Tool · Base 01 Power
Wattage calculator
Tick what you want to keep running, and see the generator or battery size that covers it. Every number is editable.
- Tick each appliance you want to run. The fridge, lights, router and phone charging start ticked.
- Swap in the watts from each appliance label if you have them. If the label shows volts and amps, multiply them to get watts.
- Set how many hours you want a battery to last, then read the results.
Wattage calculator
Tick what you want to run in an outage. The numbers are typical estimates. Your appliance label overrides them, so edit any value.
| Run | Appliance | Qty | Running W | Starting W | Duty % |
|---|
Running watts are what an appliance uses while on. Starting watts are the short burst some motors need when they start. Duty is the share of time an appliance is actually drawing power, used only for the battery estimate.
Tick at least one appliance to see results.
On a phone, swipe the appliance table sideways to see every column.
How the math works
Running watts add up. Everything you tick is on at the same time, so their running watts are summed.
Starting watts are a short burst. Motors in fridges, pumps, furnace blowers and air conditioners pull extra power for a second or two as they start. The calculator adds only the single largest extra burst, since motors rarely start at the same instant.
Headroom keeps a generator off its limit. The default adds 20 percent, then rounds up to the next 100 watts.
Duty is the share of time an appliance actually draws power; a fridge cycles on and off. The 0.85 allows for inverter losses, the same rough figure we use on the power guide.
Worked example with typical numbers: a fridge (200 running, 800 starting), a sump pump (800, 1,300), a furnace blower (600, 1,800), ten LED bulbs (100) and a router (30) run at 1,730 watts. The biggest extra start is the blower at 1,200, so the peak is 2,930 watts. With 20 percent headroom, look for about 2,100 running watts and 3,600 starting watts.
Before you run a generator
Run it outdoors only, at least 20 feet from windows, doors and vents, and never in a home or garage, even with the doors open. Keep battery-powered carbon monoxide alarms inside. The full list is in our home backup power guide, and the picks that match small and large loads are on the gear page.
Sources: CDC, Carbon Monoxide Poisoning Basics (generator distance and CO detectors); Ready.gov, Power Outages (inventory and medical devices). Appliance wattages in the calculator are typical estimates, not measurements of your equipment.