Generator Size Calculator
Select what you want to power. We add every running watt, then count only the single biggest motor surge — the way electricians actually size. Adding every surge at once is the classic mistake that sells you 2× the generator you need.
How the math works (so you can check us)
Step 1. Add the running watts of everything you'll power at once.
Step 2. Find the largest motor-driven item's starting watts — the surge. Add that once.
Step 3. Multiply by 1.25 for headroom (never run a generator flat-out for hours).
Step 4. Round up to the nearest standard size: 2, 3, 3.5, 5, 6.5, 7.5, 9, 10, 12, 15 kW.
Needed = (Σ running watts + largest single surge) × 1.25
Wattage figures are typical nameplate values compiled from DOE/ENERGYSTAR appliance data and manufacturer spec sheets (Generac, Honda, Jackery). Check your own nameplates — yours may differ. Surge estimates: 3× running watts for pumps and compressors unless noted.
Winter-storm reality check
The “Winter storm essentials” preset — furnace fan (blower motor, the surge king), fridge, a few lights, chargers, a TV — lands around 3,600 running / 4,100 peak → recommended 5.1 kW → buy a 6 kW class unit. That matches what generator manufacturers publish for the same scenario, which is the point: same math, no upsell.
Common sizing mistakes we prevent
- Summing every surge at once. Motors almost never start simultaneously. You only pay for one.
- Forgetting the furnace fan. In a winter outage the gas furnace still needs electricity for its blower (~600–1,000 W running, ~2,000+ surge). A generator without that headroom leaves you with heat that can't circulate.
- No headroom. Continuous full load shortens generator life; 25% margin is standard practice.
- Oversizing by fear. A 15 kW standby for storm essentials burns more fuel at low load and costs 3–4× the right inverter.