A friend recently asked me about how photovoltaics [PV] is best used to meet Marin County's Single Family Dwelling Energy Efficiency Ordinance, or SFDEEO. 

Designed to compensate for grid-tied energy demands from homes larger than 3,500SF, this ordinance requires that

• all homes be designed to California's 2005 Energy Efficiency Standards.
• The delta between the home's calculated energy usage and the energy budget for a 3500SF home built to Title 24 Standards be calculated, and
• this delta must be compensated for by increasing the energy efficiency of the residence and incorporating on-site pv power production.

The larger the home gets above 3500SF, the bigger the energy offset is required to be.  I went through the guide published by the county, went through the actual ordinance, but still couldn't find the relationship between size and PV. 

 Then I found the analysis that Marin County used to backstop the ordinance.  Basically, the county takes the annual production of the system, multiplies it by 4.2 [PV provides power at the same time demand on the power grid is peaking--so they estimate it is 4.2 times as valuable as an alternate power source not producing during peak times--defined as TDV, or Time Dependent Valuation] and then divides by the square footage of the home to establish the offset.

The math is different for each home, but it looks like a 5kW system would offset a prototypical 5,000SF home [1500SF larger than the design standard], an 11kW system would offset a 7,000SF home, and a 21kW system would offset a 10,000SF home.  This assumes no greater energy efficiency improvements and in climate zone 2--where 70% of us in the county live.  A lot of assumptions.

There are a multitude of ways to slice and dice the energy consumption and generation to get to the home you want.  You will want to put all options on the table--such as increasing insulation, using radiant barriers, solar shading, and glazing locations and options.  PV is a great component to any home,  and the wisdom of this ordinance is that it gives you credit for using onsite PV to minimize your effect on our old, obsolete power grid.

The good news is that harvesting your onsite available solar resource to reduce the environmental footprint of a large home makes sense for a lot of reasons. 

The bad news is that each deal must be taken on its own, the best outcome is achieved by iterating through the options, and an increasing amount of PV is needed for each additional square foot of conditioned area.