Solar Backfeed Capacity Calculator

It is the allowance that lets a busbar be supplied by two sources totalling more than its own rating, on the reasoning that the utility supply and the solar supply sit at opposite ends of the bar and the loads in between draw from both. In practice it means the busbar rating multiplied by 1.2, less the main breaker rating, gives the largest backfeed breaker the panel can accept. A 200 A busbar with a 200 A main leaves 40 A. Because a backfeed breaker is sized at or above 125% of continuous inverter output, that 40 A corresponds to about 32 A of continuous output. The allowance is conditional rather than automatic, and the conditions are where projects come unstuck.

Multiply the busbar rating by 1.2, then subtract the main breaker rating. What remains is the maximum backfeed breaker the busbar can accept under the allowance. Both inputs have to be read from the panel label rather than inferred: the busbar rating and the service size are frequently different numbers, and assuming a 200 A service means a 200 A busbar is one of the most common and most expensive assumptions in residential solar. If the result is smaller than the breaker the inverter needs, the design does not fit that panel as it stands.

No, and treating it as automatic is the usual error. It depends on where the supplies land on the busbar, which is why a centre-fed panel does not receive it. It depends on the panel being listed for the arrangement, and the manufacturer's own listing can be more restrictive than the general rule. It depends on the jurisdiction, which adopts and interprets the code, and some require the conservative reading where both supplies together stay within the busbar rating. The calculator shows that conservative figure alongside the allowance for exactly this reason.

There are four usual exits and they differ enormously in cost. Derating the main breaker is much the cheapest, and it works only where the load calculation for the property supports a smaller main. Connecting ahead of the main, on the supply side, sidesteps the allowance entirely because nothing lands on the busbar, but it brings its own requirements and the utility has a view. Reducing the inverter size lowers the required breaker at the cost of production the customer was sold. A main panel upgrade removes the constraint and is the most expensive, usually needing its own permit.

Because the busbar is the physical conductor everything lands on, and it is what the allowance is calculated against. A property can have a 200 A service feeding a panel whose busbar is rated well below that, and nothing outside the panel reveals it. This is why the label has to be photographed at the site survey rather than estimated from the meter, and it is the single input most likely to turn a sold design into a redesign after contract.

Yes. Certain discontinued load centres have no listed replacement breaker still in production, which means they cannot accept a backfeed at any size regardless of what the arithmetic says. The calculation is irrelevant in that case, because the constraint is availability of a listed device rather than capacity. Recognising those panels on sight is a survey skill worth building, since the design that follows is a panel replacement rather than a solar design.

No. It performs one arithmetic step that a designer performs, using inputs you supply, to help a sales or operations team see a constraint early rather than after contract. It does not perform a load calculation, does not evaluate whether a derate is permissible for the property, does not read the panel listing, and does not know what the local jurisdiction requires. The authority having jurisdiction decides, and a licensed designer or electrician has to confirm any of this before it is relied upon.