Your solar installer will look at your roof first, but your electrical panel decides how (and sometimes whether) the system can connect. Older and smaller panels often need work.
How does solar connect to my panel?
Most home systems connect through a breaker in the main panel, so the solar feeds the panel's busbar along with the utility. Because both sources can feed the bus, the code limits how much solar a given panel can accept.
What is the 120% rule for solar?
A common method in the electrical code lets the main breaker plus the solar breaker add up to no more than 120% of the busbar rating, with the solar breaker at the opposite end of the bus from the main. Example: a panel with a 200-amp bus and a 200-amp main can take up to a 40-amp solar breaker (240 − 200 = 40). A panel with a 100-amp bus and 100-amp main can take only 20 amps.
What if my panel cannot take the solar breaker?
- Derate the main breaker (for example a 175-amp main on a 200-amp bus) when a load calculation shows the house can live with it.
- Supply-side (line-side) connection ahead of the main breaker, where the equipment and the utility allow it.
- Panel or service upgrade, often to a 200-amp meter-main with a higher-rated bus. See how panel upgrades work (IES 200-amp service upgrade from $6,500).
Batteries add their own requirements, sometimes a backup or critical-loads panel and a disconnect.
Do problem panels need replacing before solar?
Installers and inspectors will generally not connect solar to a Federal Pacific Stab-Lok, Zinsco or damaged panel. Plan the panel replacement first.
About these prices: rough starting-at prices from the IES price book. The final price depends on what we find on site, your utility and your city or county. Estimates are free.
Common questions
Do I need a panel upgrade for solar?
Not always. Many 200-amp panels can take a solar breaker under the 120% rule. Smaller or older panels, or large systems, may need a main breaker derate, a supply-side connection or a panel upgrade.
How big a solar breaker can a 200 amp panel take?
With a 200-amp busbar and 200-amp main breaker, the 120% rule allows up to a 40-amp solar breaker at the opposite end of the bus.
General information for California homes under the California Electrical Code (based on the NEC). Local rules and inspectors can vary. Not a substitute for an on-site inspection. Never work inside a panel, at the meter or on live circuits.