Solar and your electrical panel (the 120% rule)

Your home's electrical panel is where solar physically ties in, and it comes with a hard safety limit that decides whether your system fits as-is. It's called the 120% rule, and it trips up more solar projects than almost anything else. Here it is in plain terms.

Meet the busbar

Inside your panel is a busbar — the metal bar that distributes power to every breaker. It has an amp rating, commonly 200 A on modern homes (older homes may be 100 or 150 A). That rating is the ceiling for how much current the panel can safely carry.

Normally, power flows in from the utility through the main breaker. Solar adds a second source pushing power in from its own breaker. Two sources feeding one busbar is exactly what the rule manages.

The 120% rule, simply

Code lets the main breaker plus the solar breaker add up to no more than 120% of the busbar rating. The extra 20% headroom exists because the two sources rarely peak at the same spot, so the busbar never actually sees dangerous overload.

The formula:

• Busbar × 1.20 = maximum allowed
• Subtract your main breaker
• What's left is the biggest solar breaker you can add

Example, 200 A panel:

• 200 × 1.20 = 240 A allowed
• Minus a 200 A main breaker = 40 A left for solar
• A 40 A breaker supports roughly a 7.6 kW inverter (40 A × 0.8 = 32 A continuous)

Notice the trick in row two: a smaller main breaker can leave more room for solar, as long as it still covers your household loads.

If your panel is too small

Plenty of options exist — this is routine work for an electrician:

• Downsize the main breaker (if your loads allow) to free up room.
• Line-side tap — connect solar before the main breaker, sidestepping the busbar limit entirely.
• Upgrade the panel to a higher busbar rating.
• Build a smaller system that fits the room you have.

Why it matters early

Panel capacity can quietly cap your system size, so it's worth checking before you fall in love with a big array. It also shows up on your permit set's one-line diagram, where inspectors verify the math.

Leave the connection to a pro

The 120% rule is worth understanding, but the panel work itself is high-voltage and high-stakes. A licensed electrician must evaluate your busbar, choose the connection method, and make the tie-in — this is never a safe DIY step. Learn more in the reference.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.