Do I need a panel upgrade for solar? The 120% rule, explained

The 120% rule

Your main panel has a busbar, the metal bar the breakers connect to, and a main breaker that protects it. When solar connects through a breaker at the opposite end of the busbar from the main breaker, the National Electrical Code allows the total to reach 120% of the busbar rating.

In plain terms: the main breaker plus the solar breaker can’t add up to more than 120% of what the busbar is rated for. The solar breaker is sized at 125% of the system’s continuous output.

A simple example

Many 200-amp panels fit a typical home solar system. Smaller panels, larger systems or panels that already have another power source connected often don’t.

Many home batteries can be set to different output levels, and each level needs a different breaker size. For example, a 7.6 kW setting uses a 40 A breaker. We match the setting to what your panel can handle.

Options if your panel comes up short

• Smaller main breaker. A lower-rated main breaker can make room, but only if a load calculation shows your home still has enough power.
• Supply-side connection. Solar connects between the meter and the main breaker, so the busbar limit doesn’t apply. It isn’t possible on every panel.
• Power control system. A listed system limits solar and battery current so the busbar can’t be overloaded.
• Main panel upgrade. A new panel with a larger busbar, often combined with the solar installation so you have one project instead of two.

Other signs you may need an upgrade

Older homes often have 100-amp service. Breakers that trip often, no open slots, or plans for a heat pump or EV charger are also signs. See our main panel upgrades page for more.

We check your panel during your free assessment and tell you up front if you need an upgrade.