Sizing Your System: Part II

By now you should have figured your average kilowatt hour usage over the past six to twelve months. As explained in Part I, if you're paying 8-9 cents per kWHr this is the average cost most Americans are now paying. You could still install a PV system but your motivation likely would be more altruistic than financial. Of course, it could make major financial sense longrange as conventional energy production gets more costly.

Uni-Solar sells two sizes of kits: 1.8kW/DC and 2.5kW/DC. (The former requires about 315 square feet of roof space and the latter, about 450. This general area applies to our framed modules, PV laminates for metal roofs and solar shingles.) After inversion from DC to AC watts the 1.8kW system produces up to 1.53kw/AC. If your house has a good south-facing area with a standard 4/12 roof pitch, figure 97% efficiency with Uni-Solar photovoltaics. So now we know AC production and solar orientation. Next, we need to figure average daily solar radiance.

Click on the PV Radiation Map of the U.S at left. Notice that San Diego is at the southwestern corner of California just above the Mexican border. The color code is light brown which correlates to 5.5-6 daily sunshine hours per day. Figuring conservatively, multiply 5.5 by 365 days and you get 2008 total average radiant hours per year. Now finish the equation:

1.53 (AC watts) X .97 (solar orientation) X 2008 (annual sun hours)= 2980 kWHrs annually or 248/month.

Let's try it with a 2.5kW system which produces 2218 AC watts:

2.218 X .97 X 2008= 4320 kWHrs/yr or about 360/month

If a San Diego household uses 800 kWHrs per month (fairly common) then a 5 Kw/DC system (two 2.5kW kits) will produce 720 kWHrs per month or 90% of demand.

Let's estimate a system size in Chicago. The map shows the Windy City in a bright yellow area at 4.5-5 daily average energy producing hours. The estimate for a 5kW system there is:

4.436 (two 2.5kw kits) x .97 x 1643= 7070 kWHrs/yr or about 589/month.

NOTE: If the ridge of your house is north/south your solar orientation is east/west which is 88% efficient.

These estimates are general but sufficient for sizing a typical home grid-tie system. Data utilized for estimating come from both the National Renewable Energy Labs and standard testing performed under authority of the California Energy Commission.

Now you can estimate any size system--residential or commercial--with fairly reliable results.