How it works
Motors (refrigerators, pumps, furnace blowers, air conditioners) draw a short surge when they start. Generators are rated for both running (rated) watts and starting (surge) watts.
Total running watts = Σ running watts of everything on at once
Total starting watts = total running watts + the single highest starting watts
Only the largest surge is added because motors rarely start at exactly the same moment. Your generator needs both a running rating ≥ the total running watts and a starting rating ≥ the total starting watts.
Worked example (Briggs & Stratton worksheet)
| Item | Running W | Starting W |
|---|---|---|
| Refrigerator/freezer | 550 | 1,350 |
| Furnace fan blower 1/2 HP | 800 | 2,350 |
| Sump pump 1/2 HP | 1,200 | 3,000 |
| Plasma TV 50" | 500 | 0 |
| Lights (6 × 75 W) | 450 | 0 |
| Total | 3,500 | highest: 3,000 |
Starting watts = 3,500 + 3,000 = 6,500 W. You need a generator with at least 3,500 running watts and 6,500 starting watts.
Assumptions and limits
- Wattages are typical values from the Briggs & Stratton worksheet. Values from other manufacturers (such as Champion) differ for some items, so use the nameplate on your own appliance when you have it.
- Appliances with 0 starting watts (resistive loads like heaters and bulbs) have no meaningful surge.
- Connecting a generator to house wiring should be done through a transfer system installed by a licensed electrician, as Briggs & Stratton advises. Never backfeed through an outlet.
- Home standby generators are also limited by fuel type and altitude; check the manufacturer spec sheet.