How to size a generator
A generator has two limits: the power it can supply all the time (running watts) and the short burst it can give when a motor starts (starting or peak watts). You need to stay under both. Here is the math for the default list.
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1
Add up the running watts
Refrigerator 700, sump pump 1,050, furnace fan 875, microwave 1,000, LED lights 100, TV 80, internet 30 and chargers 75. Total: 3,910 W.
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2
Add the single biggest starting surge
The furnace fan needs the most extra watts to start, 2,300. Motors rarely start at the same instant, so you add only the biggest one: 3,910 + 2,300 = 6,210 W peak.
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3
Leave a margin
Generators run cooler, quieter and longer below full load. 20% on top of 3,910 W means you want at least 4,692 running watts.
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4
Pick a size that clears both numbers
A portable rated 5,000 running watts, with about 6,250 starting watts, covers 4,692 running and the 6,210 W peak.
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5
Estimate the fuel
At half load that generator makes 2.5 kW and burns about 0.43 gallons of gasoline an hour. At $4.27 a gallon, that is about $1.81 an hour, or $44 for a full day.
Running watts vs starting watts
Anything with a motor or compressor (refrigerator, freezer, pumps, furnace blower, air conditioner) pulls two to three times its running power for a second or two as it starts. Heaters, lights, electronics and the microwave have no real surge.
Generator boxes print the bigger "peak" or "starting" number in large type. Shop by running watts, also called rated watts. A "9,500 W" model on the box might deliver 7,500 W all day. In this calculator, the generator size is always the running number.
Typical appliance wattages
These are the calculator's defaults. Your appliance's data plate is better: running watts are volts times amps, and a motor's locked-rotor amps (LRA) hint at its starting surge.
| Appliance | Running W | Extra to start |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Freezer | 700 | 2,200 |
| Sump pump, 1/2 HP | 1,050 | 2,150 |
| Well pump, 1/2 HP (often 240 V) | 1,000 | 2,100 |
| Furnace fan, 1/2 HP | 875 | 2,300 |
| Central AC, 2 ton (often 240 V) | 3,800 | 4,950 |
| Window AC | 1,200 | 2,000 |
| Space heater | 1,500 | — |
| Microwave, 1,000 W | 1,000 | — |
| Electric range, one 8" burner (often 240 V) | 2,100 | — |
| Electric water heater (often 240 V) | 4,500 | — |
| Coffee maker | 800 | — |
| Washing machine | 500 | 1,200 |
| Dishwasher | 1,200 | 1,500 |
| Garage door opener, 1/2 HP | 875 | 2,350 |
| LED lights, about 10 bulbs | 100 | — |
| TV, LED | 80 | — |
| Internet modem and router | 30 | — |
| Desktop computer | 100 | — |
| Phone and laptop charging | 75 | — |
Inverter, portable or standby?
| Type | Running watts | Best for |
|---|---|---|
| Inverter | 1,800–7,500 | Quiet, fuel-thrifty, clean power for electronics. Fridge, lights, internet. |
| Portable (open frame) | 3,600–9,500 | Most watts per dollar. Essentials plus a well pump or window AC through an interlock. |
| Home standby | 10–26 kW | Starts by itself, runs on natural gas or propane, can carry central AC and the whole house. |
Prices swing a lot by brand and season. A 2,200 W inverter costs roughly $600 to $1,200. A 9,500 W dual-fuel portable runs about $1,200 to $2,200. A 22 kW standby unit with its automatic transfer switch is about $6,000 to $7,000 for the equipment alone and commonly $10,000 to $15,000 installed, with gas line, concrete pad, permit and electrician. Standby ratings are usually for propane; on natural gas the same unit makes a little less.
Fuel use and cost
Generators burn fuel roughly in proportion to the power they make. The calculator uses fuel per kWh figures taken from manufacturer spec sheets at half load:
| Fuel | Per kWh made | Example at half load |
|---|---|---|
| Gasoline, portable | 0.17 gal | 9,500 W portable: about 0.8 gal/h |
| Propane, portable | 0.16 gal | 20 lb tank lasts 6 to 7 h on a 9,500 W unit |
| Propane, standby | 0.22 gal | 22 kW unit: about 2.5 gal/h |
| Natural gas, standby | 21 cu ft | 22 kW unit: about 220 cu ft/h |
Real use swings with load. A generator idling at a quarter load burns less per hour but more per kWh. Gasoline goes stale in storage, so add stabilizer and rotate your cans. Propane and natural gas keep, which is why standby units use them. A tank exchange costs far more per gallon than a bulk fill, so price your own source.
Safety: carbon monoxide, backfeed and grounding
Carbon monoxide kills. CPSC estimates about 85 people die each year in the U.S. from carbon monoxide made by generators. CO has no smell or color. Run a portable generator outdoors only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. Never run one inside a home, garage, basement, crawlspace or shed, and not on a porch or in a carport, even with the doors open. A fan does not make it safe.
Put CO alarms (battery-powered or with battery backup) on every level of the home and test them monthly. When you shop, look for a model with a CO shut-off certified to PGMA G300-2023 or UL 2201-2023. If anyone gets a headache, dizziness or nausea while the generator runs, get everyone outside and call 911.
Standby units are not immune. After Hurricane Ida, CPSC investigators found standby generator exhaust that traveled 25 feet or more and set off CO alarms inside homes. Keep the unit well away from windows and air intakes, past the minimum clearance if you can, and keep CO alarms working.
Never backfeed. A "double-male" cord from the generator into a wall or dryer outlet sends power back into the house wiring and out onto the utility lines. It can electrocute line workers who think the lines are dead, and it can start a fire or wreck the generator when grid power returns. The safe ways to power circuits are a transfer switch or a listed panel interlock with a power inlet, installed by a licensed electrician with a permit. The National Electrical Code (Article 702) requires transfer equipment for this kind of standby system.
Grounding. Follow the owner's manual. OSHA notes that a portable generator's frame can act as the ground when it only feeds cords plugged into its own outlets. When it feeds the house through a transfer switch, it has to be grounded to code, and the generator's neutral bonding has to match the switch (bonded-neutral generators usually need a switched-neutral transfer switch). That is a call for the electrician, not a guess.
Also: use outdoor-rated, heavy-gauge cords with no cuts. Keep the generator dry under an open canopy rather than in an enclosure. Shut it off and let it cool before refueling, and store fuel away from the house and any flame.
Tips from people who have lived through outages
- Stagger the motors. Start the biggest motor first, then the rest one by one. That keeps you under the peak.
- Watch for 240 V loads. Well pumps, central AC, electric ranges and water heaters often run on 240 V. They need a generator with a 240 V outlet and a panel connection, not an extension cord.
- Skip the heat hogs. A water heater, electric dryer or range can double your load. Run them one at a time, or not at all.
- Soft-start kits help. A soft starter on a central AC or well pump can cut the starting surge a lot, which sometimes lets a smaller generator work.
- Test before the storm. Run the generator under load for 20 minutes a few times a year so you find a stuck carburetor or dead battery in good weather.
Sources and assumptions
- Carbon monoxide guidance, 20-foot rule, CO alarms and certified models: CPSC, generator safety release (May 27, 2026). Death estimate of about 85 a year (2017–2019 average): CPSC, 2022. Standby exhaust findings: CPSC staff report on stationary generators (June 3, 2025).
- Backfeed and grounding: OSHA, Using Portable Generators Safely, OSHA, Grounding Requirements for Portable Generators, and Generac on backfeeding. Transfer equipment: NFPA 70 (National Electrical Code), Article 702.
- Appliance watts: Generac portable generator brochure wattage chart (refrigerator, sump pump, well pump, furnace fan, garage door opener, central AC, microwave, range element) and the Champion wattage chart (window AC, TV, computer, coffee maker, washer, dishwasher). Space heater 1,500 W and water heater 4,500 W are common nameplate ratings. Internet gear: 30 W rounds up the 9 to 29 W ready-state draw that Xfinity, internet equipment energy usage lists for the modems and gateways it supplies (read November 1, 2026). Chargers (75 W) are a HomeOutlay estimate.
- Fuel use: Generac 22 kW spec (221 cu ft/h natural gas and 2.45 gal/h propane at 50% load) and the Westinghouse WGen9500DF spec (8 h on 6.6 gal of gasoline and 7 h on a 20 lb propane tank at 50% load), converted to fuel per kWh.
- Fuel prices: gasoline $4.27/gal and propane $2.73/gal (before taxes), U.S. averages for the week of October 26, 2026, from EIA Gasoline and Diesel Fuel Update and EIA Heating Oil and Propane Update. Natural gas $1.61 per 100 cu ft, the 2026 residential average ($16.09 per thousand cu ft) in the EIA Short-Term Energy Outlook, October 2026.
- Hookup prices: HomeGuide, transfer switch installation cost, read November 1, 2026: an interlock kit costs $400 to $850 installed and a manual transfer switch $400 to $1,300. The $800 default sits inside both ranges.
- Equipment prices (estimates): HomeGuide, whole house generator cost (updated December 30, 2025) puts portable generators at $500 to $2,500 for a 3 to 10 kW unit, a 22 kW Generac unit at about $5,000, and installation labor at $3,000 to $5,000, or $5,000 to $12,000+ above 20 kW. Our generator prices by size and the 22 kW installed figure of about $10,000 to $15,000 are HomeOutlay estimates from 2026 retailer listings inside those ranges, as of October 2026.
- HomeOutlay assumptions: a generator's starting capacity is taken as about 1.25 times its running watts; the largest single starting surge is added to the total running load; fuel is estimated at 50% of the generator's running watts.
Common questions
What size generator do I need to run my house?
For the essentials (refrigerator, sump pump, furnace fan, microwave, lights, TV and internet) the calculator lands on a portable rated about 5,000 running watts. Adding a well pump moves you up to about 7,500 watts, and central air usually means a standby unit. Running the whole house at once, including an electric range and water heater, takes a 14 to 26 kW standby generator.
Will a 5,000 watt generator run a refrigerator?
Yes. A refrigerator needs about 700 running watts and up to 2,200 extra watts for a moment when the compressor starts. A 5,000 watt generator handles that with room left for a sump pump, a furnace fan, lights and a microwave, as long as two motors do not start at the same instant.
Can I plug a generator into a wall outlet?
No. Plugging a generator into a wall outlet with a double-male cord backfeeds the house wiring and the utility lines. It can electrocute line workers and start a fire when the power comes back. Use extension cords to the appliances, or have an electrician install a transfer switch or an interlock with a power inlet.
How far from the house should a generator be?
The CPSC says to run portable generators outdoors only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. Never run one in a garage, basement, shed, porch or carport, even with the doors open.
How long will a generator run on a 20 lb propane tank?
A 20 lb tank holds about 4.7 gallons of propane. A 5,000 watt portable at half load burns roughly 0.4 gallons an hour, so the tank lasts about 12 hours. A bigger generator or a heavier load empties it faster.
What size generator do I need for central air?
A 2-ton central air conditioner needs about 3,800 running watts and close to 5,000 extra watts to start. Added to the usual essentials, that calls for a standby unit of about 14 kW, because the starting surge is more than most portables can deliver. A soft-start kit on the AC can bring it within reach of a large portable.