Solar ADU / Granny Flat Calculator

Calculate solar panels for your ADU. California Title 24 mandate sizing, attached vs detached, roof area needed, ITC credit, and payback period.

ADU details

ADU size?

sqft

Bedrooms?

Attached or detached?

State?

Construction type?

ADU solar estimate

7 × 400W panels (2.5 kW system)

CA Title 24 minimum: 2.2 kW | Your recommended: 2.5 kW

Estimated annual usage3,600 kWh/yr (6.0 kWh/sqft)

Monthly kWh estimate300 kWh/mo

Roof area needed455 sqft

Gross system cost$7,500

Federal ITC (30%)-$2,250

Net cost after ITC$5,250

Annual production3,650 kWh/yr

Annual savings (est. $0.15/kWh)$540/yr

Payback period9.7 yrs

Detached ADU: A separate solar system is typically required for detached ADUs in California. The ADU must meet its own Title 24 requirement independently.

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How to Use This Calculator

Enter your ADU size, bedrooms, and state

Enter the conditioned floor area (interior square footage) of your ADU. Select the number of bedrooms — studio units use about 5 kWh/sqft/year while 2-bedroom units use about 6 kWh/sqft/year. Select your state to see whether a solar mandate applies. If you're in California, the calculator shows both the Title 24 minimum requirement and the energy-based recommendation — your system must meet whichever is larger.

Attached vs detached ADU matters in California

California Title 24 treats attached and detached ADUs differently. Attached ADUs (above-garage apartments, basement units, additions) may be allowed to combine their solar requirement with the main dwelling's solar system. Detached ADUs (backyard cottages, standalone granny flats) typically need their own separate solar system meeting the full Title 24 requirement. The calculator notes which situation applies and what it means for your project.

New construction vs conversion

New construction ADUs must fully comply with Title 24 solar requirements in California. Conversions of existing structures (garage-to-ADU, basement-to-ADU) have different compliance pathways and may qualify for exemptions if the existing roof has poor orientation, excessive shading from trees or neighboring buildings, or structural limitations. Your Title 24 energy consultant will determine eligibility.

The Formula

Annual kWh = ADU sqft × kWh/sqft/yr (5-6 depending on bedrooms and state) Required kW = Annual kWh ÷ (5.0 PSH × 365 days × 0.80 efficiency) CA Title 24 Minimum kW = max(1.2, ADU sqft × 0.0036) Final Required kW = max(Required kW, CA Minimum kW) — for CA only Panel Count = Final kW × 1000 ÷ 400W (round up) Roof Area = Panel Count × 65 sqft Gross Cost = Final kW × 1000 × $3.00/W ITC Credit = Gross Cost × 30% Net Cost = Gross Cost − ITC Credit Annual Savings = min(Annual Production, Annual kWh) × $0.15/kWh Payback = Net Cost ÷ Annual Savings

The California Title 24 formula uses a simplified version of Appendix B to the 2022 Building Energy Efficiency Standards: minimum system size equals conditioned floor area multiplied by 0.0036 kW/sqft, with a floor of 1.2 kW. A 500 sqft ADU requires at minimum 1.8 kW; a 1,000 sqft ADU requires at minimum 3.6 kW. The energy-based calculation may result in a larger requirement for units with high per-sqft usage.

Example

Andrea — 600 sqft 1-bedroom ADU in San Jose, CA

Andrea is building a detached backyard ADU in San Jose. It's a 600 sqft one-bedroom unit, new construction. She needs to know the Title 24 solar requirement and actual cost.

ADU600 sqft, 1BR, detached, new

StateCalifornia (Title 24)

TypeNew construction

Result

Annual energy use (est.)~3,600 kWh/yr

Energy-based required kW~2.5 kW

CA Title 24 minimum kW2.16 kW (600 × 0.0036)

Final required kW2.5 kW (energy basis wins)

Panel count7 × 400W panels

Roof area needed~455 sqft

Gross system cost~$7,500

Federal ITC (30%)-$2,250

Net cost~$5,250

Annual savings~$540/yr

Payback~9.7 years

Andrea's 2.5 kW system satisfies both the Title 24 minimum and the energy requirement. At $5,250 after ITC, the system pays back in under 10 years and provides 15+ years of free electricity after that. Since her ADU is detached, the system must be independent from the main house system — a separate inverter and meter are required per California interconnection rules.

FAQ

Is solar required for ADUs in California? ▼

Yes — California's 2022 Building Energy Efficiency Standards (Title 24) require solar on new ADUs. The requirement applies to new ADU construction and certain conversions. The minimum system size is calculated based on the ADU's conditioned floor area: approximately 0.0036 kW per square foot, with a minimum of 1.2 kW. A 500 sqft studio requires at least 1.8 kW; a 1,000 sqft ADU requires at least 3.6 kW. Exemptions may apply if: the roof has excessive shading (50%+ annual shading), the roof is north-facing and can't support sufficient panels, there are structural limitations, or a utility-side constraint prevents interconnection. Your energy consultant (HERS rater) will determine eligibility.

Can I combine the ADU solar system with my main house system? ▼

It depends on the ADU type. Attached ADUs (basement ADUs, above-garage units, room additions) may be allowed to expand or share the existing main dwelling solar system, as long as the combined system meets both the main house and ADU Title 24 requirements. Detached ADUs typically need their own independent solar system because they have separate electrical services and meters. In practice, having a combined system for a detached ADU would require a shared electrical service arrangement, which complicates rental scenarios (separate utility bills). Most detached ADU owners install a separate dedicated system for the ADU.

Do ADU solar systems qualify for the 30% federal ITC? ▼

Yes — ADU solar systems qualify for the 30% federal Investment Tax Credit (ITC) when the ADU is part of your primary residence or when you own the property. The ITC applies to the full installed cost including panels, inverter, mounting, wiring, and labor. If you rent out the ADU, the tax treatment becomes more complex — consult a tax advisor about whether the ITC applies to rental property solar installations. Generally, the ITC can still be claimed but may interact differently with passive activity rules.

How much roof space does an ADU solar system need? ▼

Each 400W solar panel requires approximately 22-25 square feet of roof area. A typical ADU solar system of 2-4 kW (5-10 panels) needs 110-250 square feet of usable roof area. "Usable" means south, east, or west-facing sections with less than 25% shading. For small ADUs with limited roof area, consider high-efficiency panels (400-500W) which produce more power per square foot. If the ADU roof can't fit enough panels due to HVAC units, skylights, or fire setbacks, the ADU may qualify for a Title 24 exemption or be allowed to use the main house roof with proper documentation.

What other states are starting to require solar on ADUs? ▼

As of 2026, California is the only state with a comprehensive solar mandate for new ADUs. However, several trends are worth watching: Oregon passed residential solar-ready requirements (conduit and panel space pre-wiring) for new construction, making future solar installation easier. Washington has aggressive clean energy standards that may lead to solar requirements. Colorado and Massachusetts have active legislative discussions about solar mandates. Several California municipalities (Los Angeles, San Francisco, San Jose) have additional local requirements beyond state minimums. Even where solar isn't mandated, it's financially beneficial in states with net metering policies and good sun hours.

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