SFOG BLOG

San Francisco Solar Maps

A Google Maps mashup of San Francisco Solar Activity was launched by the San Francisco Department of the Environment and CH2M HILL in June, 2007.  It is located at sf.solarmap.org.  There are several problems with this map.  I will comment on two important errors.

The sf.solarmap uses the same figure for solar electric output (5 kWh per day per kilowatt system) for every location in the City
as of July, 2008, more than one year after the sf.solarmap was launched.  To see this, enter the SFDOE downtown address of 11 Grove St., and you get the result:*
Estimated solar PV potential: 7 - 15 kW
Estimated electricity produced: 12775 - 27375 KWh/yr
Try a house in the outer Sunset, 2494 44th Ave., and the result is:*
Estimated solar PV potential: 2 - 3 kW
Estimated electricity produced: 3650 - 5475 KWh/yr
In each of the above examples, dividing the figures in line 2 by the corresponding figures in line 1 gives 1825 kWh/yr or 5 kWh per day per kilowatt system.

Everyone knows, however, that the west side of the City is foggier than the east side and gets less sunshine.  This has been quantified for several years by the 11 SFPUC solar monitoring stations, whose findings as of the beginning of 2008 are summarized in the San Francisco Solar Map.  The west side of the City averaged 4.1 to 4.2 kWh/day/m2 insolation and the east side 4.5 to 4.6 kWh/day/m2.  The citywide average was less than 4.5 kWh/day/m2.

But there is a larger error.  The sf.solarmap uses 5 kWh per day per kilowatt system as output.   Actually, the output, kWh per day per kilowatt system,  is only about 2/3 of the input (insolation) kWh/day/m2 according to California Energy Commission 2001 guidelines. This is demonstrated by the Moscone Center Solar Power System and other locations.

So a more accurate figure for output is about 3 kWh per day per kilowatt system. The figures on the sf.solarmap are too high.

Robert Karis

*  2/10/09  The estimated solar PV potential and electricity produced in the sf.solarmap have inexplicably been increased by a factor of 2-4 times.  However, the output is still incorrect, 1825 kWh/yr or 5 kWh per day per kilowatt system in different areas of the City.  View comments for update.