What §150.1(c)14 actually requires
California’s 2025 Energy Code requires a prescribed PV system on every
newly constructed low-rise residence. The size of that
system isn’t a fixed number — it’s computed per project from the
conditioned floor area, the number of dwelling units, and a pair of
per–climate–zone factors taken from the Title 24 baseline tables. The formula is
kWdc = CFA × CFAmult / 1000 + NDU × DUmult. Hotter,
drier climate zones use larger factors than mild coastal zones, so a 2,400 sf
home in Bakersfield carries a meaningfully larger prescribed array than the same
footprint in San Francisco.
The mandate applies to newly constructed low-rise residences and newly constructed detached ADUs. Additions, alterations, and attached ADUs fall under §150.2(a) and are not required to install PV. A non-ADU addition modeled as its own permit (“Add-Alone”) is still an addition for code purposes — the prescribed PV size only kicks in when the project is genuinely new construction.
Exception 2: if the prescribed PV size lands under 1.8 kWdc, the project qualifies for a reduced PV requirement. In practice the compliance documentation reflects the exception and no PV array is installed on the project. This commonly happens on small ADUs (under roughly 700 sf in low-PV-factor zones) where the prescribed size is too small to install economically.
The kWdc this tool returns is the code-prescribed system size that shows up on the Title 24 report (CF1R compliance form). The contractor who actually installs the array sizes the panels-plus-inverter combination to meet or exceed that kWdc using whatever module and inverter combination they prefer. For the full decoded explanation of how the prescriptive PV formula interacts with battery storage, net-billing tariffs, and the rest of the residential code, see the Title 24 explained guide.