What Changed in the 2025 Code Cycle
California's Title 24 standards update every three years. The 2025 cycle took effect January 1, 2026, bringing stricter documentation requirements — you now need a full CF1R, CF2R, and CF3R package as part of your permit submittal, not just the CF1R alone. Heat pump water heaters are also now the prescriptive baseline for new detached ADUs. If your builder is still referencing the 2022 code, raise it immediately.
Catching the solar requirement after structural plans are finalized means redesigning your roof layout, potentially upgrading your electrical panel, and re-running your permit application — costing weeks of delay and thousands in redesign fees.
Which ADUs Actually Require Solar?
The requirement depends on whether your project is classified as new construction or an alteration:
- New detached ADU — Yes, solar required (unless exempt)
- Attached ADU/addition — Usually no
- Garage conversion — No
- Basement or interior conversion — No
- Junior ADU (JADU) — No
- Manufactured or modular ADU — No
Garage conversions, basement conversions, and ADUs built within an existing footprint are classified as alterations — not subject to the solar mandate. This surprises many homeowners planning garage conversions.
How System Size Is Calculated
When solar is required, Title 24 uses Equation 150.1-C to determine the minimum system size. The formula factors in your conditioned floor area and applies climate-zone-specific multipliers. A larger ADU in a hotter inland climate zone requires a larger system than a small ADU in a mild coastal zone.
For most ADUs between 600 and 1,200 square feet, the formula produces a requirement of 1.8–3.5 kW — roughly 4 to 8 panels depending on wattage. High-efficiency panels like QCells Q.PEAK DUO (430W+) can satisfy the requirement with fewer panels, which matters when ADU roof space is limited.
The 1.8 kW Automatic Exemption
If the formula calculates a requirement below 1.8 kW, the solar mandate is waived entirely. For most California climate zones, this applies to ADUs under approximately 600–700 square feet — and is the most commonly applied exemption in the state.
The Four Official Exemptions
Even for new detached ADUs, four paths can eliminate the solar requirement:
- System size below 1.8 kW — If Equation 150.1-C produces a result below 1.8 kW, the requirement is automatically waived
- Limited Solar Access Roof Area — Fewer than 80 contiguous square feet of viable solar area due to permanent shading from trees, adjacent buildings, or hillsides, documented in the permit application
- Structural unsuitability — If the roof cannot support a solar installation and reinforcing it would be cost-prohibitive, some jurisdictions accept a structural engineering assessment
- Manufactured or modular ADUs — Factory-built ADUs placed on a foundation are generally exempt from the PV mandate
Battery Storage: What the 2025 Code Actually Requires
Battery storage is not yet mandated for most single-family ADUs — but the 2025 code added a critical pre-wiring requirement. Your ADU must include a dedicated 240V circuit and reserved electrical panel space for a future battery. You don't have to install a battery now, but the infrastructure must be roughed in during construction. Adding it later requires opening walls and costs significantly more.
Under NEM 3.0, export credits have dropped sharply — storing solar energy and using it during peak evening hours (4–9 PM) when SCE and PG&E rates are highest is now the smarter financial play. Installing a battery alongside your solar system is almost always more cost-effective than retrofitting later.
Why US Power
US Power's CSLB-licensed consultants handle the full process: system sizing per Equation 150.1-C, coordination on CF1R documentation, permit submittal, installation, and utility interconnection. As California's exclusive factory-direct QCells partner, US Power offers American-made Tier-1 panels at 15–20% below typical market pricing, backed by a 25-year comprehensive warranty covering panels, workmanship, and performance. Most ADU systems are installed within 3–4 weeks of permit approval, with $0-down financing available.
The homeowners who move through ADU permitting smoothly are the ones who resolve Title 24 compliance in the design phase — not after plan check rejects their application. Get your free consultation and find out exactly what your ADU project requires.
https://uspowersolar.com/blog/adu-title-24-solar-requirements