If you’re building an accessory dwelling unit (ADU) in California, you’ve probably come across plenty of information about solar panels for ADUs.
More California homeowners adding newly constructed ADUs are using solar power to lower electricity costs, improve energy efficiency, reduce fossil fuel use, and benefit from renewable energy solutions.
Understanding solar panel requirements, system sizing, and cost and benefits can make the difference between a smooth build, unexpected delays, and surprise energy cost overruns.
In this article, we’ll guide you through installing a solar setup for your ADU in California — from requirements to practical tips for your solar project.
Understanding California’s Solar Requirements for ADUs
Adding solar panels to an ADU isn’t just environmentally smart; it’s often required by law. Knowing when solar is needed, how many solar panels are required, and what alternatives exist helps you plan your solar project, manage energy costs, and maximize solar solutions.
What is Title 24? When Did Solar Become Required?
Title 24 (Part 6 of the California Code of Regulations) is California’s Building Energy Efficiency Standards, administered by the California Energy Commission (CEC). These standards set mandatory requirements for new construction, additions, and alterations of residential and non-residential buildings.
The standards are updated every three years and aim to:
- reduce energy consumption
- lower energy costs
- support renewable energy; and
- promote environmental sustainability while maintaining occupant comfort.
Since January 1, 2020, Title 24 has required newly built homes, including many detached ADUs, to include a solar photovoltaic (PV) system unless an exemption applies.
Which ADUs Must Have Solar Panels?
Not every ADU is required to install solar panels. Under Title 24, the mandate generally applies to:
- Newly constructed ADUs (permit submitted after January 1, 2020)
- Non-manufactured ADUs
- Detached ADUs separated from the main home and treated as new dwellings
Knowing whether your ADU falls under the solar mandate helps you plan solar equipment, manage energy costs, and anticipate your project’s electrical usage.
Early awareness also lets you evaluate roof space, orientation, unshaded solar access, and potential energy production, ensuring your ADU is energy-efficient and code-compliant.
ADU Types and Solar Requirements: Does Your ADU Need Solar?
ADU types determine solar requirements, helping you ensure Title 24 compliance.
Detached ADUs Built from the Ground Up
If you’re building a detached accessory dwelling unit with its own meter and foundation, plan a PV system that meets California’s Title 24 solar requirements. Installing solar panels can lower energy costs, improve energy independence, reduce your carbon footprint and enhance property value.
Solar panel installation (or expanding an existing solar system) may be required.
Attached ADUs (Addition Exemption)
Attached ADUs — those sharing a wall or roofline with the main house — are often treated as home additions rather than fully new dwellings. While solar panels may not be mandatory, adding them still yields long-term energy savings and efficiency benefits.
Garage Conversions and Existing Structure Conversions
Converting a garage or existing detached building typically counts as a renovation rather than new construction. These units are often exempt from Title 24 solar requirements, though you can still add solar equipment to improve energy savings.
Manufactured/Prefab ADUs
Prefab or manufactured units usually fall under Title 25 and may be exempt from mandatory solar panels. Even so, installing them supports renewable energy goals and can lower energy bills.
The Size and Shading Exemption
Some accessory dwelling units may be exempt from solar installations due to size or shading:
- If the calculated PV requirement for the unit is less than 1.8kWdc, the ADU may qualify for exemption.
- If roof shading or orientation lowers the effective annual solar access (less than 70% of unshaded potential), a solar exemption may apply.
- Smaller ADUs (typically 600-800 square feet) may fall below the threshold depending on the climate zone.
Even when exempt, installing solar panels for ADUs in California can increase energy savings, improve residential space efficiency, and reduce reliance on grid electricity and fossil fuels.
Work with an experienced builder like Apex Homes to ensure that solar panels on your ADU are fully compliant with Title 24.
How Much Solar Do You Actually Need?
The required solar system capacity for your ADU depends on CFA (conditioned floor area), climate zone, projected electrical use, and whether the accessory dwelling unit is attached or detached.
The Title 24 Solar Calculation Formula
The formula used (Equation 150.1-C) is:
kWPV = (CFA × A)/1000 + (NDU × B)
Where:
kWPV = kWdc size of the PV system
CFA = Conditioned floor area (sq ft)
NDU = Number of dwelling units
A = CFA adjustment factor (Refer to Table 150.1-C)
B = Dwelling unit adjustment factor (Refer to Table 150.1-C)
This calculation ensures your solar project meets Title 24 energy efficient standards while optimizing energy savings and projected electrical usage.
For example, a 700 sq ft detached ADU in Menlo Park (Bay Area, Climate Zone 3) may need a 1.56kW PV system, while the same unit in San Diego (Southern California, Climate Zone 14) may require 1.78kW.
Title 24 energy code sets these minimum solar system sizes based on regional energy efficiency requirements and typical sunlight. However, site-specific factors like roof shading can adjust the final system size.
Climate Zones Matter: Regional Differences
California is divided into 16 climate zones, which influence minimum solar system sizes and energy production under Title 24.
- In foggy zones (like the Bay Area, Zone 3), Title 24 may require slightly smaller or larger PV systems depending on the adjustment factors used for that region.
- In sunnier zones (Southern California, inland valleys), system size is also set by climate-zone adjustment factors, not just sunlight, so required PV capacity may differ.
For example, a 700 sq ft detached ADU in San Jose (Climate Zone 4) would need about 1.62kW of solar panels under Title 24. Installing a system of 1.6–2.0kW can help maximize energy savings, reduce electricity costs, and lower your carbon footprint, even if your ADU is just above the minimum requirement.
Understanding your climate zone helps with solar panel installation, system sizing, energy production estimates, and budgeting for your ADU solar project.
Where Can Solar Panels Be Installed?
Once you’ve decided to set up solar panels, you’ll want to think about placement and installation logistics.
On the ADU Itself
The most straightforward approach is to install your solar modules on the ADU roof. Key considerations include:
- Roof pitch and orientation: Ideally south-facing (or west-south) with minimal shade.
- Unshaded solar access: Trees, taller neighbouring structures, or obstructions reduce output — one of the reasons why Title 24 includes shading exemptions.
- Roof space: Make sure you have enough area for the required PV system; if you’re tight on space, you may need a higher-efficiency panel or consider an alternate location.
Installing panels on the ADU roof offsets the unit’s load directly and gives you a clean, renewable energy profile.
On the Primary Residence
If the ADU roof can’t accommodate the required PV system, solar panels can be installed on the main house. This option provides several benefits:
- Alternative placement: Installing panels on the main house allows flexibility when the ADU roof is too small, shaded, or poorly oriented.
- Expand existing system: If you already have a solar panel installation on your main house, expanding that system can optimize total solar energy production and reduce additional solar panel installation costs.
- Requirements for expansion: the system must meet Title 24 solar requirements and the ADU’s minimum solar capacity, ensuring compliance and maximizing energy savings.
Important: Existing Solar and Your ADU
If your property already has solar panels, that may account toward the ADU’s minimum solar capacity, but specifics depend on:
- Net Energy Metering (NEM) version: For example, NEM 2.0 VS NEM 3.0 affects how much grid electricity your existing solar system can offset for the ADU.
- System expansion needs: Larger or newly constructed ADUs may require additional solar panels to meet Title 24 solar requirements.
- Battery storage (optional): Adding a battery storage system helps maximize renewable energy use and lower energy bills, especially during grid outages or peak pricing.
Navigate ADU Solar Requirements with Confidence
Understanding solar panels for ADUs in California under Title 24 can feel complicated, but careful planning makes compliance achievable. With the right solar company — handling solar panel installation, PV system setup, and solar equipment — homeowners can focus on designing a functional, energy-efficient ADU while meeting all Title 24 energy code requirements and managing energy costs effectively.
Schedule a consultation with Apex Homes to ensure your ADU meets Title 24 solar requirements, achieves the minimum solar capacity, and maximizes energy savings.
FAQs
Are solar panels required for all ADUs in California?
No, solar panels are not required for every ADU. The mandate mainly applies to newly constructed, detached, non-manufactured ADUs permitted after January 1, 2020. Attached additions, conversions, or prefab units may be exempt.
How much do solar panels cost for an ADU?
Costs for a 1.8–2.5kW ADU solar system vary by size, roof complexity, climate zone, and installer, typically $4,500–$7,900 before incentives. The federal solar tax credit can reduce upfront expenses and increase energy savings.
Can I put solar panels on my main house instead of my ADU?
Yes. Solar panels can be installed on the primary residence roof if the system meets the minimum solar capacity for the ADU. This can expand existing solar equipment, optimize projected electrical usage, and provide energy savings without extra roof construction.
What size ADU is exempt from solar requirements?
Smaller ADUs (600-800 sq ft) or units needing less than 1.8kWdc may be exempt from mandatory solar panel installation, depending on climate zone and available unshaded solar access. Even exempt units can benefit from solar solutions for lower energy costs and improved resilience during power outages.