Panel Capacity Calculator
Will a heat pump, EV charger or water heater fit on the existing electrical service? This is a quick screening check using the NEC 220.87 arithmetic: measured peak demand at 125 percent, plus the new load, against the service rating.
1. Existing service
The rating on the main breaker. Assumes a 120/240-volt single-phase residential service.
2. Highest measured peak demand
The highest 15-minute (or hourly) demand over the last 12 months, from the utility's smart-meter data.
3. New equipment
Presets are typical values. Use the nameplate of the actual equipment wherever you can.
| Measured peak | |
| At 125% (NEC 220.87) | |
| New equipment | |
| Total against a service |
This is a screening estimate, not a load calculation for a permit. A real NEC 220.87 calculation uses the home's own 12 months of interval data, the nameplate of the equipment being installed, and a judgment about which loads run at the same time as the measured peak. It must be prepared by a licensed professional, and the local inspector has the final say.
The 9.7 kW default is the average measured peak across 11,940 homes studied by Lawrence Berkeley National Laboratory. An individual home can be well above or below it. Treats kW as kVA, which is close for most homes.
Contractors
Run the real thing. Upload the utility's interval file and TrueLoad finds the measured peak and produces an AHJ-ready report, or run the traditional calculation when there is no data. Your first project is free.
Try TrueLoad free →Homeowners
Been told you need a panel upgrade? Read what to ask first, then send it to your electrician or installer.
Read the homeowner guide →How the calculation works
NEC 220.87 lets the load on an existing service be based on measured maximum demand instead of a worst-case estimate built from appliance nameplates. The highest demand recorded over a year is multiplied by 125 percent, the new load is added, and the total must not exceed the rating of the service.
To convert: amps = kW × 1,000 ÷ 240. A 100-amp service is 24 kW. A 9.7 kW peak is about 40 amps, or 50 amps once the 125 percent factor is applied, which leaves roughly 50 amps for new equipment.
For comparison with the traditional approach, see 220.83 vs. 220.87. To find a home's actual peak, see how to get interval data from your utility.