We have been living in our new house for 1 year now. The following graph shows our energy consumption. It is very high, our house is all electric, and not very well insulated.
Out total annual usage was 19,440 KWh, an average of 1620 KWh per month, 55KWh per day.
Considering the 6 cold months, average consumption is 72 KWh/day.
With these numbers, it is now possible to determine the area of solar collectors needed to provide 50% of our energy with solar.
First, the solar insolation is Seattle (in KWh/m2/day): [ Reference ]
The average daily insolation for the 6 cold months, October through March, is:
Winter insolation = 1.8 KWh/m2/day = 0.18 KWh/sqft/day
Solar panel efficiency is about 0.6 for flat plate collectors.
[ Reference ]
I apply another 80% factor to that, due to the imperfections of a home made collector. The total insolation comes out at:
0.18 * 0.6 * 0.8 = 0.086 KWh/sqft/day
The goal is to provide 50% of our needs, which is 72/2 = 36KWh/day.
This equates to 36 / 0.086 = 400 sqft.
This is the same number I got earlier (in the Carbon Masters presentation) using a different method.
Water Heating can be estimated between 1/4 and 1/3 of heating needs, or about 120sqft. This too matched the calculations made during the hot water system design.
Now that the numbers have been cross-checked with two different methods, I can finally proceed with the construction.