What Is Title 24?
Title 24 is California's Building Energy Efficiency Standards, updated every three years by the California Energy Commission. Since January 2020, the solar mandate has required panels on most new homes. In 2023, it was formally extended to ADUs. If you're submitting a permit in 2026, you're under the 2025 Title 24 standards that took effect January 1, 2026.
Which ADUs Actually Require Solar?
The requirement depends on ADU type, size, and whether any exemptions apply:
Which ADUs require solar:
- New detached ADU — Yes (unless exempt)
- Attached ADU/addition — Usually no
- Interior conversion — No
- Garage or basement conversion — No
- Manufactured/modular ADU — No
- Detached ADU under ~600–700 sq ft — Maybe not
Garage conversions, basement conversions, and interior ADUs are classified as alterations — not new construction — and are not subject to the solar mandate. This surprises many homeowners, usually in a good way.
How Is ADU Solar System Size Calculated?
When solar is required, Title 24 uses Equation 150.1-C to size the system based on your conditioned floor area and California climate zone. Most ADUs between 600 and 1,200 square feet require 1.8–3.5 kW — typically 4 to 8 panels depending on wattage. High-efficiency panels like QCells Q.TRON (440W) mean fewer panels while still meeting the required output.
If the formula calculates a requirement below 1.8 kW, the solar mandate is waived entirely. For most climate zones, this applies to ADUs under roughly 600–700 square feet.
The Four Official Solar Exemptions
- Garage and structural conversions — Classified as alterations, not new construction. The most widely used exemption in California.
- Small system size (under 1.8 kW) — If the Title 24 formula produces a requirement below 1.8 kW for your ADU and climate zone, the requirement is automatically waived.
- Limited Solar Access Roof Area (SARA) — If your ADU's roof has fewer than 80 contiguous square feet of viable solar area due to permanent shading from trees, adjacent structures, or hillsides, the requirement can be waived with proper documentation.
- Manufactured and modular ADUs — Factory-built ADUs placed on a foundation are generally exempt from the PV mandate.
Battery Storage: What the 2025 Code Requires
Battery storage is not yet mandated for most single-family ADUs — but the 2025 code introduced battery-ready pre-wiring: a dedicated 240V circuit and panel space reserved for a future battery. You don't have to install a battery now, but the infrastructure must be there.
That said, installing a battery alongside your original solar system is significantly more cost-effective than retrofitting later. Under NEM 3.0, storing your solar energy and using it during peak hours (4–9 PM) is far more valuable than exporting it at the drastically reduced export credit rates.
The Steps to Get Title 24 Compliant
- Confirm your ADU type and exemption status — This single step determines whether solar is required at all.
- Calculate your required system size — A licensed solar installer will do this free as part of a consultation using Equation 150.1-C.
- Get a Title 24 energy report — Required for permit approval, generated by a certified consultant using approved software (EnergyPro or CBECC-Res). Typically costs $300–$800.
- Install a compliant solar PV system — Must be included in your permit application, properly sized, and installed before final inspection.
- Pass final inspection — Without it, you cannot legally occupy or rent the unit. Compliance is not optional.
Common Questions
My main house already has solar. Do I need more panels for my ADU? Yes, but not a separate system. You can expand your existing system with new panels, as long as they're included in the ADU's permit application. The 2025 code doesn't require panels to physically serve the ADU's meter.
What happens if my ADU doesn't comply? Permit denial or failed final inspection. Without final inspection approval, you cannot legally occupy or rent the unit.
Why ADU Solar Is a Smart Investment Even When Not Required
California electricity rates have more than doubled since 2015. An ADU adds load to your property — sizing your solar system to cover both the main home and ADU from the start means generating more of what you consume rather than buying it at peak rates. Homes with solar command measurable premiums at resale, and renters increasingly factor utility costs into housing decisions.
US Power works with California homeowners throughout the permitting and installation process — American-made QCells panels, factory-direct pricing 15–20% below market, and a 25-year comprehensive warranty covering panels, workmanship, and performance. Talk to a licensed consultant today and get your ADU project on the right track from day one.
Better savings for QCells solar through US Power or US Power as Qcells direct-partner
California ADU Solar Requirements Title 24 — Full Guide ☀️