How Big a Solar System Does a New Home Need Under Title 24?
The smaller of two numbers, in kWdc. Equation 150.1-C gives kWdc = (floor area × A) / 1,000 + (dwelling units × B), with A and B set by climate zone in Table 150.1-C. The roof limit is the solar access roof area times 18 watts per square foot on steep roofs or 14 on low-slope roofs. A result under 1.8 kWdc means no solar is required.
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The equation, worked
A 2,000 square foot house with one dwelling unit in climate zone 10 has A of 0.627 and B of 1.41, so 2,000 × 0.627 / 1,000 + 1.41 = 2.66 kWdc. The same house in zone 15 (A 1.56, B 1.47) needs 4.59. A 3,000 square foot duplex in zone 10 counts two units: 1.881 + 2.82 = 4.70. Find your zone on the climate zone lookup.
Where the 1.8 kWdc line falls
With one unit, the equation reaches 1.8 kWdc at a floor area of (1.8 − B) / A × 1,000. That is about 1,265 square feet in zone 5, 1,083 in zone 3, 668 in zone 1, 622 in zone 10 and 212 in zone 15. A 1,200 square foot home in zone 5 works out to 1.76 kWdc, so it needs no PV; a 1,300 square foot one needs 1.82.
Where the roof lowers it
Take a 2,400 square foot house with one unit in zone 12 (A 0.613, B 1.40): the equation gives 2.87 kWdc.
- 250 square feet of usable steep roof allows 4.5 kWdc, so the equation governs at 2.87.
- 120 square feet allows 2.16 kWdc, which is smaller, so 2.16 governs.
- 90 square feet allows 1.62 kWdc, under 1.8, so none is required.
Solar access roof area includes covered parking, carports and other new structures. It leaves out roof with less than 70 per cent annual solar access, occupied roof areas and area lost to other state code such as fire pathways. The exceptions are on solar exemptions for a new home.
The battery cut
Exception 5 lets the equation figure drop by 25 per cent when a JA12 battery with at least 7.5 kWh of cycling capacity is installed too: 2.87 becomes about 2.15 kWdc. Both must be installed, and the Energy Commission says this cannot be combined with the 1.8 kWdc exception. See JA12 battery storage.
What the code does not give
The figure is DC capacity, not a panel count, because module wattage is not set in the code. On the performance path the prescriptive system sets the budget the design is measured against. Other pages cover the requirement itself, solar-ready roof zones and ADU solar.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
The arithmetic is Equation 150.1-C and Table 150.1-C, which anyone can run. The compliance software applies it for the compliance documents.
Yes, within the roof limit. The floor area is multiplied by the zone factor A, so size rises with conditioned floor area, and the dwelling unit factor B adds a fixed amount per unit.
Yes. The Energy Commission says the system need not be on the roof of the building; it may be ground-mounted on the property or on a carport on the lot.
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