Does My ADU Need Solar? 2025 California Rules

A new detached ADU needs new solar panels under the 2025 California Energy Code, unless one of the code's exceptions applies. An attached ADU, a garage conversion or an ADU built inside space that is already heated or cooled does not need solar.

The 2025 code applies to permit applications submitted on or after January 1, 2026. Everything below comes from the Energy Code itself (Section 150.1(c)14), the CEC's 2025 Single-Family Compliance Manual and the CEC's ADU FAQ.

Which ADUs need solar?

It depends on how the Energy Code classifies your ADU. See Title 24 for ADUs for the full breakdown.

Your ADU	Energy Code type	Solar required?
New detached ADU	Newly constructed building	Yes, unless an exception applies
Attached ADU	Addition	No
Garage conversion (attached or detached)	Addition	No
ADU or JADU inside existing conditioned space	Alteration	No

For additions, the rule is direct. Exception 6 to Section 150.2(a) says: "Photovoltaic systems, as specified in Section 150.1(c)14, are not required for additions." The CEC's ADU FAQ says the same about attached ADUs. See garage conversion reports for more on that project type.

How big does the solar system have to be?

The required size is the smaller of two numbers:

The equation. Equation 150.1-C: floor area times a climate zone factor A, divided by 1,000, plus the number of dwelling units times a factor B. The factors come from Table 150.1-C.
The roof. The solar access roof area (SARA) times 18 watts per square foot for a steep roof, or 14 watts per square foot for a low-slope roof.

SARA is roof area that can hold panels and gets at least 70 percent annual solar access. It leaves out areas you cannot use because of other code rules, such as fire access pathways.

Here are the Table 150.1-C factors for the climate zones found in Los Angeles County:

Climate zone	Factor A (per sq ft)	Factor B (per unit)
6	0.594	1.23
8	0.586	1.37
9	0.613	1.36
14	0.741	1.26
16	0.59	1.22

Not sure of your zone? See California climate zones.

When is no solar required for a detached ADU?

Section 150.1(c)14 lists five exceptions. Two matter most for ADUs:

Exception 2: under 1.8 kWdc. "No PV system is required when the minimum PV system size specified by section 150.1(c)14 is less than 1.8 kWdc." The compliance manual says this applies to the equation result or the roof-area result.
Exception 1: not enough good roof. On steep roofs, north-facing areas (between 300 and 90 degrees from true north) do not count toward SARA. No PV is required if SARA is less than 80 contiguous square feet.

The others are less common in Southern California. Exception 3 covers roofs that cannot meet snow load rules. Exception 4 covers buildings approved by a planning department before January 1, 2020 with conditions that limit the roof. Exception 5 is a size reduction, covered below.

Can a small ADU fall under 1.8 kWdc?

Yes, often. Here is the math for one 400 square foot ADU in climate zone 9:

400 x 0.613 / 1,000 = 0.25
1 unit x 1.36 = 1.36
Total: about 1.61 kWdc, which is under 1.8

Using the same equation, a one-unit ADU comes in under 1.8 kWdc below about 960 square feet in zone 6, 730 in zone 8, 715 in zone 9, 725 in zone 14 and 980 in zone 16. These are our calculations from Table 150.1-C, not CEC figures. Your report runs the exact numbers.

Two catches:

Two ADUs in one building count together. The compliance manual's Example 9-8b looks at a detached building with two 250 square foot ADUs. The equation is "assessed on the building, and not on the dwelling units in that building individually," so the B factor counts twice. That example needed PV.
The performance path is different. The manual says a project assessed PV under the equation needs it "unless compliance can be achieved without PV using the Performance path." Your report shows which path was used.

Does a battery lower the solar requirement?

Yes. Exception 5 lets the equation size be reduced by 25 percent when the PV is installed with a battery that meets Joint Appendix JA12 and has a cycling capacity of at least 7.5 kWh.

Can I use the main house's solar?

Not the existing panels. The CEC's ADU FAQ says an existing solar PV system cannot be used to meet the requirement for a new detached ADU. But you can add new modules:

They can go on the existing system, and they do not have to sit on the ADU.
They must be on the same lot and part of the ADU's permit.
Your utility must allow the system to be expanded.
The code does not say whether the new modules must serve the ADU's meter or the main house's meter.

The FAQ also says the solar-ready rules apply to new homes in subdivisions of ten or more, so they do not apply to an ADU on an existing lot.

Where does the CF1R show the solar requirement?

The prescriptive CF1R has a section titled "Photovoltaic (PV) Requirements (Section 150.1(c)14)." It lists the A and B factors, the minimum size from the equation, any exception that applies, the SARA, the roof slope type and the adjusted system size. A plan checker reads that section to see whether your plans must show panels.

Need a report for your detached ADU? See our flat prices.

Frequently asked questions

Does a new detached ADU need solar under the 2025 Energy Code?

Usually, yes. The CEC says a newly constructed detached ADU must have newly installed solar PV to meet Section 150.1(c)14, unless an exception applies. The most common exception for small ADUs is that no PV is required when the calculated size is less than 1.8 kWdc.

Does an attached ADU or garage conversion need solar?

No. Both are additions under the Energy Code, and Exception 6 to Section 150.2(a) says PV systems are not required for additions.

Can a small ADU skip solar?

Sometimes. The prescriptive PV size comes from Equation 150.1-C, based on floor area and climate zone. If the result, or the roof-area result, is less than 1.8 kWdc, Exception 2 says no PV is required. For a one-unit ADU, that happens below roughly 720 to 980 square feet, depending on the climate zone.

Can I use the solar panels already on my main house?

Not as they are. The CEC says an existing system does not meet the requirement. You can add new modules to the existing system on the same lot, as part of the ADU's permit, if your utility allows the expansion.

Does a battery reduce the solar requirement?

Yes. Exception 5 to Section 150.1(c)14 lets the PV size from Equation 150.1-C be reduced by 25 percent when it is installed with a qualifying battery of at least 7.5 kWh.

Where does my Title 24 report show the solar requirement?

The CF1R has a photovoltaic section that lists the minimum PV size from Equation 150.1-C, any exception that applies, the solar access roof area and the adjusted system size.

Does my ADU need solar panels?