Watt Sizing

Français

What size generator or battery do you need?

Pick your situation, add your appliances, and get the generator or inverter size (watts) and the battery capacity (Wh) you need.

  • Free
  • No sign-up
  • Open method

Example result

  • Refrigerator700 W / 2,200 W
  • Sump pump (1/2 HP)1,050 W / 2,150 W

Recommended generator size

3,900 W

Run my numbers

Why you can trust this number

  • Open method. Every step of the math is explained below. No black box.

  • Safety first. Carbon monoxide guidance comes from US federal safety agencies.

  • Brand-neutral. The result sizes your real load, not a product we want to sell you.

Official sources: CPSC · CDC · CPSC

Updated

How the calculation works

  1. Running watts: we add up the watts of everything running at the same time.
  2. Starting surge: motors (fridge, AC, pumps) draw extra power for a second or two when they start. We add the single largest surge, since they don't all start at the same moment.
  3. Recommended size: the peak is multiplied by 1.2 to keep a 20% reserve, so the generator or inverter never runs flat out.
  4. Battery: each appliance's watts × hours per day, times the days of backup, divided by 0.85 for inverter losses and safe depth of discharge.

Wattages are typical values. For an exact result, read the rating label on your own appliances.

Frequently asked questions

Which calculator should I use?

Pick the one that matches where you need power: home backup during an outage, an RV or camper van, a CPAP or oxygen machine, or a food truck. Each one starts with the appliances typical for that use, and you can add or remove any of them. The method is the same everywhere: running watts, plus the largest starting surge, plus a 20% margin.

Can I run a generator indoors or in the garage?

No, never. The CDC reports more than 400 deaths from accidental carbon monoxide poisoning in the US each year, and the CPSC counts about 85 deaths a year from portable gasoline generators alone. The CPSC says to run a generator outside only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents.

Why are starting watts higher than running watts?

Anything with a motor or compressor (fridge, AC, pumps) draws 2 to 3 times its running watts for a second or two when it starts. The calculator adds the single largest surge, because in practice motors don't all kick on at the same moment.

Why add a 20% safety margin?

A generator or inverter running at 100% all the time runs hot, wears faster and trips on the smallest surge. Aiming for about 80% of rated output leaves room for a forgotten appliance or a real draw higher than the label.

What's the difference between watts (W) and watt-hours (Wh)?

Watts measure power at a given moment: they size the generator or inverter. Watt-hours measure energy over time: they size the battery. A 60 W CPAP used for 8 hours uses 480 Wh.