What Size Generator Do I Need?
Wattage chart and rules last checked September 30, 2026.
A home-essentials list (refrigerator, sump pump, furnace fan, 10 LED bulbs, 2 chargers and a router) needs a generator rated for about 3,970 W, using the high end of each wattage range.
Size to about3,970 Wpeak need at the high end of each range1. What do you need to run?
Enter how many of each. Numbers come from Champion Power Equipment's wattage chart and are ranges, because models differ. Your appliance's nameplate beats any chart.
Home and backup power
| Appliance | Running watts | Starting watts | Qty |
|---|---|---|---|
| 150 to 400 | 800 to 1,200 | ||
| 100 to 500 | 500 to 1,000 | ||
| 450 to 600 | 900 to 1,200 | ||
| 900 to 1,200 | 1,800 to 2,400 | ||
| 2,000 to 2,500 | 3,500 to 4,500 | ||
| 3,000 to 3,500 | 5,000 to 6,000 | ||
| 4,000 to 5,000 | 6,500 to 8,000 | ||
| 3,000 to 4,500 | none listed | ||
| 300 to 800 | 800 to 1,600 | ||
| 500 to 800 | 1,000 to 1,600 | ||
| 800 to 1,050 | 1,300 to 2,150 | ||
| 750 to 1,000 | 1,500 to 2,000 | ||
| 1,000 to 2,000 | 2,000 to 4,000 | ||
| 500 to 1,000 | 1,000 to 2,300 | ||
| 4,000 to 6,000 | none listed | ||
| 1,200 to 2,400 | none listed | ||
| 600 to 1,200 | none listed | ||
| 1,200 to 2,400 | none listed | ||
| 750 to 1,500 | none listed | ||
| 50 to 200 | none listed | ||
| 15 to 75 | none listed | ||
| 80 to 400 | none listed | ||
| 8 to 15 | none listed | ||
| 20 to 100 | none listed | ||
| 30 to 60 | none listed | ||
| 60 to 120 | none listed | ||
| 15 to 40 | none listed | ||
| 10 to 20 | none listed |
Job site
| Appliance | Running watts | Starting watts | Qty |
|---|---|---|---|
| 1,200 to 1,800 | 2,400 to 3,600 | ||
| 1,800 to 3,000 | 3,600 to 6,000 | ||
| 1,200 to 1,800 | 2,400 to 3,600 | ||
| 960 to 1,200 | 1,920 to 2,400 | ||
| 900 to 1,800 | 1,800 to 3,600 | ||
| 400 to 600 | 800 to 1,200 | ||
| 900 to 1,200 | 1,800 to 2,400 | ||
| 800 to 1,200 | 1,600 to 2,400 | ||
| 1,500 | 4,500 | ||
| 2,500 | 7,500 | ||
| 3,500 | 10,500 | ||
| 1,200 to 1,500 | 2,400 to 3,000 | ||
| 2,000 to 2,500 | 4,000 to 5,000 | ||
| 15 to 25 | none listed | ||
| 500 | none listed | ||
| 1,500 to 2,000 | none listed | ||
| 1,800 to 2,500 | 3,600 to 5,000 | ||
| 3,000 to 8,000 | 6,000 to 16,000 | ||
| 1,500 to 3,000 | none listed |
Outdoors and recreation
| Appliance | Running watts | Starting watts | Qty |
|---|---|---|---|
| 1,500 to 2,000 | 2,800 to 3,500 | ||
| 2,000 to 2,500 | 3,500 to 4,500 | ||
| 600 to 1,200 | none listed | ||
| 150 to 600 | 400 to 1,000 | ||
| 1,200 to 1,500 | none listed | ||
| 1,650 | none listed | ||
| 1,200 to 1,800 | none listed | ||
| 800 to 1,200 | none listed | ||
| 50 to 100 | 400 to 600 | ||
| 50 to 200 | none listed | ||
| 50 to 200 | none listed | ||
| 50 to 150 | none listed | ||
| 20 to 100 | none listed | ||
| 30 to 60 | none listed | ||
| 25 to 100 | none listed | ||
| 300 to 600 | 600 to 1,200 | ||
| 100 to 300 | none listed |
Something not listed? Enter its nameplate watts
If the label lists only volts and amps, multiply them: volts x amps = watts (Champion).
2. How the total adds up
- Running watts, all items added1,080 W to 2,370 W
- Highest single starting wattage1,000 W to 1,600 W
- Peak need (running + highest starting)2,080 W to 3,970 W
The high end of the range is the safer number to shop with, and Champion advises buying above the calculated figure by a margin.
Champion says propane delivers about 10% less wattage than gasoline and natural gas about 10 to 15% less. If a dual-fuel spec sheet gives a rating for your fuel, use that rating instead of this conversion.
Champion's size tier: 3,000 to 7,000 W. Typical loads: Refrigerator, freezer, window AC, TV. Best for: home essentials during outages.
Connecting to the house? Champion says a portable generator should never be connected to a home's electrical system without a properly installed transfer switch, and to consult a qualified electrician. Otherwise plug appliances into the generator with heavy-duty, outdoor-rated extension cords. Always run it outdoors, at least 20 feet from windows, doors and attached garages (Ready.gov).
3. Fuel runtime (optional)
Runtime depends on the exact generator, so this uses numbers from its spec sheet. Fill in what you have.
Enter the tank size, the 50% runtime and the generator's running watts to see fuel use.
Shopping in November? How to tell a real generator deal from an inflated one.
How we got this
- Running watts for each item (low and high figures from Champion's chart), times how many: Refrigerator: 150 to 400 W; Sump pump (1/3 HP): 500 to 800 W; Furnace fan (1/2 HP): 300 to 800 W; LED lights (per bulb): 10 × 8 to 15 W = 80 to 150 W; Phone/laptop charger: 2 × 20 to 100 W = 40 to 200 W; Internet router/modem: 10 to 20 W (Champion wattage chart)
- Add every running watt, low end with low end and high end with high end: 150 to 400 W + 500 to 800 W + 300 to 800 W + 80 to 150 W + 40 to 200 W + 10 to 20 W = 1,080 to 2,370 W (Champion sizing guide)
- Find the single highest starting wattage on the list. Only that one is added, not every item's: Sump pump (1/3 HP) starting watts: 1,000 to 1,600 W (Champion sizing guide, Champion wattage chart)
- Running watts plus that starting wattage gives the peak need: 1,080 to 2,370 W + 1,000 to 1,600 W = 2,080 to 3,970 W (Champion sizing guide)
- On gasoline there is no output cut, so the size to shop for is the high end of the peak need: 3,970 W
- 3,970 W falls in the 3,000 to 7,000 W row of Champion's generator sizes table. (Champion sizing guide)
The idea behind it: Starting watts last only a fraction of a second, so Champion's method adds every running watt but only the single highest starting wattage on top. Its chart gives a low and a high figure for each appliance, and this calculator sizes to the high end; Champion also advises choosing a generator above the calculated wattage by a safe margin.
Pick the appliances you want to run in an outage and get running watts, starting watts and a size to shop for, for gasoline, propane or natural gas. Then estimate fuel use from your generator's spec sheet.
How this is calculated
The method is the one Champion Power Equipment, a generator manufacturer, publishes. Add the running watts of everything you want on at once. Find the single highest starting wattage among those items. Add that starting wattage to the running total. That sum is the peak your generator has to survive, and the generator's rated running watts have to cover the running total on its own.
Champion's chart gives a range for every appliance, because a 15-year-old refrigerator and a new one do not draw the same. The calculator carries the range through, so you see a low and a high peak. Shop with the high one unless you have read the nameplates. The U.S. Department of Energy notes that the wattage stamped on an appliance is its maximum power draw, and that many appliances run below it depending on the setting.
Champion's steps add the highest starting wattage in full, and so does the calculator. Champion also advises buying above the calculated figure by a safe margin, so treat the result as a number to size up to, not a target to hit exactly.
A worked example: a short essentials list
A refrigerator, a 1/3 hp sump pump, a furnace fan, ten LED bulbs, two chargers and a router. Every figure comes from the chart and is added by the same code the calculator uses.
| Item | Qty | Running watts (each) | Starting watts (each) |
|---|---|---|---|
| Refrigerator | 1 | 150 to 400 | 800 to 1,200 |
| Sump pump (1/3 HP) | 1 | 500 to 800 | 1,000 to 1,600 |
| Furnace fan (1/2 HP) | 1 | 300 to 800 | 800 to 1,600 |
| LED lights (per bulb) | 10 | 8 to 15 | none listed |
| Phone/laptop charger | 2 | 20 to 100 | none listed |
| Internet router/modem | 1 | 10 to 20 | none listed |
| Running total | 1,080 W to 2,370 W | ||
| Highest single starting wattage | 1,000 W to 1,600 W | ||
| Peak need | 2,080 W to 3,970 W | ||
The high end, 3,970 W, sits comfortably inside Champion's 3,000 to 7,000-watt "home essentials" tier. Add a window air conditioner or a well pump and the total climbs quickly, because each brings its own starting surge. That is the reason to enter the real list instead of picking a size from a headline.
What this estimate leaves out
- Your exact appliances: the chart gives a range per appliance, so only the nameplates show the true figures. The custom row takes nameplate watts for anything not listed.
- Several motors starting at the same moment: like Champion's method, it adds only the single highest starting wattage.
- Fuel burn at your own load and conditions: runtime comes from the spec-sheet numbers you enter, plus our stated modelling choices.
- Electrical work: a transfer switch, a standby installation and any permit are for a licensed electrician or dealer, and a load calculation replaces this estimate.
Size tiers
Champion's own grouping of what each size class typically handles.
| Size | Typical loads | Best for |
|---|---|---|
| 1,000 to 3,000 W | Lights, phone chargers, small appliances | Camping, tailgating, short outages |
| 3,000 to 7,000 W | Refrigerator, freezer, window AC, TV | Home essentials during outages |
| 7,000 to 12,500 W | Central AC units, electric water heater, well pump, sump pump | Larger homes, job sites, extended outages |
| 12,500 to 22,000 W | Whole-home coverage, multiple large appliances at once such as compressors and air conditioning | Whole-home portable backup, large job sites |
| 8,500 to 26,000 W, home standby | Permanently installed, starts automatically through an automatic transfer switch | Whole-home backup, installed by a qualified installer |
Fuel: propane and natural gas put out less
Champion says propane typically delivers about 10% less wattage than gasoline and natural gas about 10 to 15% less. A generator rated 7,500 running watts on gasoline is therefore a smaller machine on propane. The calculator converts your need to a gasoline-basis rating so you can compare it with the big number on the box, but if the spec sheet publishes a rating for your fuel, that is better evidence than our conversion.
Fuel runtime, and what we assume
A manufacturer's spec sheet usually lists a runtime at 50% load, sometimes also at full load. Fuel use does not fall in proportion to load, so at or below 50% we keep the 50% burn rate from the spec sheet, which gives a conservative, shorter runtime. Between 50% and full load we interpolate between the two spec points, or, if you only have the 50% figure, scale burn in proportion to load, which overstates fuel use. Both are our own modelling choices and the calculator labels which one it used. The load it uses is the high end of your running watts, not the starting surge, because the surge lasts a fraction of a second.
Portable or standby, and where an electrician comes in
A portable generator you plug appliances into needs no electrician. The moment you want it to feed the house wiring, Champion says a properly installed transfer switch is required, and that is licensed-electrician work under your local electrical code. A standby generator is a different purchase: Champion says its units are installed by a qualified dealer or installer, usually need a permit, and are sized by square footage, the circuits or appliances you want covered, and whether you want everything or only essentials. If your total lands above about 12,500 watts, or you want it to start without you, that is the conversation to have with a standby installer. Nothing here replaces an electrician's load calculation.
Safety, in the sources' words
- Use generators outdoors only, and keep them and their fuel at least 20 feet from windows, doors and attached garages (Ready.gov).
- Install working carbon monoxide detectors on every level of the home. Carbon monoxide is colorless and odorless (Ready.gov).
- Never run a portable generator in a shed, garage or home, even with fans or open doors (Champion).
- Keep the generator dry. Touching a wet generator or devices connected to it can cause electric shock (Ready.gov).
- Let it cool before refueling, and follow the manufacturer's instructions (Ready.gov).
Keep going
- Black Friday generator deals: how to tell a real discount from an inflated "was" price, after you know your size.
- Hot tub running cost calculator: another load worth pricing before you buy.
Questions people ask
What size generator do I need for my house?
Champion Power Equipment says most homes need a 5,000 to 12,500-watt generator to keep essential appliances running in an outage. For a 2,000 square foot house, it says 5,000 to 7,500 watts covers essentials such as the refrigerator, lights, outlets and a window air conditioner, and that running central air takes 10,000 watts or more. Your own number depends on what you list, which is what the calculator above adds up.
What is the difference between starting watts and running watts?
Champion defines running watts as the continuous power a device needs once it is running, and starting watts as the extra burst that motor-driven items such as refrigerators and air conditioners need to start. Champion says starting watts are typically 1 to 2 times the running wattage and last a fraction of a second. Simple loads such as light bulbs and laptops only need running watts.
How do I add up the wattage?
Champion's five steps: list everything you want to power, find the running watts for each (from the label, or volts times amps if only those are listed), add all the running watts together, find the highest starting wattage among the items, and add that starting wattage to the running total. The result is your peak need. The calculator follows those steps and shows a low and a high figure because the chart gives a range for each item.
Will a 3,500-watt generator run a refrigerator?
Champion says yes. It lists a refrigerator at 150 to 400 running watts and 800 to 1,200 starting watts, which leaves room on a 3,500-watt unit for lights and small devices. The catch is everything else you add: a sump pump, furnace fan or window air conditioner each brings its own running and starting watts to the total.
Does running on propane or natural gas change the size I need?
Yes. Champion says propane typically delivers about 10% less wattage than gasoline, and natural gas about 10 to 15% less, and tells buyers to factor that into sizing. The calculator does the arithmetic for you: if a generator's rating is on a gasoline basis, it divides your need by 0.90 for propane and by 0.90 to 0.85 for natural gas. If a dual-fuel spec sheet lists a rating for your fuel, use that one.
Do I need a transfer switch?
Champion says yes if you are connecting a portable generator to your home's electrical system: a manual transfer switch installed by a licensed electrician routes generator power to the circuits you choose and prevents backfeeding, where power flows back onto utility lines and endangers utility workers. Without one, Champion's guidance is to plug appliances into the generator with heavy-duty, outdoor-rated extension cords. Follow local electrical codes and ask a qualified electrician.
Where can I safely run a generator?
Outdoors only. Ready.gov says generators and fuel should be used outdoors and at least 20 feet from windows, doors and attached garages, and recommends working carbon monoxide detectors on every level of the home. Champion says there is no safe reason to run a portable generator in a shed, garage or home, even with fans or open doors, because carbon monoxide can build up to deadly levels within minutes, and not to run one under a canopy-style cover unless the manufacturer gives instructions for it.
How long will a generator run on one tank?
It depends on the model, so the calculator asks for the tank size and the runtimes from the spec sheet rather than guessing. Champion says a generator can run as long as it has fuel, stopping only for refueling and cooling, and to follow the maintenance intervals in the manual during a long outage. Home standby generators on a natural gas or propane line do not have a tank to refill, so the limit is your fuel supply.
What is the difference between a portable and a home standby generator?
Champion describes portable generators as manually operated and set up by you, on gasoline or alternate fuels, and home standby generators as permanently installed, running on propane or natural gas, and starting automatically within seconds of an outage through an automatic transfer switch. Champion lists standby sizes from 8,500 to 26,000 watts, says they must be installed by a qualified installer and usually need a permit, and says the right size depends on square footage, which circuits you want backed up, and whether you want the whole home or just essentials.
How long does food last in a power outage?
Ready.gov says a closed refrigerator keeps food cold for about four hours and a full freezer for about 48 hours, and to throw out food that has been at 40 degrees or higher for two hours or more. That is a reason the refrigerator and freezer usually lead a backup list.
Should I buy bigger than the number I calculate?
Champion advises choosing a generator that exceeds the calculated wattage by a safe margin to cover surges and things you add later. The calculator's headline figure already uses the high end of each range plus the highest starting wattage, so it is on the cautious side. Your appliance nameplates, and an electrician for anything wired to the house, are the final word.
Sources and references
- Champion Power Equipment: Generator Wattage Chart, running and starting watts by appliance (checked Sep 30, 2026)
- Champion Power Equipment: What Size Generator Do I Need? Method, fuel output, size tiers (April 24, 2026; checked Sep 30, 2026)
- Champion Power Equipment: FAQ, transfer switches, backfeeding, standby generators (checked Sep 30, 2026)
- Champion Power Equipment: Generator Safety (checked Sep 30, 2026)
- Ready.gov (FEMA): Power Outages, generator safety and food storage (checked Sep 30, 2026)
- U.S. Department of Energy: Estimating Appliance and Home Electronic Energy Use, nameplate wattage (archived copy, checked Sep 30, 2026)
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