ADU solar requirements in California under the 2025 Energy Code

Which ADUs need solar: the answer by project type

The Energy Commission sorts ADUs by what the project does to the property, not by size. A new, detached ADU is a “newly constructed building” and must meet the rules for a new single-family home. Attached ADUs and conversions of unconditioned space, such as a garage, are additions. Converting space that is already heated or cooled, such as a finished basement or pool house, is an alteration. CEC·checked Sep 23, 2026

The CEC is direct about attached units: “No, since attached ADUs are considered additions, solar PV is not required.” A junior ADU of up to 500 square feet built inside a new home falls under that home’s own requirements. And an ADU that was built without a permit and is being permitted now must meet the Energy Code in force when the application is filed.

How much solar a new detached ADU needs

According to the CEC’s 2025 single-family solar PV guidance (checked September 2026), Section 150.1(c)14 gives two ways to size the system, and the required size is the smaller of the two. CEC·checked Sep 23, 2026

• Solar access roof area (SARA): 18 watts per square foot on steep-sloped roofs, or 14 watts per square foot on low-sloped roofs. SARA can include carports and other new structures on the property, and excludes roof area with less than 70 percent annual solar access.
• Equation 150.1-C: based on the building’s climate zone, conditioned floor area and number of dwelling units.

The CEC says the minimum is meant to offset roughly the building’s electricity use as if it had gas appliances, so choosing a heat pump or electric appliances does not raise the required size. Your ADU designer or energy consultant runs this calculation for the permit set; ask to see it rather than accepting a round number. For sizing a system to your own usage rather than the code minimum, see how big a solar system you need.

The five exceptions

According to the CEC’s 2025 Energy Code guidance, five exceptions remove or reduce the requirement:

1. No PV is required if the solar access roof area is less than 80 contiguous square feet.
2. No PV is required if the minimum size from Section 150.1(c)14 is less than 1.8 kWdc.
3. No PV is required if the enforcement agency finds the system cannot meet ASCE 7-16 snow load requirements.
4. Buildings approved by the local planning department before January 1, 2020, with mandatory conditions of approval.
5. With a qualifying battery, the size from Equation 150.1-C may be reduced by 25 percent.

Exceptions can’t be stacked: the CEC says the under-1.8 kWdc exception cannot be combined with the battery reduction. For steep-sloped roofs, roof area facing between 300 and 90 degrees from true north is left out of SARA, which is how a north-facing or heavily shaded ADU roof can end up with no requirement at all.

Can the main house’s solar cover the ADU?

Not the panels you already have. “Newly constructed detached ADUs must have a newly installed solar PV to meet the prescriptive requirements of Section 150.1(c)14 unless an exception applies,” the CEC says. CEC·checked Sep 23, 2026

You can, however, add new panels to the existing system, and they don’t have to sit on the ADU. The CEC allows it if the added panels are on the same residential lot, are part of the ADU’s permit application, are sized to the Energy Code, and your load-serving entity allows the expansion. The Energy Code does not care whether those panels serve the ADU’s meter or the main house’s meter. On a shaded ADU lot, putting the new panels on a sunnier main roof can make sense. Check whether that roof is ready for solar first.

Separate meter or shared meter

The Energy Code leaves metering to you and your utility. A separate meter makes sense when a tenant pays the ADU’s electricity; a shared meter is simpler when the household pays one bill. Either way, adding panels or a new interconnection is your utility’s process. For PG&E, SCE and SDG&E customers, the CPUC says anyone applying for interconnection since April 15, 2023 takes service on the net billing tariff, which credits exports at values “usually lower than the retail rate.” CPUC·checked Sep 23, 2026 How that changes the value of ADU solar is explained in NEM 2.0 vs NEM 3.0 in California.

Battery-ready rules and optional batteries

According to the CEC’s 2025 ADU guidance, a new detached ADU must meet the battery-ready requirements of Section 150.0(s) unless the ADU’s electric service is 125 amps or less. A 200-amp panel can qualify only if its busbar is rated 225 amps and marked. Installing a complete battery system instead also satisfies the rule. Separately, a battery of at least 7.5 kWh that meets Joint Appendix JA12 lets you cut the Equation 150.1-C solar size by 25 percent. The California home battery guide covers what a battery costs and does.

Two related rules also apply to new detached ADUs: electric-ready wiring if you install gas appliances, and, for units of 500 square feet or less, a prescriptive option for an electric water heater with point-of-use distribution where a heat pump water heater won’t fit.

Cost and the tax credit

Required solar is part of the ADU’s construction cost. Ask the builder to price it as its own line so you can compare it with a separate solar bid. Don’t count on the federal residential credit to offset it: the IRS says the Residential Clean Energy Credit “is not available for any property placed in service after December 31, 2025.” IRS·checked Sep 23, 2026 How solar fits into the rest of the financing is covered in cash, loan, lease or PPA.

Steps that keep an ADU permit moving

1. Tell your ADU designer early whether the unit is detached, attached or a conversion; that decides everything above.
2. For a detached unit, get the SARA and Equation 150.1-C numbers shown on the plans, with any exception claimed.
3. If you want new panels on the main house instead, ask your utility whether it allows expanding the existing system.
4. Decide on metering before the electrical design is final.
5. Use licensed contractors and check their licenses; see how to verify a California solar contractor.

HOA rules can also come up on an ADU roof; California’s HOA solar rights explains what an association can and can’t require. To compare installers for the work, start with how to compare solar companies in California.