From the press release (emphasis added):

Moving to cut energy use in new homes by more than 50 percent, the California Energy Commission [CEC] today adopted building standards that require solar photovoltaic systems starting in 2020. The building energy efficiency standards, which are the first in the nation to require solar, will reduce greenhouse gas emissions by an amount equivalent to taking 115,000 fossil fuel cars off the road.

I've read a few different articles about the new standards, and spent some time reviewing the standard itself, but I have one big question:

For a specific home, how will the builder determine the exact capacity of solar system that must be installed?

An infographic from the CEC gives a figure of $9,500 for the initial cost, but what is this based on? Will the amount of solar required vary depending on the size of the house, number of bedrooms, location or location type (urban, rural, etc), or some other factor?

up vote 3 down vote accepted

From Greentech Media's recent article, Everything You Need to Know About California’s New Solar Roof Mandate:

The solar mandate will be climate zone-specific and based on the floor area of the dwelling unit. The PV system must be sized to net out the annual kilowatt-hour energy usage of the dwelling.

This gave me a starting point to dig into the code -- a text search on "floor area" finally led to the section with the formula used to determine the exact capacity, detailed below.

tl;dr: For an average-sized 100 square meter house, a system of 1.7 to 3.1kW will be required, depending on climate zone.

The formula

Digging in to the code, I found the formula on page 304, in Part 6, Chapter 8, Section 150.1(c)14:

kWPV   = (CFA x A)/1000 + (NDwell x B)

kWPV   = kWdc size of the PV system
CFA    = Conditioned floor area
NDwell = Number of dwelling units
A      = Adjustment factor from Table 150.1-C
B      = Dwelling adjustment factor from Table 150.1-C  

NDwell is 1 when looking at a single home, and CFA is the total floor area that will be heated and/or cooled year round (so excluding porches and balconies, etc).

Scaling and adjustment factors

The table of values for A and B is on page 306. Based on climate zone (see below), A scales the floor area up or down, and B is an adder, or minimum requirement per dwelling, regardless of floor area.

Table 105.1-C -- CFA and Dwelling adjustment Factors

Climate zone definitions

Climate zones are detailed in Joint Appendix JA2-1:

California Building Climate Zones

Average system size

Assuming an average home size of 100 m2, the range of values for required size of a PV system will be 1.7 to 3.1 kW. Before adding B, the dwelling adjustment factor, the requirement is 6.2 to 16.8 watts of PV capacity per square meter of floor area.

It is an estimation, made by "The California Energy Commission" not considering a specific product. They calculate: "Single-family homes built with the 2019 standards will use about 7 percent less energy due to energy efficiency measures versus those built under the 2016 standards. Once rooftop solar electricity generation is factored in, homes built under the 2019 standards will use about 53 percent less energy than those under the 2016 standards. This will reduce greenhouse gas emissions by 700,000 metric tons over three years, equivalent to taking 115,000 fossil fuel cars off the road. Nonresidential buildings will use about 30 percent less energy due mainly to lighting upgrades.


On average, the 2019 standards will increase the cost of constructing a new home by about $9,500 but will save $19,000 in energy and maintenance costs over 30 years. Based on a 30-year mortgage, the Energy Commission estimates that the standards will add about $40 per month for the average home, but save consumers $80 per month on heating, cooling and lighting bills. "

  • Hmm I clarified the question a bit. I understand the averages, but if I were building a house, how would I know how big a solar system I must install? Is there some formula that is used to determine this? – LShaver May 14 at 20:49
  • They are not only considering solar power, but also improvements with other components like Building insulation, inside ventilation, other energy consumers, solar water heating AND Solar Photovoltaic Systems. They estimate an improvement in power consumption of 7% less. Half of electric energy (53%) consumed in the household shall be produced by additional photovoltaic systems. These people calculate with estimates and statistic figures (average household consumption, market price reduction, etc.). – Salt May 14 at 21:08
  • 1
    This doesn't seem to answer the question asked. – EnergyNumbers May 15 at 19:25
  • I worked with big international companies and developed projects for them. Forecasts are never made on individual figures, but only on assumptions, estimates and statistical figures. Governments work the same way. It is unlikely to receive an other answer to that question in general. If to build a house the formula to confirm with the objectives would be: Calculate the power consumption based on 2016 standards and then reduce that value by 53%. They assume that the required additional costs will be around $9,500. This should include all additional effort and not just solar photovoltaic. – Salt May 15 at 20:13
  • Please see my answer -- from a recent news article I was able to find the specific requirement, which depends on the size of the home and the climate zone where it's located. – LShaver May 22 at 12:54

You will more than likely need as much power as you need to consume, actual standards for CA going 100% renewable do not start until 2045 so this has not been defined yet.

In 2020 all new homes built in the state of CA will require rooftop solar in their construction, to accommodate on grid net-metering, as well as off-grid living standards for those who can not meet a grid. However, the answer to the question may very well be something between bi-partisan and non-partisan. It's non-conclusive, and destabilizes any argument.

After all 20 + years is a long time. Ultimately, look at the benefits of home solar.

However, new homes in zoning require solar panel systems. This does not mean you need excess power, because the rest is going onto the grid. But the homes will need to be able to satisfy an electricity bill.

System size can be judged by sizing your total system, based on your average utility bill and the amount you will need to reach this number. This quantity is assessed by the size of your home in combination with the bill.

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