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Once you’ve estimated your system’s kilowatt-hour production (see box at left), you need to determine the cost of your utility electricity. This usually is a flat rate (the national average cost for electricity is almost 10 cents a kilowatt-hour) but some areas, such as California, have penalty-pricing structures in which high-usage households pay more for their electricity. A long-term financial analysis should include an estimated inflation factor for the cost of electricity. The more expensive electricity becomes in the future, the faster your system will pay for itself.

The savings on your electric bill, unlike a stock market investment, will not be taxed. By investing in a solar system, you may get a higher rate of return on your investment than with an average stock market investment. If you are lucky enough to have Time Of Use metering (rates are lower for nights, weekends and off-season usage — available in parts of California, for example), your rate of return will be even better.

Doug Livingston is a former physics instructor who has designed renewable energy systems for a decade. He also is an instructor for the Solar Living Institute in Hopland, Calif. Scott Hollis is an associate editor at Mother Earth News.

People Worry too much about …

Photovoltaic orientation

Changing the PV angle is uncommon — it’s usually restricted to off-the-grid systems in which every little bit of winter sun is more precious than summer sun. For the lower 48 states, any inclination between 5 and 45 degrees will work fairly well. In the absence of shading problems, pointing your PV at any orientation between southeast and southwest also will work nearly as well as due south. Solar-tracking mounts, which automatically follow the daily movement of the sun, rarely are used on grid-tied residential systems.

Backup power from batteries

Batteries reduce the amount of useable electricity your system will produce, as well as make your system more expensive and complicated to install and maintain. “Store” your excess electricity by sending it back into the utility grid for a credit, and if you need power during an outage, buy a generator — they’re cost-effective and probably more environmentally friendly than batteries.

People Don’t Worry Enough about…

Shading

Crystalline modules usually consist of 36 or 72 cells wired together in a series. If just one cell becomes covered by a shadow, perhaps from a vent pipe or tree branch, it can block the flow of the entire module. Built-in bypass diodes help minimize loss, but a shadow covering just 3 percent of a module can equal up to a 50-percent power loss. If you can’t avoid partial shading, consider amorphous PV because it is not as affected by shading as crystalline modules.

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