Main Panel Capacity Calculator

How to use this calculator

1. Enter the main rating (your main breaker or service fuse, in amps) and the supply.
2. Enter the connected load in kW (the sum of everything that could run) and a demand factor for diversity.
3. Click Check capacity to see the demand load, capacity used and spare.

How service capacity is calculated

Voltage and current determine apparent capacity in kVA. Power factor converts that to real-power capacity in kW; the demand load applies a diversity factor to the connected load:

Not everything runs at once, so a demand factor below 100% reflects realistic simultaneous use. If the demand load exceeds the capacity, the service must be upsized or load shifted.

Service rating to capacity

Adding an EV charger or heat pump can push a small service over its limit — check before installing.

Frequently asked questions

How do I know if my panel is big enough?

Compare your demand load (connected load × a realistic demand factor) with the service capacity (rating × voltage). If demand approaches or exceeds capacity, you need a bigger service or load management.

What is a demand factor?

A factor that reflects the fact that not all loads run at once. A whole house rarely uses every appliance simultaneously, so 100% connected load overstates the real peak.

Is this a code load calculation?

No. This is a quick check. A formal calculation (NEC Article 220 in the US, or IEC/local rules) applies specific demand factors to each load category.