How much solar does California require on a new home? The 2025 Title 24 formula, worked

Equation 150.1-C and the sixteen pairs of factors behind it

Section 150.1(c)14 of the 2025 Building Energy Efficiency Standards (Title 24, Part 6) says every newly constructed single-family building must have a newly installed PV system meeting Joint Appendix JA11, and that its annual electrical output must be no less than the smaller of two numbers: the size from Equation 150.1-C, or the building's total Solar Access Roof Area (SARA) multiplied by 18 for steep-sloped roofs or by 14 for low-sloped roofs. The equation itself has two terms. The first scales with conditioned floor area (CFA, the enclosed, conditioned square footage), multiplied by factor A and divided by 1,000. The second is a fixed amount per dwelling unit, NDU multiplied by factor B. For an ordinary detached house NDU is 1, so B simply adds to the floor-area term.

A and B change by climate zone because the modeled electricity use of a code-built home changes with climate, and the CEC manual ties the standard PV size to that modeled use. Climate Zone 15 has an A of 1.56, more than twice the 0.627 of Climate Zone 10, and the CEC's own Single-Family Compliance Manual uses exactly that pair to show a 2,000 square foot house needing 2.66 kW in Zone 10 and 4.59 kW in Zone 15. The B factors sit in a narrower band, from 1.06 in Zone 5 to 1.51 in Zone 13.

The table prints the factors exactly as Table 150.1-C gives them, the result for a 2,000 square foot, one-unit house, the same result after the 25 percent battery reduction in Exception 5, and the floor area below which the equation falls under 1.8 kW and no PV is required at all under Exception 2. That last column comes straight from the equation: solve (CFA x A)/1000 + B = 1.8 for CFA.

The spread in the last column is wider than most people expect. In Climate Zone 5 a one-unit house has to pass about 1,265 square feet of conditioned area before the equation reaches 1.8 kW, so a compact 1,200 square foot house there needs no PV under the equation at all. In Climate Zone 15 the same threshold is about 212 square feet, which means practically every new dwelling there carries a requirement. Zones 11 and 13 sit at about 431 and 324 square feet.

Climate zone is set by the building's location, not the mailing address or the utility. The CEC publishes a lookup by ZIP code, and the zone used for compliance is printed on the builder's Certificate of Compliance. If the zone on that form does not match the zone the CEC tool gives for the lot, every number downstream of it is wrong.

Five house sizes, sixteen zones: the minimum in kW DC

Because the B term is a fixed amount per dwelling, small homes get proportionally larger requirements per square foot, and the required size grows more slowly than floor area. A 3,000 square foot house is 50 percent larger than a 2,000 square foot one, but in Climate Zone 12 its minimum rises from 2.63 kW to 3.24 kW, only 23 percent more. Where the equation lands below 1.8 kW, Exception 2 applies and the table says so instead of printing a number.

Two results in the table catch builders and buyers out. First, only Climate Zone 5 drops a 1,200 square foot house below the floor, although Zones 3 and 7 come within 0.1 kW of it, at 1.87 and 1.84 kW. Second, Zone 15 is in a different league: a 3,000 square foot house in Zone 15 needs 6.15 kW, close to the 8 kW system that DOE uses as its benchmark, while the same house in Zone 5 needs 2.81 kW.

None of these numbers says anything about how much electricity the household will use. They are the code's estimate for a standard, all-electric-ready building with standard occupancy, and the CEC manual notes that the standard design PV size in the performance method is set from the modeled electrical use of a mixed-fuel version of the proposed home. A family with two electric vehicles, a pool pump and a home office will use more. For a bill-based size, the panel sizing method divides your annual kilowatt-hours by a production factor; the code minimum is the starting point the builder had to meet, not the answer to that division.

When the roof sets the number: the Solar Access Roof Area cap

The required size is the smaller of the equation and the roof calculation, so on a house with little usable roof the SARA figure governs. SARA is the area of roof that can structurally support PV, including the roofs of covered parking, carports and other new structures on the site that are compatible with supporting a PV system. The 2025 text then removes four kinds of area. On steep-sloped roofs, planes facing a northerly azimuth between 300 and 90 degrees from true north are excluded. Any area with less than 70 percent annual solar access is excluded. Occupied roof areas, as defined in the California Building Code, are excluded. So is roof area that other state building code requirements take up, such as fire access pathways, and roof area taken by local requirements once the CEC's Executive Director has confirmed them.

Steep-sloped means a rise of 2 in 12 or more (9.5 degrees from horizontal); low-sloped means less than that. The 2025 code multiplies the remaining SARA by 18 watts per square foot on steep roofs and 14 on low-slope roofs, a change from earlier cycles that the CEC's "What's New for 2025" summary lists as an update to PV sizing. On a steep roof, only shading from permanent external objects, such as trees, hills and neighboring buildings, counts toward annual solar access. On a low-slope roof, everything counts, including parapets and the building's own design features.

The practical question for a buyer is how much SARA a house needs before the roof stops being the limit. Divide the equation result in watts by 18 or 14.

Outside Zone 15, a 2,000 square foot house needs only about 124 to 183 square feet of qualifying steep roof for the equation to govern, so the cap matters mainly on narrow infill lots, on houses shaded by a hillside or a neighbor's tall building, on three-story townhouse-style homes with small roofs, and in Zone 15 where a steep roof needs about 255 square feet of qualifying area to carry the full 4.59 kW.

The CEC manual works two cases. In Example 7-3, a 1,410 square foot Climate Zone 13 home has 310 square feet of SARA: the roof could carry 5.58 kW, the equation asks for 2.77 kW, and 2.77 kW is required. In Example 7-4, a mature tree to the south leaves 133 square feet with at least 70 percent solar access; 133 x 18 = 2,394 watts, so 2.4 kW is required instead of 2.77. Had the shading report shown less than 80 contiguous square feet, no PV would have been required. Solar access has to be shown with a solar assessment tool approved by the CEC's Executive Director, so a SARA claim should come with a report, not a sketch.

All five exceptions in Section 150.1(c)14, and what each one changes

The exceptions are short and worth reading whole, because the 1.8 kW floor is only one of five. The wording below follows the CEC's 2025 text; the plain-language column is ours.

Two more routes out of the requirement sit outside Section 150.1(c)14. Exception 6 to Section 150.2(a), in the CEC's 2025 rulemaking text, says PV systems as specified in Section 150.1(c)14 are not required for additions, so building a new bedroom wing or a second story onto an existing house does not trigger the equation. And Section 10-109(k) of Part 1 lets the Commission declare the PV or battery requirements inapplicable where a utility's rates, export compensation or interconnection charges make the Commission's cost-effectiveness findings fail. The Commission has used it: on June 11, 2025 it ordered (Order No. 25-0611-3c) that the 2025 PV and battery storage requirements do not apply to newly constructed buildings in the Trinity Public Utilities District service area, as it had for the 2022 single-family PV requirement in February 2023.

A third point is about scope rather than exemption. The CEC manual's Example 7-2 says a newly built conditioned building with no dwelling unit on a residential lot, such as a pool house, recreation room or art studio, still has to meet the PV requirement, with NDU entered as zero. With B dropped, such a building in Climate Zone 10 reaches the 1.8 kW floor only at about 2,870 square feet, so most of them fall under it in practice: a 600 square foot studio there comes out at 0.38 kW.

A Zone 12 house run through every rule

A 2,000 square foot, one-unit house sits in Climate Zone 12 under a steep 5-in-12 roof. The equation gives 2,000 x 0.613 / 1,000 + 1 x 1.40 = 1.226 + 1.40 = 2.63 kW (2.626 before rounding).

The roof survey then finds a south plane with 120 square feet of qualifying area after fire pathways and vents, and a 40 square foot patch on a west dormer. The north plane does not count at all. Under the CEC manual's roof-area-by-roof-area reading of Exception 1, the 40 square foot patch is below 80 contiguous square feet and drops out, leaving 120 square feet of SARA. 120 x 18 = 2,160 watts, or 2.16 kW, which is smaller than 2.63 kW, so 2.16 kW is the requirement. If the plan checker instead counts total SARA, 160 square feet x 18 = 2.88 kW, the equation is smaller, and 2.63 kW applies. The difference is 0.47 kW, about $1,375 on the benchmark line, and it turns on one sentence of interpretation.

Now the builder adds a JA12-listed battery commissioned with 7.5 kWh of compliance cycling capacity. Exception 5 cuts the equation result to 2.626 x 0.75 = 1.97 kW, below the 2.16 kW roof figure, so 1.97 kW becomes the requirement. That is still above 1.8 kW, so Exception 2 does not apply. The panel saving against the roof figure is 0.19 kW, about $562 at $2.95 per watt.

At the Los Angeles planning factor of 1,677 kWh per kW, 2.16 kW makes about 3,620 kWh a year and 1.97 kW about 3,300 kWh. Both are illustrations, not a Zone 12 model; a PVWatts run with the house's own location, tilt and azimuth is the number to trust. The buyer's CF1R should show one of these three sizes, the reasoning that produced it, and, if it is the 1.97 kW figure, a battery certificate to match.

The battery trade in Exception 5, priced against the DOE benchmark

Exception 5 lets a builder install 25 percent less PV if a qualifying battery goes in with it. JA12 sets the qualification: the product must be self-certified to the CEC and listed with Y(2025) on the CEC's solar equipment list, and the CEC manual explains that the 7.5 kWh threshold is compliance cycling capacity, the part of usable capacity commissioned for daily cycling after any backup reserve is set aside. A 10 kWh battery with half of it held in reserve does not qualify.

In money, the trade is lopsided. For a 2,000 square foot home in Climate Zone 10, the reduction is 0.67 kW of panels (2.66 kW down to 2.00 kW). At $2.95 per watt that is about $1,965 on the benchmark line. DOE's same Q1 2025 table prices the 8 kW system with 13.5 kWh of storage at $4.59 per watt, $36,720 in total, which puts the storage at $13,120 above the PV-only system. Even allowing that a 7.5 kWh unit costs less than a 13.5 kWh one, no builder adds a battery to save $2,000 of panels. In Zone 15, where the reduction is largest at 1.15 kW, it is still worth only about $3,385 at the benchmark.

So when a new house arrives with a battery and a smaller array, the battery was chosen for something else: buyer demand, backup, a performance-path credit, or a local requirement. The CEC manual notes that in the performance method, PV coupled with at least 5 kWh of compliance cycling capacity earns a modest credit usable against efficiency measures. What it means for you is that the array was legally sized smaller because of the battery, and if the battery is later removed or fails, the remaining PV is below what the equation alone would have required. The battery itself should arrive with its JA12 control settings and a certificate of installation. For what storage is worth to a household on its own terms, the home battery cost guide works the economics without any code credit.

One edge case the text does not settle in words: whether a battery-reduced size that falls below 1.8 kW triggers Exception 2. In Climate Zone 5 a 2,000 square foot house goes from 2.23 kW to 1.67 kW with a battery; in Zone 16 it lands at exactly 1.80 kW. The exception refers to the minimum size "specified by Section 150.1(c)14", and the 25 percent reduction is an exception to that same section. That is a question for the plan checker, and the answer should be written on the compliance form, not assumed.

Prescriptive minimum or performance budget: why your house may not match the equation

The equation is the prescriptive path. A builder can instead use the performance path, where CEC-certified software compares the proposed house with a standard design house, and the PV size on the standard design comes from the prescriptive rule. The proposed house must use no more energy than that budget on the CEC's long-term system cost (LSC) metrics. Efficiency is scored separately: the manual says PV has no effect on the Efficiency LSC metric, so panels cannot buy back a leaky envelope, but a more efficient building can comply with a smaller system.

Three consequences matter to a buyer. First, the final array can be smaller or larger than the equation result, and the number that counts is the one on the registered Certificate of Compliance. Second, orientation rules loosen: the prescriptive path requires steep-roof modules to face between 90 and 300 degrees from true north and limits shading to a weighted 98 percent annual solar access, while the performance path allows any azimuth but rewards good orientation with a smaller required size. The simplified California Flexible Installation options, CFI1 (150 to 270 degrees) and CFI2 (105 to 300 degrees, at roughly 10 percent lower modeled output), still require 98 percent solar access. Third, the performance path is where batteries, solar water heating and other measures trade against each other.

There is also a community shared solar route under Section 10-115 of Part 1. A builder can offset some or all of the on-site requirement with a share of an off-site solar or storage project, but the CEC manual sets conditions: the share has to deliver bill reductions greater than its cost to the building, the system must deliver benefits for at least 20 years, and the building stays in the program for 20 years regardless of who owns it, unless the owner installs an on-site system meeting the code in force when the original permit was applied for. If a new house has no panels on the roof and the builder says it complied, ask whether it was an exception or a community solar enrollment, because the second one binds the next owner.

What the code minimum produces, and what it is worth on the benchmark line

A planning factor turns kilowatts into kilowatt-hours. Our saved NREL PVWatts version 8.5.0 run for Los Angeles (8 kW, 20 degree tilt, due south, 14 percent losses) produced 13,417 kWh a year, about 1,677 kWh per kW. Used as a rough California coastal-basin planning factor, not a roof-specific model, that puts the 2.66 kW Climate Zone 10 minimum at about 4,470 kWh a year. Sunnier sites produce more per kW, foggier ones less, and an array split across east and west planes less again; the production estimate guide explains which inputs move the result.

On the cost side, the benchmark is a comparison line built for an 8 kW system. A 2.66 kW system carries roughly the same permitting, design, interconnection and inspection effort as one three times its size, so per-watt costs for code-minimum arrays run above $2.95 on most job sites. In new construction, though, the builder spreads some of those costs across the subdivision and installs before the drywall goes up, so the trade is not one-directional. The table below uses the benchmark only to show scale, not to say what a builder paid or what you paid for the panels inside the house price.

What those kilowatt-hours are worth depends on the tariff, not the code. A new home in an investor-owned utility territory goes onto net billing, where exports earn hourly avoided-cost values far below retail; the California net billing guide covers how those export values and the monthly true-up work. A small array that serves daytime load directly is the configuration net billing favors, which is one reason a code-minimum system can make more economic sense than its size suggests.

Adding panels during construction is usually the cheapest time to do it, because the roof, conduit and inverter location are all being designed anyway. If the builder offers an upgrade, price it per added watt against the benchmark the same way you would a retrofit quote, using the cost-per-watt method, and check that the inverter and main panel can carry the larger array. If you plan to add later, the guide to expanding an existing system covers the interconnection steps a second array triggers.

The paperwork trail: CF1R, CF2R-PVB-01-E and the registry

Everything above ends up on three documents. The Certificate of Compliance (CF1R) is the design-stage document; it records the climate zone, conditioned floor area, compliance path and the PV size the house was approved with, and the installation is checked against it. The Certificate of Installation for PV, form CF2R-PVB-01-E, is signed by the installer and certifies that the system installed matches the CF1R and meets JA11; a battery gets its own form, CF2R-PVB-02-E. The local building department checks that the installation certificate is complete and correct and uploaded to the registry.

The installer also has to provide a solar assessment report using a CEC-approved tool. Either its satellite, drone or other image must be dated after the panels went up, or on-site photos must be attached showing that the installed array matches what was modeled. JA11 also requires remote monitoring: a web portal and a mobile app showing the system's nominal kW, the number and wattage of modules, hourly or 15-minute production, daily and running totals, daily peak power and current output. Inverters must meet CPUC Electric Tariff Rule 21 for interconnection with the utility.

For a buyer, those documents answer the questions a sales office often cannot: what size the code required, what was actually installed, whether a battery reduced the requirement, and whether the system was exempted. If the CF1R and CF2R forms are not in the closing documents, ask the builder for them, and if that fails, the building department's permit file is the next place to look.

Documents and numbers to collect before closing

• Get the CF1R and confirm its climate zone against the CEC ZIP code lookup for the lot.
• Check the conditioned floor area on the CF1R against the floor plan, and rerun (CFA x A)/1000 + B yourself.
• Find the code year on the CF1R: 2025 if the permit application was filed on or after January 1, 2026.
• If the installed size is below the equation result, find out which reason applies: SARA cap, battery reduction, performance path or exception.
• Ask for the solar assessment report and check that its image is dated after installation or comes with on-site photos.
• Get the CF2R-PVB-01-E for the panels and, if there is a battery, the CF2R-PVB-02-E and its JA12 control settings.
• Confirm the monitoring portal and app log-in transfer to you, with module count and wattage visible.
• Confirm whether the system is owned outright, leased, under a PPA, or a community solar enrollment, and get any contract.
• Ask what an upgrade to a larger array costs per added watt during construction, and compare it with $2.95 per watt.

Rules that shape the roof besides the size equation

Fire access rules from the State Fire Marshal, carried in the California Residential Code (Title 24, Part 2.5, Sections R324.3 and R324.6), decide where the array can go. The CEC manual's summary: each roof plane with panels needs two pathways at least 36 inches wide from the lowest roof edge to the ridge, arrays must stay 18 or 36 inches below the ridge depending on how much of the roof they cover, and no array may exceed 150 feet in either dimension. Those pathways come out of SARA, which is why a hip roof with many small planes can end up with a lower requirement than a simple gable. The fire setback guide shows how much roof the pathways actually take.

Homes that end up with no PV, through an exception, are not off the hook entirely. In subdivisions of ten or more single-family homes with a tentative map deemed complete or approved, a house without PV must meet the solar-ready rules: a solar zone of at least 250 square feet on the roof or overhang, made of areas no narrower than five feet and no smaller than 80 square feet each, with smaller zones allowed under listed exceptions; a pathway for conduit; and a main panel with a 200 amp busbar and a reserved breaker space marked "For Future Solar Electric".

Battery readiness is separate again. A new home with one or two dwelling units and electrical service above 125 amps must be battery-ready unless a battery is already installed. That means either interconnection equipment with at least 60 amps of backed-up capacity, or a raceway of at least one inch to a subpanel; in both cases at least four identified backup circuits (refrigerator, a lighting circuit near the main exit and a bedroom outlet among them), a main panelboard busbar of at least 225 amps, and space reserved for a transfer switch within three feet of the main panel. That infrastructure is what makes adding a battery later a smaller job than on an older house.

Which code cycle governs a given house, and what comes after 2025

The trigger is the permit application date, not the move-in date. The CEC's 2025 Standards page says buildings whose permit applications are applied for on or after January 1, 2026 must comply with the 2025 Energy Code, which the Commission adopted on September 11, 2024 and the Building Standards Commission approved on December 17, 2024. A house finished in 2026 under a permit applied for in 2025 was built to the 2022 code, which took effect January 1, 2023; the CEC's What's New summary lists the 18 and 14 watt roof multipliers as a 2025 change. Ask which code year is on the CF1R; it is printed there.

Two later documents could confuse a search. In November 2025 the CEC published a restructured version of the 2025 code that renumbers Section 150.1(c)14 as Section 702.3.1 and Table 150.1-C as Table 702.3-A. The CEC labels it "for information only": it uses the adopted language, has not been formally adopted, and should not be used in place of the code. The factors in it match the adopted table. And a June 9, 2025 correction notice changed battery capacity factors in the 2025 code, but only in Tables 140.10-B and 170.2-V, which apply to nonresidential and multifamily buildings, not to the single-family equation.

The next cycle is the 2028 Energy Code. The CEC's page for it says the standards will be proposed for adoption in 2027 with an effective date of January 1, 2029, so the 2025 factors govern permit applications through the end of 2028 unless the Commission makes a determination under Section 10-109(k) for a particular utility, as it did for Trinity PUD. Cities and counties can also adopt local ordinances that go beyond the state code, and the CEC lists approved local ordinances exceeding the 2025 Energy Code on the same Standards page. For California-wide context on incentives and export rules, the California state page collects what applies to existing homes too.