Panel headroom
Measured demand (preferred) or a simplified optional dwelling-unit estimate. For adding a charger, heat pump, or other large load.
OpenCalculators
The three tools
Each calculator is a hub: use it with your numbers, then read the question pages that match your panel size, loads, or climate.
Generator size
Honda-attributed running and starting watts, plus your own nameplates. Sizes the generator, not the transfer equipment.
OpenHeat pump vs furnace
EIA 2025 state-average prices, AFUE and HSPF2, dollars per MMBtu of delivered heat, optional annual scale-up from your bill.
OpenThis is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.