What size generator do I need? (simple worked example)
Champion Power Equipment says to add up the running watts of everything you want to run at the same time, then add the highest starting watts of any one item. Using Honda's appliance figures, a fridge, a 1/2 HP sump pump, a furnace fan, a TV and five LED bulbs come to about 4,400 watts at peak and 2,200 watts running.
An average of about 100 people in the US die every year from carbon monoxide from portable generators, according to the CPSC, so the right size matters less than where you put it.
- Running watts and starting watts
- How to size a generator in 4 steps
- Worked example
- Typical appliance wattage
- The big loads that change everything
- Carbon monoxide warning
- Gear
Running watts and starting watts
Running watts are what an appliance draws while it runs. Starting watts are the short surge some appliances need to start, mostly anything with a motor. The gap can be huge: Honda lists an Energy Star fridge at about 132 to 192 running watts but 1,200 starting watts (Honda). A generator has to cover both.
How to size a generator in 4 steps
- List what you must run at the same time in an outage (Champion).
- Find each item's running watts on its label or in its manual. If the label only shows volts and amps, multiply them: volts x amps = watts. A 120 V, 3.5 A item uses about 420 watts (Champion).
- Add up the running watts.
- Add the single highest starting watts on your list. That total is your peak need. Champion suggests buying a generator with some margin above it (Champion).
Honda's tip: start the big motor loads one at a time, not all at once, and you can get by with a smaller generator (Honda).
Worked example
A typical outage list for a house with a basement and a gas furnace. Figures from Honda's wattage guide, LED bulbs from Champion's chart.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator (Energy Star) | 192 | 1,200 |
| Sump pump, 1/2 HP | 1,050 | 2,150 |
| Furnace fan, 1/3 HP | 700 | 1,400 |
| Flat screen TV, 46 in | 190 | 190 |
| 5 LED bulbs at 15 W | 75 | none |
| Total running | 2,207 |
- Total running watts: 192 + 1,050 + 700 + 190 + 75 = 2,207.
- Highest starting watts on the list: the sump pump at 2,150.
- Peak need: 2,207 + 2,150 = 4,357 watts.
Answer: look for a generator rated for at least about 2,200 running watts and 4,400 starting (peak) watts, with some room to spare. Add a 10,000 BTU window air conditioner (1,500 running, 2,200 starting, from Honda) and the numbers become 3,707 running and 5,907 peak.
Typical appliance wattage
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer (Energy Star) | 132 to 192 | 1,200 |
| Sump pump, 1/3 HP | 800 | 1,300 |
| Sump pump, 1/2 HP | 1,050 | 2,150 |
| Furnace fan, 1/4 HP | 600 | 1,000 |
| Furnace fan, 1/2 HP | 875 | 2,350 |
| Window AC, 10,000 BTU | 1,500 | 2,200 |
| Microwave, 1,000 W | 1,500 | 1,500 |
| Coffee maker, 4 cup | 600 | 600 |
| Laptop | 200 to 250 | 200 to 250 |
| Garage door opener | 720 | 1,420 |
| Automatic washer | 1,200 | 1,200 |
| Electric water heater | 4,500 | 4,500 |
| Electric clothes dryer | 5,400 | 6,750 |
Approximate figures from Honda's generator wattage estimation guide. Your own appliance label is always the better number. Champion lists a 1 HP well pump at 1,000 to 2,000 running watts and 2,000 to 4,000 starting watts (Champion).
The big loads that change everything
An electric water heater (4,500 W) or electric dryer (5,400 W running) on its own uses more than everything in the worked example combined (Honda).
To power anything wired into the house, like a furnace or well pump, you need a transfer switch fitted by a qualified electrician. Never plug a generator into a wall outlet. It back feeds power into the lines and can electrocute utility workers (Washington State Dept. of Health).
Carbon monoxide warning
A bigger generator makes more exhaust. Whatever size you buy, the rules are the same.
The CDC says more than 400 Americans die every year from accidental carbon monoxide poisoning not linked to fires, more than 100,000 go to the emergency room, and more than 14,000 end up in the hospital. You cannot see or smell the gas. People who are asleep or drunk can die before they ever feel sick.
- Never run a generator, grill, camp stove or charcoal inside your home, basement or garage, even with the doors and windows open (CDC).
- Run generators outside, more than 20 feet from windows, doors and vents (CDC). Not on a porch or in a carport (CPSC).
- Never heat your home with a gas oven (CDC).
- Have a battery-powered or battery backup carbon monoxide alarm on every level and outside sleeping areas, and test it monthly (CPSC).
- Headache, dizziness, weakness, upset stomach, vomiting, chest pain or confusion can mean carbon monoxide poisoning (CDC). If the alarm sounds or anyone feels this way, get everyone outside, then call 911 or Poison Control at 1-800-222-1222.
Read the full rules on portable generator safety.
Common mistakes
- Adding up running watts only and forgetting the starting surge.
- Starting the fridge, sump pump and furnace at the same moment instead of one at a time.
- Reading the big starting (peak) number on the box as what the generator can run all day.
- Plugging the generator into a wall outlet instead of using a transfer switch.
- Buying a big generator with no plan for where it can sit more than 20 feet from the house.
Gear for this
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Generator with automatic CO shutoff
The CPSC says look for portable generators with a CO shut-off safety feature. See our researched picks.
Plug-in watt meter
Shows the real running watts of plug-in appliances, so you do not have to guess from a chart.
Heavy-duty outdoor extension cord
Long enough to keep the generator more than 20 feet from the house. Check it is rated for your load.
Battery-powered carbon monoxide alarm
The CDC says use a battery-powered or battery backup alarm when you run a generator.
Questions people ask
What size generator do I need to run a refrigerator?
Honda lists an Energy Star fridge at about 132 to 192 running watts and 1,200 starting watts. Add those starting watts to the running watts of everything else you want on at the same time.
How do I work out watts from volts and amps?
Multiply them. Champion's example: 120 volts x 3.5 amps = about 420 watts.
Can a portable generator run a window air conditioner?
Usually, if it is big enough. Honda lists a 10,000 BTU window unit at about 1,500 running watts and 2,200 starting watts.
Sources
- Honda: Generator Wattage Estimation Guide
- Honda: What Size Generator Do I Need? (power management)
- Champion Power Equipment: What Size Generator Do I Need?
- Champion Power Equipment: Generator Wattage Chart
- CPSC: Winter storm carbon monoxide and fire safety tips (2026)
- Washington State Department of Health: Generator Use During a Power Outage
- CDC: Carbon Monoxide Poisoning Basics
Related entries
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- G-02Best portable power stations for power outagesBattery backup with no fumes. How to size one for a fridge or CPAP, and 5 picks.
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- P-02What to do when the power goes outThe first steps, food and medicine rules, and the carbon monoxide mistakes that kill people in outages.
General safety information only, not medical or professional advice. Rules can change, so check the linked sources and follow your local officials. As an Amazon Associate I earn from qualifying purchases.