Multifamily Solar and Battery Requirements

Two methods, split at three habitable stories

The 2025 Energy Code sizes multifamily PV in two different ways depending on the number of habitable stories, a count explained on low-rise vs high-rise multifamily. In both, the required capacity is the smaller of a formula result and the solar access roof area (SARA) times 18 watts per square foot for steep-sloped roofs or 14 for low-sloped roofs. SARA counts roof that can structurally carry PV, including covered parking, carports and other new structures on the site, and leaves out area with under 70 per cent annual solar access, occupied roofs and roof taken up by other state or approved local code requirements. Systems meet Joint Appendix JA11 and batteries JA12.

Up to three habitable stories: Section 170.2(f)

Equation 170.2-C adds a floor-area term and a dwelling-unit term: kWdc = CFA × A / 1000 + NDU × B, with A and B from Table 170.2-T by climate zone. In climate zone 9, A is 0.613 and B is 1.36, so a 12-unit building of 9,600 square feet needs about 5.9 + 16.3 = 22.2 kWdc, unless SARA gives less. The exceptions:

• No PV where the required size is under 4 kWdc.
• On steep-sloped roofs, north-facing area between 300 and 90 degrees is left out of SARA. No PV is needed if SARA is under 80 contiguous square feet.
• No PV on roofs the building department finds cannot carry it under the ASCE 7-16 snow load chapter. For projects approved by planning before 1 January 2020, roof barred from PV by the conditions of approval is left out of SARA.
• The PV size may be cut by 25 per cent when installed with a JA12 battery of at least 7.5 kWh.

A battery is otherwise not required at this height; the manual describes it as an option for sites short of PV space.

Four or more: Sections 170.2(g) and (h)

Equation 170.2-D uses conditioned floor area alone, times a factor from Table 170.2-U. For multifamily over three stories it is 1.82 W/ft² in climate zones 1, 3, 5 and 16, 2.21 in zones 2, 4 and 6 to 14, and 2.77 in zone 15. No PV is needed where SARA is under 3 per cent of the conditioned floor area, under 80 contiguous square feet, or the result is under 4 kWdc.

Every building required by Section 170.2(g) to have PV also needs a battery under Section 170.2(h). Equation 170.2-E sets its usable energy from floor area, a Table 170.2-V factor (1.88 to 2.85 Wh/ft² for multifamily) and the battery's round-trip efficiency; power capacity is the energy capacity divided by four. No battery is needed if the installed PV is under 15 per cent of the Equation 170.2-D capacity, or if the battery would be under 10 kWh.

The virtual energy bill credit test

Exception 5 to Section 170.2(g) removes the PV requirement where the building's load-serving entity offers no program crediting PV generation to occupants through virtual energy bill credits. It does not apply where the Energy Commission has approved a community solar program for the site. The manual says to check the CEC's list of load-serving entities, and that solar readiness applies when PV is not required. With no PV required, no battery is required either.

Readiness, additions and forms

Without PV, Section 110.10 solar readiness applies to low-rise multifamily buildings and to high-rise ones of up to ten habitable stories. Additions are exempt from PV and batteries (Exception 6 to Section 180.1), and the manual lists the rules as not applicable to alterations. Prescriptive compliance is shown on LMCC-SAB-01-E up to three stories and NRCC-SAB-E from four.

Common Title 24 Questions

1. Do apartments need Title 24?
2. Does a duplex need Title 24?
3. Do triplexes and fourplexes need Title 24?
4. Which Title 24 forms does a multi-family project use?
5. What is an LMCC certificate?
6. How much does a multi-family Title 24 report cost?

Frequently Asked Questions

Do low-rise apartment buildings need a battery?

Can the PV go on carports instead of the roof?

Does an existing apartment building need solar when it is remodelled?

What if the roof is too small?