Is My Roof Right?

Determining whether solar makes sense…

How Much Roof-Space Do I Need?

A rule of thumb is 100-150 square feet per kilowatt. Since the average residential system is 5 kilowatt (kW), that means 500 – 750 square feet. And the space really needs to be contiguous — roofs look pretty silly if panels are scattered about as if they were dropped from a plane. But if you don’t have enough space to eliminate your whole electrical bill, that’s not a problem. That’s because PG&E’s base rate is actually pretty low; it’s the pricing for “above-average” use that’s expensive.

E-1 Rate Rate per Kilowatt-hour
Baseline $0.13
101% – 130% of baseline $0.15
131 – 200% of baseline $0.31
201% and up $0.35

Let’s say, with your big house and three teenagers, you’re using three times your baseline. That first third of your electricity is not the problem; it’s 13 cents per kilowatt hour. Even if you had the room on your roof, it might not be worth eliminating that portion of your bill. But that last third of your bill is almost three times as expensive: 35 cents per kilowatt hour! There’s no excuse not to eliminate this portion of your bill by adding solar. There’s more on this at, but the take-away is that, even if eliminating your electric bill requires a 6,000 kW system, a system half that size will remove the most expensive electricity from your power bill.

Earlier I said that solar makes sense because we’re in PG&E territory, and energy’s expensive. That remains true. PG&E’s cheapest tier is still more expensive than the average charge for electricity almost anywhere else in America.

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