Generator Size Calculator — Running & Starting Watts
Who this is for: For homeowners sizing standby or portable backup — and the electricians quoting the transfer switch — who want the motor-starting math done honestly instead of by brochure.
List what must run in an outage with each item's starting watts — get running watts, peak demand and the suggested nameplate kW.
Quick answer: Generator sizing: add up running watts of everything on the backup list, add only the largest single motor-start increment, then size the nameplate at about 50% loading. Fridge + sump + lights = 1,800 W running, 3,200 W with the biggest start → a 6.5 kW class generator.
| Appliance | Running W | Starting W | |
|---|---|---|---|
| Running watts | Straight sum of the list |
|---|---|
| Peak | Running + largest (start − run) increment |
| Nameplate target | Peak ÷ 50% loading, 0.5 kW steps |
| Typical surges | Fridge ~2.2 kW · well pump ~2–3 kW · 3-ton AC ~9–13 kW LRA |
| Example | 1,800 W running / 3,200 W peak → 6.5 kW |
| Status | Figures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code. |
How much generator do I actually need?
Two numbers decide it. Running watts are the steady-state draw of everything on your list, added straight up. Starting watts are the surge when a motor kicks on — a refrigerator may run at 800 W but need 2,200 W for half a second. You never add every starting figure; motors starting one after another means only the biggest surge rides on top of the running total. This calculator takes the list, sums running watts, adds the single largest start-up increment, and suggests a nameplate size at a 50% loading target — generators hate running flat-out for hours, and headroom is what keeps voltage stable when the well pump kicks in. Example: fridge 800/2,200 W + sump 600/1,800 W + lights 400 W = 1,800 W running, 3,200 W peak → a 6.5 kW class unit.
Common uses
- Sizing a portable unit for fridge, well pump, furnace and a few circuits
- Quoting a standby generator for partial-home (load-shed) coverage
- Deciding what NOT to back up — the fastest lever on generator cost
- Checking whether a 7 kW portable really carries a heat pump start