Why small panels limit solar
Under the National Electrical Code’s common “120% rule,” the main breaker plus the solar breaker can’t exceed 120% of the panel’s busbar rating. For a 100-amp panel with a 100-amp main, that leaves room for a 20-amp solar breaker — enough for roughly 3.8 kW of inverter output. That’s smaller than many homes need.
Older Colorado homes, especially 1950s ranches and earlier bungalows, often still have 100-amp service. Some have panels that should be replaced for safety reasons regardless of solar; if we find one of those, we’ll tell you.
Ways around a full upgrade
A supply-side connection ties the solar in between the meter and the main breaker, so the busbar limit doesn’t apply. It’s common, though it depends on the equipment and the utility’s rules.
Derating the main breaker — swapping a 100-amp main for a smaller one — frees busbar capacity, but only works if a load calculation shows the house can live with it. Adding a heat pump or EV later can make that a bad trade.
With batteries, some Powerwall 3 and SigEnergy configurations can back up a set of chosen circuits or manage loads so a small service isn’t overloaded. Whether that avoids an upgrade in your particular house depends on your loads and the panel, and we won’t promise it before we’ve seen it.
When an upgrade is the right call
If you’re planning a heat pump, an EV charger, an induction range or an addition, the panel may need to grow anyway. Doing it alongside solar puts the electrical work, permits and utility coordination into one project. A service upgrade also involves the utility, which can add lead time.
What to do next
- 1Take a photo of your main panel label and the main breaker.
- 2List the big loads you have now and plan to add (EV, heat pump, hot tub).
- 3We do a load calculation and pick between a supply-side tap, derate, battery load management or an upgrade.
- 4If an upgrade is needed, we include it in the quote and coordinate with your utility.
FAQ
Questions we hear
How much does a panel upgrade add to a solar project?
It varies a lot with whether only the panel is replaced or the whole service, including the meter base and utility work. We quote it after seeing the panel rather than giving a number that might be wrong by thousands.
Can I add a Level 2 EV charger to a 100-amp panel?
Often, with a load calculation and sometimes a smaller charger circuit or load management. It depends on what else the house runs. Our charger installs run about $2,500–$3,000 before any panel work.
Will a battery let me skip the panel upgrade?
In some houses, yes; in others, no. Certain battery setups can back up selected circuits or manage loads, but a site assessment decides. We won’t sell a battery as a workaround until we’ve done the load math.
My panel is an old brand people warn about. Does that matter?
Some older panel models have known reliability concerns. If we find one, we’ll recommend replacing it regardless of solar, and we won’t tie new equipment into a panel we don’t trust.