Generator size calculator
Tick everything you want running at the same time during an outage. The calculator adds running watts, finds the single biggest motor surge, and suggests a rated output with headroom.
1. Pick what must run at the same time
Defaults are the high end of published ranges. Replace them with the numbers on your appliance labels if you have them.
| On | Appliance | Qty | Running W | Starting W |
|---|---|---|---|---|
| Kitchen | ||||
| Refrigerator 150–400 W run, 800–1,200 W start | ||||
| Freezer (upright or chest) 100–500 W run, 500–1,000 W start | ||||
| Microwave 600–1,200 W | ||||
| Coffee maker 900–1,200 W | ||||
| Toaster 800–1,400 W | ||||
| Electric range, one burner (240 V) 1,200–2,400 W | ||||
| Dishwasher 1,200–2,400 W | ||||
| Heating and cooling | ||||
| Furnace fan, gas or oil (1/2 HP) 300–800 W run, 800–1,600 W start | ||||
| Space heater 750–1,500 W | ||||
| Window AC (5,000 BTU) 450–600 W run, 900–1,200 W start | ||||
| Window AC (10,000 BTU) 900–1,200 W run, 1,800–2,400 W start | ||||
| Central AC (2 ton, 240 V) 2,000–2,500 W run, 3,500–4,500 W start | ||||
| Central AC (3 ton, 240 V) 3,000–3,500 W run, 5,000–6,000 W start | ||||
| Central AC (4 ton, 240 V) 4,000–5,000 W run, 6,500–8,000 W start | ||||
| Window fan 50–200 W | ||||
| Ceiling fan 15–75 W | ||||
| Water | ||||
| Sump pump (1/3 HP) 500–800 W run, 1,000–1,600 W start | ||||
| Sump pump (1/2 HP) 800–1,050 W run, 1,300–2,150 W start | ||||
| Well pump (1/2 HP) 750–1,000 W run, 1,500–2,000 W start | ||||
| Well pump (1 HP, 240 V) 1,000–2,000 W run, 2,000–4,000 W start | ||||
| Electric water heater (240 V) 3,000–4,500 W | ||||
| Laundry | ||||
| Washing machine 500–1,000 W run, 1,000–2,300 W start | ||||
| Electric clothes dryer (240 V) 4,000–6,000 W | ||||
| Lights and electronics | ||||
| LED bulbs (per 10 bulbs) 8–15 W per bulb | ||||
| TV (32 to 55 inch) 80–400 W | ||||
| Internet modem and router 10–20 W | ||||
| Phone and laptop chargers 20–100 W | ||||
| Home security system 15–40 W | ||||
| Desktop computer and monitor CPU about 120 W, monitor about 150 W awake | ||||
| Medical | ||||
| CPAP (no heated humidifier) 30–60 W | ||||
| CPAP with heated humidifier 60–120 W | ||||
| Other | ||||
| Garage door opener Typical 1/2 HP opener; check the label | ||||
| Vacuum cleaner 1,000–1,440 W | ||||
| Dehumidifier About 785 W | ||||
| RV | ||||
| RV air conditioner (13,500 BTU) 1,500–2,000 W run, 2,800–3,500 W start | ||||
| RV refrigerator (electric mode) 150–600 W run, 400–1,000 W start | ||||
2. Your result
Running watts (everything on together)
4,120 W
Largest single starting surge: 1,100 W extra (Sump pump (1/2 HP)).
Peak demand at the moment that motor starts: 5,220 W. Your generator’s starting (peak) rating should be at least this.
Look for a running (rated) output of at least 5,150 W and a starting rating of at least 5,220 W.
Math: running total + largest (starting − running) = peak. Running total ÷ (1 − headroom) = suggested rated watts. Start big motors one at a time.
How this calculator works
Every appliance has a running wattage, the steady power it uses once it is on. Appliances with a motor or compressor (refrigerators, freezers, pumps, furnace blowers, air conditioners, washers) also need a brief burst of extra power to start, called starting or surge watts. Resistive loads such as space heaters, toasters, coffee makers, and electric water heaters have no meaningful surge.
Motors in a house rarely all start in the same second. So the standard sizing method, used in manufacturer charts, is to add up the running watts of everything that will be on together, and then add only the extra starting watts of the single largest motor. That gives the peak your generator must handle for a second or two. The calculator does exactly that, and it tells you which appliance set the peak.
Why we add headroom
A generator’s running (rated) output is what it can supply continuously. Running any engine at its limit for hours is hard on it and leaves no room for a load you forgot, such as a well pump that kicks on while the fridge is running. The headroom slider divides your running total by 80% by default, so the suggested rated watts leave about a fifth of capacity spare. This is a common rule of thumb, not a code requirement. Set it to zero to see the bare minimum.
Where the default numbers come from
Defaults are the high end of the ranges in Champion Power Equipment’s published wattage chart, with a few household items from Virginia Cooperative Extension’s appliance energy guide. Appliances vary widely. A new ENERGY STAR refrigerator can run on a fraction of an older model’s wattage. The nameplate sticker (usually inside the door or on the back) or the manual is the real answer. If a label lists only amps, multiply amps by volts (120 or 240) to get watts, or use our watts-amps-volts converter.
Reading the result
- Compare the peak number with the generator’s starting (sometimes called peak or surge) rating.
- Compare the suggested rated watts with the running (rated or continuous) rating. This is the more important number.
- If you picked any 240-volt loads (well pump over 1/2 HP, central air, electric water heater, dryer, range), the generator must provide 240 V and your hookup must carry it. Most inverter generators under 3,000 W are 120 V only.
- Too big? Drop loads you can run in turns. A water heater or dryer alone can double the size you need. See load management in an outage.
Safety first
The U.S. Consumer Product Safety Commission warns that portable generator exhaust contains carbon monoxide, which can kill in minutes. Run generators outside only, far from windows, doors, and vents, never in a garage even with the door open. Do not plug a generator into a wall outlet (backfeeding); it can electrocute utility workers and neighbors. Use a transfer switch or interlock installed to code. Read our carbon monoxide safety guide before your first outage.