How is the minimum solar PV size calculated for a new detached ADU?

The Title 24 report calculates it. Under Section 150.1(c)14 of the 2025 Energy Code, the required PV size for a newly constructed ADU is the smaller of two numbers: a formula based on conditioned floor area and climate zone, or a cap based on how much usable sunny roof the building has. If the result is under 1.8 kWdc, no PV system is required.

Step 1: the floor-area formula

Equation 150.1-C sets the size as (conditioned floor area x A) / 1000 + (number of dwelling units x B). A and B are adjustment factors from Table 150.1-C, and they change by climate zone. That's why two identical ADUs in different parts of California can get different PV sizes. Your climate zone is set by the parcel address.

Step 2: the solar access roof area cap

The second number comes from the solar access roof area (SARA). The code multiplies SARA by 18 for steep-sloped roofs or 14 for low-sloped roofs, which the CEC's ADU training expresses as watts per square foot. SARA includes roof area that can structurally support PV, including carports and other newly constructed structures on the site. It excludes:

• roof area with less than 70 percent annual solar access, as calculated with an approved solar assessment tool;
• occupied roof areas; and
• roof area that other state or approved local code requirements make unavailable.

Whichever result is smaller is your minimum PV size.

Step 3: check the exceptions

• Under 1.8 kWdc: no PV is required (Exception 2). This is the exception small ADUs most often use.
• North-facing steep roofs: roof areas facing between 300 and 90 degrees from true north are left out of SARA, and no PV is required if SARA is under 80 contiguous square feet (Exception 1).
• Battery storage: the formula size may be reduced by 25 percent when a qualifying battery system of at least 7.5 kWh is installed (Exception 5).
• Other exceptions cover high snow loads and certain buildings approved by planning before January 1, 2020.

What the calculation does not do

• It does not apply to additions. An attached ADU or a garage conversion has no Energy Code PV requirement.
• It does not give credit for panels already on the main house. New modules are required, although they can be added to an existing system on the same lot.
• It sets a minimum, not a maximum. You can install more PV if your utility allows it.

What this means for your MEP plans and Title 24 report

We run the PV calculation in the compliance software for your parcel's climate zone and record the result or the exception on the CF1R. The electrical plans then show the PV interconnection and any battery equipment the report relies on. A solar contractor designs the array to meet that number. See the ADU Title 24 guide, why your climate zone matters, and Title 24 reports.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.