Key takeaways
- Peak watts = total running watts + the largest motor's extra starting surge.
- Size the generator to the peak, not the running total — motors spike when they start.
- Add a safety margin (~20%) to the running load for headroom and voltage stability.
- 4,000 running W + a 2,200 W surge = a 6,200 W peak, so pick a 7,000 W generator.
How to size a generator
A generator is sized for two different numbers: the steady running watts it must hold all day, and the brief peak watts it must survive when a motor kicks on. Total the running watts of everything you'll run at once, add a margin for headroom, then add the extra starting surge of your single largest motor. The peak is what the generator must cover.
Only one motor surges at a time, so you add just the biggest surge — not every motor's spike. The starting surge figure here is the extra watts above that motor's running draw, which is already counted in your running total. The result is rounded up to the next standard generator size.
Worked example: 4,000 W running, 2,200 W surge, 20% margin
Running needed = 4,000 × 1.2 = 4,800 W with the safety margin. Peak watts = 4,000 + 2,200 = 6,200 W. The next common generator size at or above 6,200 W is 7,000 W, so a 7,000-watt generator handles both the steady load and the surge with room to spare.
Running vs. additional starting watts by appliance
| Appliance | Running watts | Extra starting surge |
|---|---|---|
| Refrigerator | 150 W | 1,050 W |
| Well pump (1 HP) | 1,000 W | 2,000 W |
| Air conditioner | 1,500 W | 3,000 W |
| Microwave | 1,000 W | 0 W |
| Sump pump | 800 W | 1,500 W |
Start with the load, then size the surge
Every sizing job starts from an honest running total — add up the appliances you'll run at once with the off-grid load calculator first. Then identify the single largest motor and use its starting surge as the spike to cover. Choosing the next common size up keeps the generator out of the wet-stacking zone while leaving headroom for that one big surge.
Frequently asked questions
What size generator do I need?
Total your running watts, add a margin, then add the largest motor's extra surge. Size to that peak — 4,000 W running + a 2,200 W surge = 6,200 W peak, so a 7,000 W generator.
What is the difference between running watts and starting watts?
Running watts are the steady draw once a device is going; starting (peak/surge) watts are the brief spike a motor pulls at startup — often 2–3× its running draw.
Why size for the largest motor's starting surge?
Motors surge one at a time, so you only add the single biggest surge on top of the running total. Adding every motor's surge would oversize the generator.
Can a generator be too big?
Yes — an oversized unit runs lightly loaded, wastes fuel, and can wet-stack a diesel. Aim to use ~25–75% of rated output, with headroom only for the surge.
How do I find an appliance's wattage?
Check the nameplate, manual, or a plug-in meter for running watts. For motors, look up the starting surge too — often shown as LRA or separate starting watts.
Will this generator run my house, well pump, and AC?
If the peak watts fit. Total the running watts of every circuit at once, add a margin, then add the largest motor's surge — stagger big motor starts.
Running- and starting-watt figures follow generator manufacturer sizing data — see generator sizing guidance. The watt arithmetic here is exact.
Last reviewed June 2026