To size a generator, add the running watts of everything you'll run at once, then add the single largest starting surge above that item's own running watts. The preset (1,900 W running, a 3,000 W well-pump surge that runs at 1,000 W) peaks at 3,900 watts — about a 4.7 kW generator with headroom.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Continuous (running) watts
1,900 W
Peak (surge) watts
3,900 W
Recommended generator size
4.7 kW
Formula
Total running watts = the sum of the running watts of every load on at the same time. Peak watts = total running watts + (largest single starting surge − that surge item's own running watts). Recommended generator size in kW = peak watts ÷ 1,000 × 1.2, adding 20% headroom so the unit is not run at its limit. Only one starting surge is counted, because motors almost never start at the same instant.
Worked example
Say you run a refrigerator (700 W), some lights (200 W) and a 1 HP well pump (1,000 W running, 3,000 W starting). Total running watts = 700 + 200 + 1,000 = 1,900 W. The well pump has the biggest surge, so peak watts = 1,900 + (3,000 − 1,000) = 3,900 W. Multiply by 1.2 for headroom: 3,900 × 1.2 ÷ 1,000 ≈ 4.7 kW, so a 5,000-watt generator covers it comfortably.
It depends on which loads run at once. Add the running watts of the essentials — refrigerator, furnace blower, well pump, a few lights and outlets — then add the single largest starting surge. An essentials-only setup typically lands around 3,500–7,500 running watts. To back-feed a whole panel you generally need 10,000 watts or more, plus a transfer switch installed by a licensed electrician. Size for what you actually run simultaneously, not the nameplate total of everything you own.
What is the difference between running watts and starting watts?
Running (or rated) watts is the continuous power a device draws once it is up to speed. Starting (or surge) watts is the brief, higher spike a motor pulls as it overcomes inertia at startup — often two to three times the running figure. Resistive loads like heaters and bulbs have no surge. A generator must supply the total running watts continuously and absorb the largest single surge momentarily, which is why both numbers matter.
Why is only one starting surge added instead of all of them?
Motor-driven appliances rarely start at the exact same instant. Adding every surge at once would oversize the generator and waste fuel and money. The realistic worst case is your full running load plus the one biggest surge happening on top of it, so the calculator adds only the largest single surge above its own running watts. If you genuinely expect two large motors to start together, size for both surges to be safe.
Should I add a safety margin to the generator size?
Yes. This calculator multiplies the peak by 1.2, leaving 20% headroom. Running a generator continuously at its maximum output shortens its life, increases fuel burn and leaves nothing for an unexpected load. Headroom also helps cover altitude and temperature derating — engines lose roughly 3–4% of output per 1,000 feet of elevation and more in high heat. Round up to the next standard generator size after applying the margin.
Do I size by running watts or starting watts?
Both. The generator's rated (running) watts must be at least your total continuous load, and its surge (starting/peak) watts must cover your largest single startup spike. Manufacturers list both numbers; this tool's peak figure should fit within the generator's surge rating, and your running total within its rated output. If only the surge fits but the running rating is too small, the unit will overload during steady operation.
How do I convert generator watts to kilowatts?
Divide watts by 1,000. A 3,900-watt peak is 3.9 kW; with the 20% margin it becomes about 4.7 kW. Generators are sold by both running watts and kW, so a "5 kW" unit supplies roughly 5,000 running watts. Remember the advertised figure is often the surge rating — check the rated/running number against your continuous load, not just the headline peak.
Source: Total running watts + the single largest motor starting surge, with 20% headroom (standard backup-power sizing practice). · All sources