So, we had replaced our old light bulbs with compact fluorescents, nagged the kids about turning off the lights, installed extra insulation, turned down the thermostat, bought a Prius. What else could we do to fight global warming? It was time to take the next step – direct use of solar energy.
By the end of September, 2008, a roof-mounted solar photo-voltaic system at our San Francisco home should be generating much of the electricity our household uses, with a net savings of about $35 per month on our electric bill. The best part: our total up-front cost was $29, thanks to currently generous federal, state and San Francisco subsidies. Over its lifetime the system will save the equivalent of 135,000 pounds of CO2 emissions, alleviating a major source of global warming. Solar energy – touted since the 1970’s as an alternative to the environmental problems of fossil fuels – has finally become economic for the average homeowner.
Even in foggy coastal areas, much of the Bay Area is well suited to making use of the sun’s energy for generation of electricity (solar electric or photo-voltaic systems). Our generally mild and dry climate and clean air allows for plenty of energetic sunshine. Because photo-voltaic systems generate power using light (not heat), fog only cuts down about 10% of their efficiency. The cooler temperatures that accompany fog actually improve performance, as photo-voltaic cells work best at cold temperatures.
Photo-voltaic systems generally involve the use of flat, dark colored, roof-mounted panels, to absorb solar energy. To be most effective, the panels generally want to face the south to southwest or even west, at an angle of about 30 degrees from horizontal, and be largely free from shading by nearby trees or buildings. The great majority of homes have a large area of roof that fits these criteria. Flat roofs work too, since panels can be installed on racks to raise their angle. Where the panels are located on sloped roofs they generally are installed on racks a few inches above the roofing material, to allow ventilation for cooling. The racking allows for easy removal and replacement of panels if maintenance on the roof is needed, and minimizes structural penetrations of the roofing.
The panels themselves are composed of semi-conducting silicone wafers – essentially the same material that computer chips are made of – to turn sunlight directly into electricity. The panels produce direct current (DC) electricity, so an electronic inverter is also needed to change the electricity into the AC current that our houses use. In rural areas, expensive batteries are sometimes necessary to store electric power so as to be “off grid”; here in the Bay Area it makes more sense to feed the power back into the electric grid. The power generated reduces the electric meter reading, sometimes even making it run backwards. “Time-of-use” meters are also installed, so that energy generated in the afternoon, when power is most valuable, is credited at a higher rate. Conversely, power used at night and in the early morning, when general usage is less, is billed at lower rates.
Typically, systems are sized to generate enough power to offset about 60% of a homeowner’s electric bill, with the remaining amount falling under the least expensive, lowest tier in PG&E’s rate structure. A typical household needs between 200 and 300 square feet of panels. The inverter, a cut-off switch, a new time-of-use meter and electric conduits to connect everything comprise the rest of the system. The life span of these systems is expected to be 25 to 30 years, with the inverter probably needing to be replaced once during this period. Very little maintenance is required.
Government subsidies have never been better for these systems. The Federal government currently offers a 30% Investment tax credit (capped at $2,000 for residential installations, subject to limitations imposed by the alternative minimum tax) and accelerated depreciation. The California Solar Initiative is offering incentives up to $250 per watt of installed capacity, depending on system parameters. And San Francisco is, while it lasts, portioning out $1,500,000 for residential installations, up to (see Solar Subsidy)
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