Do you need a main electrical panel upgrade for solar? The 120% rule explained
Solar feeds power into your main electrical panel. Older or smaller panels may not be allowed to accept a large system without an upgrade, which can add $2,000–$4,000.
The electrical code limits how much current can flow into a panel’s busbar from all sources. A common rule of thumb, the “120% rule”, allows solar back-feed up to 20% of the busbar rating (with the breaker at the opposite end). A 200-amp panel with a 200-amp main breaker can typically take a 40-amp solar breaker, about a 7–9 kW system.
Homes with 100-amp panels, or panels already full, often need an upgrade or an alternative connection such as a supply-side tap. Electric cars and heat pumps may push you to upgrade anyway.
What this means for you: Ask each installer whether your panel needs work, whether it is included in the price, and whether their design leaves room for an EV charger or heat pump later.