In Solar-Power Surprise, Clouds Can Raise Energy Output

This week the Seattle Times reran a Washington Post story about utilities balking at the popularity of “net energy metering” — where a solar-powered home that produces more electricity than it needs, feeds that power back to the utility’s power grid, and is paid for it, so the monthly power bill works out to be less.

“On sunny days, particularly at midday,” goes the story “those panels can generate more electricity than the home needs. […] But at night, on cloudy days and at times of high demand, that customer still needs power from the utility company, which must provide it instantaneously, as it does for other homes.”

Yet, as more and more people install solar panels on homes in the Northwest, reports UW meteorologist Cliff Mass, they’re noticing something strange. One homeowner sent Mass a note asking why he was seeing “higher peak solar output from his panels during partly cloudy days than clear ones.” It sounds wrong, but it’s not. Mass confirmed the effect by checking the University of Washington’s own solar radiation measurements. Same thing: partly cloudy days delivered more spikes of “sun punch” than perfectly clear days.

Essentially, the clouds help keep a lid on the solar energy that arrives. Explains Mass: “On partly cloudy days, we have periods with breaks in the clouds when you get direct solar radiation from the sun. But you ALSO get solar radiation that is reflected off nearby clouds…solar radiation that is not getting to someone else because the cloud is intercepting it.” Those puffy white clouds are supercharging your solar panels.

This past May, Solarize Washington announced they had solarized the homes of 100 Northwest Seattle residents, providing nearly 450 kW of electricity. (They’re now up to 625 kW. For context, the average energy consumption of a Washington home is 1,068 kWh per month. To meet something of that demand, you might want a 1.2 kW-generating system.) By arranging group purchases of the solar power systems, they achieve discounts of ten to twenty percent.

In Washington, in addition to net metering, there’s also a production incentive to help defray the system’s installation cost: 15 cents per kilowatt-hour (kWh) capped at $5,000 per year baseline, 30 cents per kilowatt-hour if it’s part of a community solar program, and up to 54 cents per kilowatt-hour) if the PV panels or inverter are manufactured in-state. Solarize Washington says that, counting a federal tax credit as well, most participants recoup about half the cost of their solar power system the first year. Right now, a drive is on to solarize central and southeast Seattle. The state’s sales tax exemption for solar power systems expires on June 30, 2013.