs are not prone to "power factor" he way CFLs are....

Chasing the Elusive Power Factor

Volt-amperes are a measure of apparent power (as in false and tricky). Watts are a measure of true power (as in honest and upright, like Carol). When AC power hits reactive loads like CFLs or TVs or air conditioners or surge protectors or computers, electrical current and voltage tend to get jostled out of phase with each other, causing voltamperes (sneaky false power) to rise above watts (foursquare honest power).

So our reactive 25 watt CFL still uses an honest 25 watts, even though amps and volts go out of phase and the sneaky VA rise to 50, making energy consumption appear to double. The extra 25 volt-amperes is not lost or consumed, just stored or borrowed. It goes to work somewhere else. It may offset the power factor of another appliance or it may go back out to the utility line where industrial capacitors tweak it back into phase. Utility meters may not read the extra volt-amperes that CFLs suck into the house, but neither do they read the extra voltamperes that blow back into the grid from Carol’s house. Everything gets canceled out, and the only thing actually consumed is the 25 watts.

Inductors (which cause current to lag voltage) and capacitors (which cause current to lead voltage) store or borrow volt-amperes that are not consumed as watts. They return those unused volt-amperes to the circuit later. For example, pure inductance does not consume energy; it stores it as a magnetic field. When that magnetic field collapses, the energy is returned to the circuit.

Utilities like the power factor of loads to be close to 1.0 (unity) because that makes it cheapest to distribute energy. These companies constantly adjust loads to bring current and voltage back into phase without using significant extra power. Commercial fluorescent lights usually have power factors of 0.9 or higher. Utilities lobbied to get residential CFLs to have similar power factors, but bulb manufacturers revolted because high power factor bulbs are more expensive to make. But the issue was power management, not energy consumption.

To prove this point, I set up an experiment with CFLs vs. incandescent lights (resistive loads with a power factor of 1.0), and measured watts and volt-amperes delivered by my inverter. Then I measured the amps from my power source, a bank of L-16 batteries, which are DC sources wise to the tricks of reactive power.

A 75 watt incandescent bulb measured 70 watts and 70 volt-amperes out of the inverter, which is what you would expect with a power factor of 1.0. The TriMetric meter measuring true power from the batteries measured 71.8 watts (volts times amps). The slightly higher wattage was due to inefficiency in converting 12 volt DC to 120 volt AC.

Then I lit a 25 watt CFL. It measured 25 watts and 52 volt-amperes out of the inverter, which indicated a power factor of about 0.5. The TriMetric, however, showed the true power needed by the batteries was 26.5 watts. The low power factor did not require the batteries (or the utility) to produce any extra energy.

This is not to say that volt-amperes are not important. Household wiring must be sized for the highest volt-ampere load it will carry, not for the highest watt load. Inverters, too, must be able to handle the highest volt-ampere load thrown at them, not the highest watt load.

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40W bulbs

I used my "Kill A Watt" to measure the power of 3 bulbs:

40W Incandecent Bulb: Measured 38.4W

40W equivalent CFL: Measured ~8.0W (measure was varying from 7.9 to 8.2W).

40W equivalent LED: Measured 7.7W (rated at 8W)

All pretty close to spec.

 

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Metering

I'm using a Greenlee meter, but it sounds like what you're using is close enough.

DTM, thanks for the refresher.  I always had a hard time understanding VARs (I work at a power plant).  Clearly, I need to revisit my measurements!

One of the reasons my power usage is high is that I am running a well pump (380 ft head) and a booster pump for system pressure.  One thought I had is to convert to a Grundfos DC pump with its own dedicated solar array (minus inverter), and downsize my house solar array accordingly.

Thoughts????

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DC well pumps...
tictac1 wrote:

One of the reasons my power usage is high is that I am running a well pump (380 ft head) and a booster pump for system pressure.  One thought I had is to convert to a Grundfos DC pump with its own dedicated solar array (minus inverter), and downsize my house solar array accordingly.

We have roughly the same issue, however we are pumping from 500-600' - don't really know since we didn't put in the well.   The problem is the flow rate from DC pumps at that depth are much lower than for an AC pump.  For us we don't have a storage tank so the well needs to put out our desired flow rate.  Now if you can put in a large tank and fill it, then pump from it for your house (which you may have do to your comment about a booster pump), that would work well.  I've been considering that course simply for resiliency, but it's expensive...

Also, have you compared the cost of pulling the pump, the additional array, etc with just making a larger grid-tied solar array?  With the incentives the way they are it may be cheaper to just go bigger.  Not saying it's sustainable, but it is what it is.....

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pumping
tictac1 wrote:

One of the reasons my power usage is high is that I am running a well pump (380 ft head) and a booster pump for system pressure.  

Thoughts????

When you say booster pump. I assume you mean one that pressures your taps (Orstrayan for faucet!) so that when you open one, the water comes out even though the water source is lower than the tap?  This is what we use to pump water up from our lower water tank to the house; head difference, ~12 feet depending on how full the tank is, can get as high as 18 feet......

I came across an interesting thing when I was selling PVs in 2010....  I sold this couple with a HUGE house (~5000 sq ft/500m2 - more than three times the size of this house) that had two kitchens, several toilets, at least two bathrooms from memory, and ran on water tanks just like our place.  Because the system had to be designed to run "everything" and possibly with several taps on at once over considerably longer pipe runs than we have here in our hyper efficient house, instead of the 450W pump we have, their tank was fitted with a 1.8kW one!

What this means of course (and they lived there on their own...) is that if they open just one tap, a whole 1800W gets consumed to feed it water!  FOUR TIMES as much power as we use.  A classic example of how a large house uses/consumes way more energy than a small one...

Mike

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wow!

Here's a better explanation of my set up-

Well pump puts water into a 2K gallon tank, booster pump takes suction off that tank to pressurize the main to about 50 psi, which has to run about 200 feet to the house and feed the taps.  This also feeds all the garden irrigation that is not gravity fed from a rainwater tank.  At least I got that part right...:)

My thought was that a dedicated solar well pump from Grundfus would run something around $4500 (assuming I do the work), and would then allow me to-

- reduce the needed size of the house solar array

- have water for consumption and plants should the grid go belly-up for an extended period of time

On that note, what do our solar peeps think of this set up?

http://www.solarpanelstore.com/solar-power-packages.cse-medium-grid-tie.enphase-grid-tie-packages.gte_3784.info.1.html

Thanks to a bitchin excel spreadsheet made by a coworker, I have calc'd that under my actual conditions this system would produce roughly 16-22 kWh/day, resulting in system pay back in 4.7 to 3.4 years.  At a 12 year lifespan, this would represent a 155% to 245% ROI.  Seems too good to be true...

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4 times the faucets doesn't mean 4 times the energy
DamnTheMatrix wrote:

What this means of course (and they lived there on their own...) is that if they open just one tap, a whole 1800W gets consumed to feed it water!  FOUR TIMES as much power as we use.  A classic example of how a large house uses/consumes way more energy than a small one...

I doubt that.  I'm sure they are pumping into a pressure tank with a bladder.  It means the pump draws 1800W but would only run 1/4 as often.  Probably not completely linear but certainly not what you are claiming. 

Tictac, since you already have the storage tank, it seems like  a pretty reasonable solution, however,  you still have a problem that your booster/pressure pump will be AC - so you would be able to get water from the storage tank with a hand pump in an emergency, but you still wouldn't have household water without the grid with your solution.

That system should probably last 20 years. Keep in mind you will have to add $1-2K for components and connection to the grid plus labor to install if you don't do it yourself. The output claims look accurate depending on the tilt angle and how much solar available you have.  The numbers match with PV watts online calculator if it was install in NM (very sunny).  Also, ROI is really in the teens, which is reasonable - you calculate it on a yearly basis including some factor for the cost of the money.   Our system without the batteries would have had an ROI of 7% or so (very expensive mounting).  I know others with systems with ROIs in the 10-15% range.  A lot depends on the incentives available to you.

 

 

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I'd like to see how the ROI

I'd like to see how the ROI is calculated, I'm fuzzy on that part.  Yes on the pressure tank, forgot to mention that!  Another aspect is the size of pressure tank, since fewer starts = less wear & tear and less watts for gallons used due to starting current.

And yes, the booster pump issue is what made me nix the independent well array idea.  Maybe a diesel generator is better, but you have fuel storage issues...

A little more tinkering and figuring on this, and hopefully I'll be ready to pull the trigger.  I really appreciate the continued feedback from everyone.

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rhare wrote: DamnTheMatrix
rhare wrote:
DamnTheMatrix wrote:

What this means of course (and they lived there on their own...) is that if they open just one tap, a whole 1800W gets consumed to feed it water!  FOUR TIMES as much power as we use.  A classic example of how a large house uses/consumes way more energy than a small one...

I doubt that.  I'm sure they are pumping into a pressure tank with a bladder.  It means the pump draws 1800W but would only run 1/4 as often.  Probably not completely linear but certainly not what you are claiming.

In my experience, all those "bladders" do is allow pressure to the taps so that you have water to the tap as soon as you turn it on, and when the pressure in the bladder drops to a preselected pressure, the pump cuts in.  The pump will not cut in only on rare occasions when you may only want to draw one glass of water, but in any case, next time it starts, it will repressurise the bladder to account for the glass!

Mike

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Pressure tanks...
DamnTheMatrix wrote:

In my experience, all those "bladders" do is allow pressure to the taps so that you have water to the tap as soon as you turn it on, and when the pressure in the bladder drops to a preselected pressure, the pump cuts in.  The pump will not cut in only on rare occasions when you may only want to draw one glass of water, but in any case, next time it starts, it will repressurise the bladder to account for the glass!

Actually there is a start pressure, when it falls below a certain point, the pumps starts, when it reaches a certain pressure it cuts off.  If you draw only a small amount, there is a good chance the pump will not start. So if you have an 1800W pump it  will take less time than the 400W pump to repressurize the system.  So it uses more power when it runs, but runs less because it brings up the pressure up that much quicker.  We can actually get several gallons out of the system before our pump kicks in, I can tell because I can watch the ~4000W well pump kick in on our electric meter - since we pump directly into the pressure tank.  The well will cycle and I can't keep it running unless I open up multiple faucets on full and exceed the output from the well.

Here is a link about it.

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ROI - tough to get right

 

tictac1 wrote:

I'd like to see how the ROI is calculated, I'm fuzzy on that part.

It's actually quite complicated to do it right.  What you need to do is figure out all your costs (equipment, install) and returns (REC credits, energy savings, tax credits) and when those will occur.  Then you put them in giant spread sheet with all the amounts, dates and use the XIRR function to calculate it.   I did this but the spread sheets tend to be very use specific.  Don't forget things like:

  • Energy Savings - what you are not paying the utility - you probably want to include a value you can adjust based on what you think energy costs will do.  Ie. for each element on your table increase the amount you save by an amount you think energy costs will rise - for us the untility is increasing prices about 5%/year consistently.  Don't forget to include taxes/fees that you pay to the utility - some of those will be fixed and not based on usage.  To do this you need to look at your usage for the last couple of years to get a good average per month, then use something like PV watts to estimate the average power you will get on a monthly basis to calculate your savings.  This gets really nasty if your utility does different tiers based on your usage.  If you have time of use billing this gets really really complex.  I gave up trying to figure out if TOU billing was beneficial - although I may revisit it now that we have a Nissan Leaf.
  • REC credits - Any REC credits you will be paid, including income taxes on those credits.
  • Degradation of the array output over time - your usage will drop about 1%/year.
  • Costs of repairs - you use mean failure rates if you can find them for the panels and inverters.
  • Residual value of the array for resale if you move - prorated based on life of array.

I actually accounted for almost all of this when we were investigating our system.  I worked on the spreadsheet for several months, including a lot of reverse engineering of our electric bill.  I can accurately calculate what our utility bill would be without solar to compare how we are doing - but it was a lot of work (regulatory agencies make utility billing incredibly complex) and only works for our utility, with our solar setup, etc.

Here is a link describing the actual ROI calculation a bit:

   http://www.fatpitchfinancials.com/392/how-to-calculate-your-return-on-in...

My recommendation - don't worry about it.  If you think energy prices are going to go up significantly in the future or we may have outages, just put in a system.  With the prices of panels and inverters today, unless you have complex mounting, live in a non-sunny area, or have poor incentives, you will probably have a ROI of 10% - which is far far better than a savings account right now.  In addition, the system is also an inflation hedge against rising energy prices.  When I did our system (expensive mounting), assuming zero inflation in energy prices, without battery backup I came up with about a 6% return over 20 years, and about a 2% return with a battery backup.  The first year of our actual costs were within 2% of my estimates (most likely by luck - since weather could easily have swayed it more than that).

I can tell you, our system without batteries, pre tax credit installed was about $6.47/W and post tax credit $3.88/W.  If you can get the system for about $13,000 (shipping, additional parts, grid hookup).  Then you are looking at $3.42/W pre tax credit and $2.39/W post tax credit.  Even without state or utility incentives, your return will be good.

I think the only thing you probably need to consider is do you want batteries for resiliency? If yes, then you may want to make different choices which will cost considerably more than the grid tied micro inverter system you priced, probably at a minimum 25% more.

If you haven't done so already, I would call several installers, get quotes, and see what they say.  Many will do at least a simplified ROI calculation for you based on your utility usage, solar isolation, and incentives available.

 

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Damnthematrix
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Pressure tanks...
rhare wrote:

Actually there is a start pressure, when it falls below a certain point, the pumps starts, when it reaches a certain pressure it cuts off.  If you draw only a small amount, there is a good chance the pump will not start. So if you have an 1800W pump it  will take less time than the 400W pump to repressurize the system.

How so?  IF the bladder/tank is bigger, it will still require more time to repressurise.....  the biggest advantage is that if it starts fewer times, then the larger start up currents are needed less frquently, but in reality, start up currents only last half a second or less, and the energy consumed is very minimal...

Mike

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rhare
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Joined: Mar 30 2009
Posts: 1323
Pressure tanks and pumps...
DamnTheMatrix wrote:

How so?  IF the bladder/tank is bigger, it will still require more time to pressurize.....  the biggest advantage is that if it starts fewer times, then the larger start up currents are needed less frequently, but in reality, start up currents only last half a second or less, and the energy consumed is very minimal...

It's not just the start up time.  It's the run time of the pump.  You have a pump that is taking more power, that means it's producing more work - ie. pumping more water over a given period of time than a smaller pump.  Either via higher pressure or more volume.  Either way you re-pressurize the system faster so the pump runs less.  A larger or smaller pressure tank only determines how frequently the pump runs.  If you have a larger pressure tank you can draw more water before the pump has to start.  But over all the pump still pumps the same amount of water, it just may start/stop more or less frequently.

Let's try this another way.  Say you have a 400W pump that pumps 5gal/min (or liters if you prefer) and another pump that is 1600W that pumps 20gal/min.  You turn on the faucet and run 5 gallons of water out.  The 400W pump will run for 1 minute consuming 400W/min.  The 1600W pump will run for 15secs, consuming 400W/min.  Now, it's probably not perfectly linear, but should be realively close depending on the efficiencies of the pumps.

Why have the bigger pump?  Well if you have 4 faucets turned on, the house with the bigger pump will be able to feed all 4 facets 5 gal/min, where as the house with the smaller pump will only be able to supply 1.25g/min to each faucet.  When you only have 1 faucet on, they both produce the same results and consume the same amount of energy.  When you try to use more than 1 faucet the smaller pump will not keep up.

 

 

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Damnthematrix
Status: Diamond Member (Offline)
Joined: Aug 10 2008
Posts: 3998
Pressure tanks and pumps...
rhare wrote:
DamnTheMatrix wrote:

How so?  IF the bladder/tank is bigger, it will still require more time to pressurize.....  the biggest advantage is that if it starts fewer times, then the larger start up currents are needed less frequently, but in reality, start up currents only last half a second or less, and the energy consumed is very minimal...

It's not just the start up time.  It's the run time of the pump.  You have a pump that is taking more power, that means it's producing more work - ie. pumping more water over a given period of time than a smaller pump.  Either via higher pressure or more volume.

That's a sure way of wasting water AS WELL as electricity! Neither of which are desirable.... It's because the water comes out the hot water tap/faucet slowly, for instance, that we never boost our solar water heater.

I'll stick to my way of doing things efficiently.....

Mike

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rhare
Status: Diamond Member (Offline)
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Posts: 1323
Are faucets in OZ binary?
DamnTheMatrix wrote:

That's a sure way of wasting water AS WELL as electricity! Neither of which are desirable.... It's because the water comes out the hot water tap/faucet slowly, for instance, that we never boost our solar water heater.

Just because you can open a faucet on full and get high pressure or a lot of water doesn't mean you have too.   You use the same amount of electricity for the same volume of water whether you pump it slow or fast.  It's not that your running it slow that means you don't have to heat your hot water (assuming that what you mean by boost versus booster pump for pressure), it's because you are using less hot water over a given period of time available to heat the water with solar.  You get the same result out of a pressurized system if you just don't run the faucet on full - and it will use about the same amount of electricity.

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Woodman
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Joined: Sep 26 2008
Posts: 1028
electricity costs

 

The return on investment in a solar PV system seems to depend a lot on how much you think electricity costs will rise.

I found price data here  from the EIA, which shows about 3%/year average for my state after sorting out the data. But given our monetary challenges ahead one might assume a higher rate but I also expect utilities have hedged some costs several years ahead also.  I should dig out my old electricty bills too.

www.eia.gov/electricity/data.cfm#sales

A spreadsheet of historical data by state is under the historical section near the bottom of the page linked above

www.eia.gov/cneaf/electricity/epa/average_price_state.xls

 

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JAG
Status: Diamond Member (Offline)
Joined: Oct 26 2008
Posts: 2492
Woodman

Woodman, thanks for the spreadsheets. I've been looking for this information for my state for some time.

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