Friday, March 22, 2013
I am starting to work on getting solar power at our house, so the blog will probably have a lot of solar power calculations in coming weeks. The above is a NREL map of the solar resource in the United States. You can get the full (poster) size image here. I think it's a gorgeous map. Pretty much the whole US (with the partial exception of Alaska and parts of the Pacific northwest) has a solar resource ranging from good to great. Where I am in upstate New York, there's round about 4 kWhr/day/m2 for a flat panel tilted at latitude (around 42o in my case) and facing south. Multiplying by 365, that gives 1460 kWhr/yr/m2.
A typical panel with peak output of 240W has an area of 1.64 m2 and an efficiency of 13.4%. The efficiency would take my 1460 kWhr/yr/m2 down to 196 kWhr/yr/m2. Multiplying by the panel area, this implies an output of 320 kWhr/yr per panel. Another way to think about this is 1330 kWhr per kW of peak power (320/0.24). This is the theoretical value before we start taking account of dirt, imperfect alignment, shading, inverter losses, and so forth. PVWatts (a calculator from the National Renewable Energy Labs) can do these calculations, and for my location and situation (in particular I have roofs that face south-east and south-west), I get about 1050 kWhr per kW.
It looks like the house currently is on track to consume about 10000 kWhr per year, suggesting a current need for about 9.5kW of panels to provide all our energy usage. However, we also want to power electric cars in the next couple of years. Between my wife and I, we drive about 30000 miles/year. The Nissan leaf gets about 3 miles/kWhr and the Tesla Model S is similar but a little less efficient (38 kWhr/100 mi). So for 30000 miles of electric vehicle usage, we will need about another 10kW of panels, making 20 kW total, in round numbers. This corresponds to around 83 of the typical modules above, and will consume a roof area of 136m2 (or 163 sq yd, if you'd rather think of it that way).
Luckily, I have a barn as well as a house to put these things on.
Lastly, it may be of interest to see PVWatts estimate of monthly energy production from a 10kW installation at my location:
I'm a bit sceptical of the winter values - around here, everything is covered in snow most of the time in January and February, and I can't see the panels generating a lot of power then (and I certainly have no intention of going up on the barn roof to clear them off). So maybe we should call our final need 22kW to be safe.