The average solar insolation for San Francisco in the past year was 1,624 kWh/year per m², or 4.45 kWh/day/m². The range was from 1,492 kW/year, 4.1 kWh/day, in the Sunset District in the west side of the city, to 1,689 kWh/yr, 4.6 kWh/day, in the Mission District in the east side. A one kilowatt solar photovoltaic system costs approximately $6,000 to install after rebates and tax credits. There are system losses of about 30% in the process of generating AC power, so that a 1 kW system in an area with 1,600 kWh/yr/m² of solar insolation will produce about 1,120 AC kWh/yr.

PG&E electricity rate is 11.4 cents per kWh up to 100% of baseline. At this rate, it costs about $128 per year to purchase 1,120 kWh from PG&E. If the $6,000 cost of a solar system is instead invested at an APR of 4% tax free, the yield is $240 a year, which pays the PG&E electricity bill. The extra money is reinvested every year to pay for future rate increases. Assuming a 4% yearly increase in electricity rates, the $6,000 investment will pay for this amount of electricity every year for the next 50 years. Or alternatively, you will have about $8,000 remaining at the end of 25 years. Here is a spreadsheet illustrating the above Solar sense. You may have read that PG&E rates go up more than 4% a year on "average" but that may not apply to you. Our PG&E bills show that our baseline usage was billed at $0.1159/kWh in January, 2000, and $0.1143/kWh in January, 2007, a 1% decrease in seven years.

If instead you buy a solar photovoltaic system, the expected lifespan is 25 years. There may be additional expenses for maintenance of the system. At the end of 25 years, there will be no money left from your initial investment to pay for a new system. The break even point, where investing your money will pay for the electricity for the same length of time (25 years) as the life expectancy of the photovoltaic system, occurs in this example at a yearly electricity cost of $260, or 23 cents a kilowatt hour. The PG&E rate is 21 cents per kWh for 131% to 200% of baseline, and 29 cents per kWh for 201% to 300% of baseline. If your electrical usage is in this range, it may make financial sense to install a solar power system that decreases your electrical usage to a lower price range, if you can not achieve this goal by improving your energy efficiency. Here are some recent letters to the San Francisco Chronicle about these issues in Business, Home & Garden and Real Estate.

As the performance at both the Moscone Center and the Urban School illustrates, the output is not guaranteed. Even with an expensive system, if your output is less than expected, the usual contract does not include repairs or upgrades to the system after installation. Most smaller systems do not measure output, all that is known is net usage, your total usage minus what is generated, which is what you are billed for. So if your system output begins to decline, you may not even be aware of the problem. Large commercial systems are more cost effective than smaller systems and are more likely to have a technician available to make sure that they are functioning properly, so it is not unreasonable to ask a government agency or the electric utility to install their own systems, if they can demonstrate that these systems make financial sense.

San Francisco insolation

Solar power links.

Solar sense spreadsheet

Urban School solar power system

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