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All calculatorsMethodology

Generator Wattage Calculator

Add up running watts and the largest starting surge for up to six appliances from their nameplates, then apply your own headroom margin to both totals.

Interactive calculator · Method GENERATOR-WATTAGE-1.0 · Last reviewed: 2026-11-04

Calculate your scenario

Total running watts = Σ running watts × quantity for loads with a quantity above 0.

Your inputs

Try an example

Default worked example. Edit the inputs to calculate your scenario.

Default worked example

Total running watts 2,060 W
Largest single starting surge (extra) 1,500 W (Load 1)
Peak watts with one load starting 3,560 W
Running watts with headroom 2,472 W
Peak watts with headroom 4,272 W

Peak = 2,060 W running + 1,500 W largest extra surge = 3,560 W; × 1.2 headroom = 4,272 W.

How to use the calculator

For each appliance enter running watts, starting (surge) watts and how many you will run together. Use 0 quantity to leave a row out, then set the headroom margin you want to keep.

What the result means

The example loads run at 2,060 W; when the 2,200 W-start load kicks on the peak is 3,560 W, or about 4,272 W with a 20% margin.

Formula and units

Total running watts = Σ running watts × quantity for loads with a quantity above 0. Extra surge for a load = starting watts − running watts. Peak watts = total running watts + the single largest extra surge, assuming only one motor load starts at a time while the rest run. Both totals are then multiplied by (1 + headroom % ÷ 100).

Worked examples

Four example household loads with 20% margin: Load 1 running watts (from nameplate): 700 W; Load 1 starting watts (0 or same as running if no surge): 2200 W; Load 1 quantity (0 = leave out): 1 units; Load 2 running watts (from nameplate): 1000 W; Load 2 starting watts (0 or same as running if no surge): 2000 W; Load 2 quantity (0 = leave out): 1 units; Load 3 running watts (from nameplate): 10 W; Load 3 starting watts (0 or same as running if no surge): 10 W; Load 3 quantity (0 = leave out): 6 units; Load 4 running watts (from nameplate): 300 W; Load 4 starting watts (0 or same as running if no surge): 300 W; Load 4 quantity (0 = leave out): 1 units; Load 5 running watts (from nameplate): 0 W; Load 5 starting watts (0 or same as running if no surge): 0 W; Load 5 quantity (0 = leave out): 0 units; Load 6 running watts (from nameplate): 0 W; Load 6 starting watts (0 or same as running if no surge): 0 W; Load 6 quantity (0 = leave out): 0 units; Headroom margin you want to keep (example 20%): 20 %.
Example result: Total running watts: 2,060 W; Largest single starting surge (extra): 1,500 W (Load 1); Peak watts with one load starting: 3,560 W; Running watts with headroom: 2,472 W; Peak watts with headroom: 4,272 W.

Two motor loads and two heaters with 25% margin: Load 1 running watts (from nameplate): 1200 W; Load 1 starting watts (0 or same as running if no surge): 3600 W; Load 1 quantity (0 = leave out): 1 units; Load 2 running watts (from nameplate): 1500 W; Load 2 starting watts (0 or same as running if no surge): 1500 W; Load 2 quantity (0 = leave out): 2 units; Load 3 running watts (from nameplate): 800 W; Load 3 starting watts (0 or same as running if no surge): 2400 W; Load 3 quantity (0 = leave out): 1 units; Load 4 running watts (from nameplate): 0 W; Load 4 starting watts (0 or same as running if no surge): 0 W; Load 4 quantity (0 = leave out): 0 units; Load 5 running watts (from nameplate): 0 W; Load 5 starting watts (0 or same as running if no surge): 0 W; Load 5 quantity (0 = leave out): 0 units; Load 6 running watts (from nameplate): 0 W; Load 6 starting watts (0 or same as running if no surge): 0 W; Load 6 quantity (0 = leave out): 0 units; Headroom margin you want to keep (example 20%): 25 %.
Example result: Total running watts: 5,000 W; Largest single starting surge (extra): 2,400 W (Load 1); Peak watts with one load starting: 7,400 W; Running watts with headroom: 6,250 W; Peak watts with headroom: 9,250 W.

Limitations and common mistakes

Uses wattages you enter from appliance nameplates or manuals; the example rows are placeholders. If two motor loads can start together, add both surges. Wattage totals do not size a generator, transfer switch, inlet, cords or wiring, and do not account for altitude or temperature derating, voltage, or 120/240 V circuit balance; follow the generator maker’s ratings and a qualified electrician.

Adding every appliance’s starting watts together. Motors rarely start at the same instant, so add running watts and only the largest extra surge, unless loads really start together.

Sources and assumptions

Method GENERATOR-WATTAGE-1.0; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.