In California, solar panels are required on a new accessory dwelling unit only when the ADU is detached and built from the ground up. Garage conversions, basement conversions, and ADUs attached to the main house are exempt because the Energy Code (Title 24, Part 6) treats them as alterations or additions rather than new construction.1California Energy Commission. 2025 Energy Code Accessory Dwelling Units (ADU) FAQs If your project does trigger the mandate, the required system size depends on the ADU’s floor area and climate zone, and several exemptions can reduce or eliminate it.
Which ADU Projects Trigger the Mandate
Three categories cover almost every project:
- New detached ADUs. Solar PV is required. Pouring a foundation and framing a standalone structure counts as new low-rise residential construction, which carries the full solar obligation.
- Garage, basement, or similar conversions. No solar required. Converting existing space into a dwelling is an alteration, not new construction.
- Attached ADUs. No solar required. An ADU sharing a wall with the primary residence is classified as an addition.1California Energy Commission. 2025 Energy Code Accessory Dwelling Units (ADU) FAQs
Your local building department makes this classification during plan check, not after construction. Confirm how your project will be classified before you finalize design, because the answer changes your equipment list, your budget, and your electrical scope.
How the Required System Size Is Calculated
There is no single panel count that satisfies the code for every ADU. The Energy Code uses Equation 150.1-C, which takes the conditioned floor area of the ADU, applies a climate-zone adjustment factor, divides by 1,000, and adds a per-dwelling adjustment to produce a minimum system size in kilowatts of direct current.2California Energy Commission. 2022 Single-Family Solar PV
California has 16 climate zones, and the adjustment factors differ meaningfully between them. A 500-square-foot ADU in foggy San Francisco produces a smaller required system than the same unit in sunny Riverside. For a typical one-bedroom detached ADU of 400 to 750 square feet, the calculation usually lands between 1.8 and 3 kWdc, though climate zone swings the number in either direction.
An energy consultant or plan designer runs the calculation in compliance software approved by the California Energy Commission, and the result appears on the project’s CF1R energy compliance form. The system in your permit application must meet or exceed that number.
Exemptions That Shrink or Eliminate the Requirement
Section 150.1(c)14 of the Energy Code builds in several exceptions, and any of them can apply even to a new detached ADU.
Required System Below 1.8 kWdc
If Equation 150.1-C produces a required size under 1.8 kWdc, no PV installation is needed.3California Energy Commission. 2025 Single-Family Solar PV Compact studios in mild climate zones often slip under this floor. The calculation still has to be run and documented; the exemption just means nothing has to be installed.
Insufficient Solar Access
If permanent obstructions such as trees, hills, or neighboring buildings leave less than 80 contiguous square feet of roof with adequate solar access, no PV system is required.3California Energy Commission. 2025 Single-Family Solar PV The usable area must also receive at least 70 percent of what an unshaded system would produce annually. You can’t just point at a tree. The exemption requires a professional solar assessment using a CEC-approved tool, with dated satellite, drone, or other imagery documenting the shading.4California Energy Commission. Solar Assessment Tools
Battery Storage Reduction
Installing a battery storage system of at least 7.5 kWh alongside your panels reduces the required PV size by 25 percent. This shrinks the mandate rather than eliminating it, but it can cut both equipment cost and roof space demand.
Conversions and Attached Units
Garage conversions and attached ADUs remain exempt because the code never treats them as new construction in the first place.1California Energy Commission. 2025 Energy Code Accessory Dwelling Units (ADU) FAQs
Every exemption lives or dies on documentation. Your building department reviews the claim during plan check, and approval depends on the right calculations, shading reports, or project description showing the exemption applies.
Putting the Panels on the Main House
The panels don’t have to sit on the ADU itself. The 2025 Energy Code allows new solar modules to be added to an existing system on the main house, or anywhere else on the same lot, to satisfy the ADU’s requirement.1California Energy Commission. 2025 Energy Code Accessory Dwelling Units (ADU) FAQs The code doesn’t even require the new modules to feed the ADU’s meter.
That flexibility matters because ADU roofs are often small, awkwardly oriented, or shaded by the primary residence. Expanding an existing array on a south-facing main-house roof is frequently cheaper and more productive. The new modules still have to be sized to meet the ADU’s Equation 150.1-C result, included in the ADU’s permit application, and allowed by your utility. Check with your load-serving utility before counting on an expansion, because some have interconnection limits that complicate the plan.
Battery-Ready Wiring Under the 2025 Code
Projects permitted under the 2025 Energy Code, which takes effect January 1, 2026, must meet BESS-ready infrastructure requirements for newly constructed ADUs of any size.5California Energy Commission. 2025 Single-Family Battery Energy Storage System (BESS) Ready You don’t have to buy a battery. The ADU just has to be wired so a battery can be added later without major electrical rework.
The requirement applies when the ADU’s electric service exceeds 125 amps. At 125 amps or less, BESS-ready provisions don’t apply.5California Energy Commission. 2025 Single-Family Battery Energy Storage System (BESS) Ready When they do apply, the practical pieces are:
- A main panelboard with at least a 225-amp busbar.
- Reserved space for a transfer switch or isolation equipment within three feet of the main panel.
- At least four branch circuits a future battery could back up, including a refrigerator, egress lighting, and a sleeping-room outlet.
- Either battery-ready interconnection gear supplying at least 60 amps of backed-up capacity, or a dedicated one-inch raceway running from main service to a labeled subpanel.
Installing an actual battery system automatically satisfies the BESS-ready provisions. Running conduit and sizing panels during construction is far cheaper than opening finished walls later.
What Compliance Costs Now
Installed residential solar in California runs roughly $2.30 to $3.50 per watt before incentives. For a 2 to 3 kWdc system on a typical detached ADU, that works out to about $4,600 to $10,500 all in, covering equipment, labor, and permit fees. Smaller systems tend to cost more per watt because permitting fees and inverter hardware don’t scale down with array size.
The bigger shift for 2026 is on the tax side. The federal Residential Clean Energy Credit under Section 25D of the Internal Revenue Code applied to solar systems placed in service through December 31, 2025, and the statute terminates the credit for expenditures after that date.6Office of the Law Revision Counsel. 26 USC 25D – Residential Clean Energy Credit If your ADU permit is issued in 2026 and the panels go in after that cutoff, the 30 percent federal credit is no longer available.
Even while it existed, the credit had limits that excluded many ADU owners. Landlords who didn’t live in the home couldn’t claim it, which ruled out property owners planning to rent both the main house and the ADU.7Internal Revenue Service. Residential Clean Energy Credit With the federal credit gone, the financial case for mandatory ADU solar rests on long-term energy savings and whatever state or utility programs still apply.
One further billing wrinkle: solar that was voluntarily installed by PG&E and SCE customers before the end of 2027 qualifies for a nine-year export compensation adder on top of standard Net Billing Tariff credits. Solar installed to satisfy the building code, which covers most new detached ADUs, does not get that adder, and SDG&E customers are excluded from it entirely.8California Public Utilities Commission. Net Energy Metering and Net Billing
Permits, Inspection, and What Happens If You Skip It
Solar plans are part of the ADU building permit application. A standard package includes a PV design sheet showing panel layout and mounting, along with the CF1R energy compliance form confirming the system meets the Equation 150.1-C result. Some jurisdictions accept the solar component as a deferred submittal, letting structural plans move ahead first, but the solar design still has to be approved before final inspection.
The building department reviews the design for structural capacity, electrical code compliance including wiring, inverter placement, and rapid shutdown, and fire safety setbacks from roof ridges and edges. After construction, a physical inspection confirms the installed system matches the approved plans. Separately, you need Permission to Operate (PTO) from your electric utility before connecting to the grid. Utility interconnection review often runs several weeks after installation, so expect a gap between passing final inspection and seeing credits on your bill.
Skipping compliance isn’t really an option because enforcement happens at the permit counter. A building department will not issue a permit for a new detached ADU without an energy compliance report showing the solar requirement is addressed, either by an installed system or by a documented exemption. If an ADU is built without complying, it won’t pass final inspection, which means no certificate of occupancy, which means the unit can’t be legally occupied or rented. Fixing it later means retrofitting solar onto a finished structure, which costs more than building it in and can delay the project by months.