energy · generator sizing
local · auto-updatingTick the appliances you need on generator power. Get total running watts, peak surge watts and the recommended generator size.
the formula
Generator size = (sum of running watts + the single largest starting surge) × 1.20. Add only the largest surge once, then round up to the next common generator size. Adding every appliance's surge would oversize the unit.
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Once you know your wattage, these three pieces complete a safe setup — the unit itself, a way to connect it to your panel, and a cord rated for the load.
main unit
Match the running and surge figures from the calculator above. A unit sized to your peak load with the × 1.20 margin keeps motors from stalling on startup and leaves headroom for one more circuit.
connection
The safe, code-friendly way to feed generator power into your home panel or a dedicated inlet. Back-feeds through a dryer outlet are dangerous for lineworkers — a transfer switch isolates the load properly.
cord
A 10-gauge or thicker cord rated for continuous current at your peak wattage. Undersized extension cords trip breakers, overheat, and waste the capacity you just paid for.
how the math works
One running total, one starting surge, one margin. Here is exactly what the calculator does with the appliances you tick.
Two numbers matter for every appliance: its running watts (steady draw) and its starting watts (the brief surge many motors pull for a second or two at switch-on). Add up the running watts of everything you selected. Then find the single largest starting surge in the list — the difference between that item’s starting and running watts. Add that surge to the running total. Finally, multiply by 1.20 so the generator is not running at 100% load all evening.
A refrigerator that runs at 150 W but starts at 600 W contributes 150 W to the running total and a 450 W surge premium. Two refrigerators still only add one 450 W surge — compressors do not all start on the same tick.
A refrigerator, eight LED bulbs, network gear, two chargers, a microwave, a furnace blower, a sump pump and a TV add up to 2,680 W running. The largest surge comes from the furnace blower: 500 W above its running draw. Add them, apply the 1.20 margin, then move up to the next common size.
| Step | Calculation | Result |
|---|---|---|
| Running load | Sum of all running watts | 2,680 W |
| Largest single surge | Furnace blower 900 W start − 400 W run | 500 W |
| Peak starting load | 2,680 W + 500 W | 3,180 W |
| With 1.20 margin | 3,180 W × 1.20 | 3,816 W |
| Recommended size | Next common size above 3,816 W | 4,000 W (4 kW) |
Values reflect the built-in storm preset. Changing a quantity or switching a load updates the arithmetic in the panel above.
Both. Compare the generator’s continuous rating against your running total and its surge rating against your peak starting load. A unit that only clears the running watts can stall when a motor restarts.
Refrigerators, furnaces and pumps cycle on independently; adding every surge at once would oversize the generator. Stagger the startup of heavy motors. If they must start together, add those surges manually before applying the 1.20 margin.
Only if you use its actual wattage and it is in the list. The calculator handles watts, not circuit voltage, phase or locked-rotor amps. For a hard-wired pump or AC, confirm the nameplate LRA and have a licensed electrician verify the generator’s 240 V output and transfer equipment.
It can. A soft starter reduces a motor’s inrush, but the calculator uses typical values. Add the appliance as a custom load using the manufacturer’s reduced starting watts, then compare that result with the standard estimate.
No. Backfeeding through a dryer outlet can energize the utility lines and endanger lineworkers. Use a listed transfer switch or a dedicated power inlet installed by a licensed electrician, and keep the generator outdoors away from doors and windows.