Battery Cost Analysis Spreadsheet

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Brandon's picture
Brandon
Status: Martenson Brigade Member (Offline)
Joined: Oct 6 2008
Posts: 143
Battery Cost Analysis Spreadsheet

I didn't want to start a new thread for this, but unfortunately my other file hosting alternative are down at the moment, and you can't attached files to comments...only thread starters.

Attached is a file that I created to help me evaluate different options for batteries to use for a (currently hypothetical) home solar/wind installation.  I have no experience with building such systems; this spreadsheet was created to help me get an understanding of costs.  Many assumptions are made, though I tried to add notes where relevant.

The spreadsheet was made in Open Office.  I do not know how good it will look in Excel, though it should at least open it.

Make sure to show comments for individual cells that are marked as having comments.  If you only rollover a cell with a comment, it will likely exclude all URLs I placed in the comment.

My target total AHs was around 750-1100 at 12V.  Your needs are likely different, and as such, adjustment of calculations may need to take place.  In fact, I am now considering a 24V system, which would change the calculations, not to mention my initial selection of batteries to begin with had I know that I would later lean towards 24V.  However, for the most part, the patterns in this spreadsheet would also be reflected at 24V, I think, but have not verified.

There are a few highlighted batteries.  These represent ones I highlighted for myself as having the best balance between initial investment I may be able to afford, and overall value (AH vs. $ vs. Lifespan).  I suppose one could create a new value that factors in initial investment as a factor in overall value, but I prefer to look at these values seperately.

The concorde batteries are AGM...they are the only AGM batteries in the spreadsheet I think.  I included them because, I am still not quite clear on my individual risks of hydrogen gas emissions.  Nor am I clear on how much maintenance I can commit to doing with my batteries.  Because of this, I am still considering AGM, despite their relatively high costs and short life.

I modified this in a bit of a hurry for the CM forums.  I apologize for any mistakes/typos/etc.

As noted in the spreadsheet and in my initial paragraph here, there are quite a few assumptions made to get these numbers.  I'm not even sure if I can remember all the assumptions I made, but hopefully they're inherently clear.

See attached file.

rhare's picture
rhare
Status: Diamond Member (Offline)
Joined: Mar 30 2009
Posts: 1323
Re: Battery Cost Analysis Spreadsheet

Make sure you consider how the batteries will be used.  If you are off-grid versus on-grid battery backup.  In the off grid scenario the batteries will be regularly cycled.  In the grid tie battery backup, if you assume they will only be needed for true crisis, then you can expect the batteries to sit at float most of the time.  This can make a huge differece in the batteries you choose. 

It is my understanding that using off-grid type batteries like the Surrettes, Dekas if you don't discharge them regularly then when you need them, they may die very quickly (I think this is called false charge).  My installer has a battery crew as they do data centers, and their battery experts said they had seen this exact scenario.  Of course if you pick batteries that are designed to sit at float most of their lives and you discharge them regularly, then they will have a much shorter life....

Looking at your spreadsheet the NiFe batteries were quite a bit more.  I'm also surprised the waranty isn't better given the talk about the batteries lasting forever. :-)

For us, we went with the C&D msEndur II, they are aimed at telco and data centers with the assumption they won't be used much and you want long life.  However, looking at your spreadsheet, I think they would be the most expensive battery on there.  It was about $9500 for 480AH @ 48V.  However, if they do last 20 years and have basically no maintenance then not too bad.  I also liked that the battery company had been around 100 years, figure they must be doing something right.  :-)   In 20 years I'll let you know if it was a good plan, maybe sooner if the SHTF.

barrt's picture
barrt
Status: Silver Member (Offline)
Joined: Aug 25 2008
Posts: 171
Re: Battery Cost Analysis Spreadsheet

Thanks for that Brandon, that will come in very handy, im just trying to understand all this and invest in batteries & solar panels myself

Can you help me with my basic question; If I want to go 12v and off grid (thanks rhare) i want to replace approximatly 300 Kilowatt/hours (based on my Electricity bill minus 25% which is a rough guess at the easy saving i could make on that with low energy light bulbs etc)

Is that enough information to work out the amount of 12v batteries i need to go off grid? im not too concerned to get the power rating of the system 'just right' to match current use, just a 'ball park' system that i can expand on.

I think i need to understand that before your chart becomes useful! its all very complicated this stuff!

Brandon's picture
Brandon
Status: Martenson Brigade Member (Offline)
Joined: Oct 6 2008
Posts: 143
Re: Battery Cost Analysis Spreadsheet

Hi barrt....I'm not ignoring you....It's a good question...I should have some time to repond by Wednesday.

-Brandon

rhare's picture
rhare
Status: Diamond Member (Offline)
Joined: Mar 30 2009
Posts: 1323
Re: Battery Cost Analysis Spreadsheet
barrt wrote:

Can you help me with my basic question; If I want to go 12v and off grid (thanks rhare) i want to replace approximatly 300 Kilowatt/hours (based on my Electricity bill minus 25% which is a rough guess at the easy saving i could make on that with low energy light bulbs etc)

Is that enough information to work out the amount of 12v batteries i need to go off grid? im not too concerned to get the power rating of the system 'just right' to match current use, just a 'ball park' system that i can expand on.

I can give you a few pointers on things to work on:

  1. Figure out what your production capacity is in your area.  The size of your array will vary considerable depending on where your located and the amount of sun you get.  It can also vary widely throughout the year (seasonal changes) and also on your equipment (adjutable tilt angle, trackers, fixed).
  2. Figure out how long you want to be able to go without solar input.  How critical is it that you have power, if it's highly critical then you are going to want more/larger batteries or potentially a generator incase you have unexpected bad weather for a while.

I would suggest playing with PVWatts (might only be for the US).   You  can put in different size arrays, tilt angles (see here and here for optimum angles), tracking to see the different outcomes.  Notice if gives you output by month. For us, adjusting the angles more than 2x/year wasn't worth it, nor were trackers since for the cost of trackers you could buy more panels - but YMMV. This will allow you to come up with a rough size of the array and equipment you probably want (problem #1).  The you have to give #2 some thought as that may have a significant effect on how big you need your array since if you plan on having supply for lots of cloudy days, you will need a larger array to charge up the batteries when you do have sun. 

Also, why do you want off-grid?  Are you currently on grid power or is this a new place?  If you have grid it sure looks like a grid-tie with battery backup is the way to go. That way you can collect incentives to help pay for it with the grid tie system and then if TSHTF you have the batteries.

 

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