Starting watts vs running watts
Every generator and inverter lists two power ratings, and every appliance with a motor has two power draws. Mixing them up is the most common reason a generator stalls when the fridge kicks on — or why people buy one twice as big as they need.
The two numbers
- Running watts (also continuous or rated watts) — what the appliance draws while it works, and what a generator or inverter can supply for hours.
- Starting watts (also surge or peak watts) — the brief extra a motor or compressor needs for a moment to get moving, and the short burst a generator or inverter can supply on top of its running rating.
Champion’s 3500 W dual-fuel generator, for example, is rated 3,500 running and 4,000 starting watts on gasoline. Its 2000 W inverter generator is 1,700 running and 2,000 starting. On propane both ratings are lower.
Which appliances have starting watts
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 150–400 | 800–1,200 |
| Freezer | 100–500 | 500–1,000 |
| Sump pump, ½ hp | 800–1,050 | 1,300–2,150 |
| Well pump, 1 hp | 1,000–2,000 | 2,000–4,000 |
| Window AC, 10,000 BTU | 900–1,200 | 1,800–2,400 |
| RV AC, 13,500 BTU | 1,500–2,000 | 2,800–3,500 |
| Circular saw, 7-1/4 in | 1,200–1,800 | 2,400–3,600 |
| Air compressor, 1 hp | 1,500 | 4,500 |
| Space heater | 750–1,500 | no extra |
| Microwave | 600–1,200 | no extra |
| Coffee maker | 900–1,200 | no extra |
Resistive loads are simple by comparison: a 1,500 W heater draws a steady 12.5 amps at 120 volts from the moment it is switched on until it is switched off, with no surge at all. That is also why a heater can share a generator with a fridge more easily than a second compressor can — its draw never spikes, so it only adds to the running total. Plan generator and inverter sizes so the running total plus one start still fits, and keep heaters on their low setting when a compressor is about to cycle on.
The pattern: anything with an electric motor or compressor surges; anything that just heats, lights or runs electronics doesn’t. The full list is in the appliance wattage chart.
How to add them up
running watts needed = sum of all running watts starting watts needed = sum of all running watts + the largest single (starting − running)
You add every running load, but only one start — the biggest — because two motors rarely start in the same instant, and you can make sure they don’t by switching things on one at a time, biggest first.
| Load | Running W | Starting W | Start-up extra |
|---|---|---|---|
| Refrigerator | 200 | 1,200 | 1,000 |
| Sump pump, ½ hp | 1,050 | 2,150 | 1,100 |
| Lights and router | 120 | — | — |
| Total | 1,370 | largest: 1,100 |
So a generator rated at least 1,370 running and 2,470 starting watts runs this set, and more running headroom makes it work less hard. Adding all the starting watts together instead (3,350 + lights) would oversize it. The generator size calculator does this for any list; what size generator to run a refrigerator and what size generator to run a well pump are worked examples.
Inverters work the same way
A battery inverter also has a continuous rating and a surge rating, and the surge figure is what starts a fridge or pump. The surge may last only a moment, so check the spec sheet for how long it is held. What size inverter for a refrigerator shows why a fridge running on 200 W can still need a 1,000 W-class inverter.
Reducing starting watts
- Start the biggest motor first, with everything else off, then add loads.
- Soft-start devices for air conditioners and some pumps reduce the start surge; check the maker’s figures for your model.
- Stagger thermostats so two compressors don’t cycle on together.
- Don’t confuse starting watts with daily energy. The surge lasts seconds, so it barely affects fuel or battery capacity — it only sizes the generator or inverter.
Frequently asked questions
Do I need to add starting watts together?
No — add all the running watts, then only the largest single start-up extra, and start motors one at a time.
Does a microwave have starting watts?
Champion’s chart lists no extra starting watts for a microwave; size on its input (running) watts from the label.
How long do starting watts last?
Only a moment, while a motor comes up to speed — which is why a generator or inverter can supply it briefly above its running rating.
Sources
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- Champion Power Equipment — 3500-Watt Dual Fuel Generator (model 201213) specifications (opens in a new tab) — 3,500 running / 4,000 starting watts on gasoline; 14 h at 50% load on 4.7 gal; 10.5 h at 50% load on a 20 lb propane tank; 120 V 30 A TT-30R RV outlet.
- Champion Power Equipment — 2000-Watt Dual Fuel Inverter (model 201230) specifications (opens in a new tab) — 1,700 running / 2,000 starting watts on gasoline (1,530 / 2,000 on propane); 11.5 h at 25% load on 1.05 gal; 34 h at 25% load on a 20 lb propane tank.
- Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.