I live in the San Francisco area, does the fog interfere with using solar? — several readersThe National Renewable Energy Laboratory experts say, “The rule of thumb is that the solar resource during foggy or low-cloud conditions is approximately 10% of the value under clear sky conditions.” And that fit’s my rule too. I size the systems with 10 percent more photovoltaics or solar thermal panel area, and the same for storage (battery bank or water tank). And where applicable, I add small wind or some other clean distributed energy to potentially compensate for less sunlight. Now solar has to work in this City; in 2004 city officials announced their use of the City’s “Community Choice” law in conjunction with the City’s 2001 voter-approved solar bond authority to support more solar and wind installations, and a great database exists so you can review the type of solar installations in the SF area (see link below). — Scott Sklar Scott Sklar is President of The Stella Group in Washington, DC, a distributed energy marketing and policy firm. Scott, co-author of “A Consumer Guide to Solar Energy,” uses solar technologies for heating and power at his home in Virginia.