PVWatts: A Performance Calculator for Grid-Connected PV Systems

Wendy S. Delmater
By Wendy S. Delmater on Thu, Jul 25, 2013 - 8:21am

This site, which we ran across while listening to a lecture on residential alternative energy,  has performance calculators for both the USA and around the world for grid-tied photovoltaic solar energy systems. It suggests useful things like the angle your solar panels should be at your lattidue. You click on the part of your state or country where you want to use PVWATTS to calculate the electrical energy that will be produced. Choose the site nearest to your location that has similar topography.

We live near Columbia, SC, so we chose:

Station Identification:
    WBAN Number: 13883
City: Columbia
State: South_Carolina

PV System Specifications:  
  DC Rating DC rating (kW):
DC to AC Derate FactorDC to AC Derate Factor: derate1_72.jpg  
Array Type:  
Fixed Tilt or 1-Axis Tracking System:   fixed tilt
        Array Tilt Array Tilt (degrees): (Default = Latitude)
Array Azimuth Array Azimuth (degrees): (Default = South)
Energy Data:
  Cost of Electricity (cents/kWh):

...and the results came in like this:

pvwatts1.jpg * * *
AC Energy
Cost Savings

(Type comments here to appear on printout; maximum 1 row of 80 characters.)

Station Identification
City: Columbia
State: South_Carolina  
Latitude: 33.95° N
Longitude:     81.12° W
Elevation: 69 m
PV System Specifications
DC Rating: 4.0 kW
DC to AC Derate Factor: 0.770
AC Rating: 3.1 kW
Array Type: Fixed Tilt  
Array Tilt: 34.0°
Array Azimuth: 180.0°
Energy Specifications
Cost of Electricity:     8.1 ¢/kWh
Month Solar Radiation
AC Energy
Energy Value
3.81      356    28.84   
4.48      373    30.21   
4.98      450    36.45   
5.95      507    41.07   
5.66      487    39.45   
5.81      467    37.83   
5.65      469    37.99   
5.41      452    36.61   
5.38      436    35.32   
10  5.71      499    40.42   
11  4.54      396    32.08   
12  3.80      348    28.19   
Year  5.10      5240    424.44   

Disclaimer and copyright notice

homepg.gif Return to RReDC home page (http://www.nrel.gov/rredc )

The bottom right box shows how very slowly this system pays back the investment. Still, a grid-tied system has no cost for a massive battery system, so it's much cheaper. The systems are reaching Grid Parity (see chart), which means they are getting cost-effctive.

Understand that the lecture this came from was done in 2011 and this was a projection - it seems it was accutrate. Aslo accurate was this: price drops

Gee, it's almost like Moore's Law is in effect here.

Okay, the class was my effort; the research into systems we could use was my husband's effort. I will try to sumarize what he told me here and he will be happy to answer any questions to the best of his ability. While he is not an expert on solar systems, he has been a Honeywell systems technician for 37 years, so he does understand how things work.

Here is what we learned that you might want to know about. Because solar panels are getting more and more efficient, those suppliers who bought quantities of solar panels a couple of years ago are now trying to unload them at unreasonable prices per kiliwatt-hour. By no means should you buy PV panels from big box stores: Lowes and Home Depot, for example, bought older panels a couple of years ago and are stuck with them. The new ones are twice as efficient and will only get more efficient. We have chosen Grangier as our PV panel supplier, as we have access to them (they supply businesses, not homeowners) and they had the best price.

Another innovation to watch for is how panels handle shade. A PV system used to lower it's total efficiency based the amount of light it got in a shaded area. Newer panels are better about that.

We are going with a net-meter, grid-tied system. So far we have bought the inverter and our existing little PV cell phone charging station is not exactly making the meter run backwards, but it slows the meter a little bit.

Oh! One more thing. If you are going to have enough PVs to sell power to the cower company, get your own meter to check if theirs is accurate. The lecturer for the linked course (jump to slide 318) stated that he was getting shortchanged, and based on his meter they knew how much he had been shortchanged, and issued him a credit when they fixed the utility company's meter.

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