Generator Wattage Calculator
Add your appliances → get the running watts, the honest surge number, and the generator size that actually covers both.
1. Pick your appliances
Wattages are typical values from generator-manufacturer sizing worksheets — always verify on your appliance’s nameplate. Tick what you’ll run at the same time.
Add a custom appliance
Starting watts ≥ running watts. If the label only lists amps: watts ≈ volts × amps.
2. Headroom
20–25% is the industry rule of thumb: generators shouldn’t run at 100% load for hours, and loads grow over time.
Your generator size
- Total running watts
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- Required surge watts
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- Surge driver
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- Running needed (with headroom)
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Fuel runtime estimator
How long one tank lasts at your load. Efficiency default (5 kWh per gallon) is typical for a portable gas generator near rated load — inverter units often do better, light loads do worse. Check your manual.
- Fuel burn
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- Cost per hour
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- Cost per tank
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Why surge watts are not added up
Motors almost never start at the same instant. The honest math used above:
surge needed = total running watts + the single biggest (starting − running) gap
Worked example. Refrigerator (700 W run / 2,200 W start) + microwave (1,000 W) + 100 W of lights: total running = 1,800 W; the biggest gap is the fridge’s 1,500 W; surge needed = 1,800 + 1,500 = 3,300 W. With 20% headroom you need 2,160 W of continuous capacity → a 3,500 W peak / 2,800 W running generator covers it. Simply summing every starting watt (2,200 + 1,000 + 100 = 3,300 W here) looks the same in this small case, but with two motors it over-counts badly: add a well pump (1,000 W run / 3,200 W start) and naive summing says 5,500 W of surge while the honest number is 4,000 W — you’d buy a generator a full size too big.
This matches the published Generac/Norwall sizing method. Spot-check: their worked example (sump + well pump + furnace + toaster + fridge + hair dryer) totals 5,850 W running and 8,000 W starting — exactly what this calculator produces from the same inputs.
Fuel types at a glance
| Fuel | Pros | Cons |
|---|---|---|
| Gasoline | Most power per dollar; available everywhere | Goes stale in months — use stabilizer |
| Propane | Stores indefinitely; cleaner burning | Typically ≈10% less power than gasoline |
| Dual-fuel | Flexibility to switch fuels | Costs more; still derates on propane |
| Diesel | Most efficient; long engine life | Louder, heavier, pricier units |
Noise: inverter vs conventional
Open-frame conventional generators typically run 68–78 dBA — about as loud as a vacuum cleaner. Inverter generators throttle the engine to the load and typically run 48–60 dBA (normal conversation is ≈60 dBA), plus they output a clean sine wave with low THD that is safer for laptops, TVs and other sensitive electronics. Typical figures at quarter load, measured 23 ft away — check the spec sheet.
Safety — non-negotiable
- Carbon monoxide kills. Run outdoors ONLY, at least 20 feet from the house with exhaust pointed away. Never in a garage, basement or near windows — not even with the door open.
- Never backfeed a wall outlet to power your house. It can electrocute utility workers and destroy the generator. Use a transfer switch installed by an electrician.
- Let the generator cool before refueling. Store fuel in approved containers, away from living spaces.
- Generators lose roughly 3–3.5% of rated power per 1,000 ft of altitude (typical) — at 5,000 ft that is about 15% less.
Scope boundary
This tool sizes combustion generators from running vs starting watts. For related jobs:
- Watts to VA calculator — UPS sizing with power-factor math (apparent vs real power).
- Wattage calculator — P=VI solver, PC PSU sizer and appliance energy cost.
Frequently asked questions
What size generator do I need for my home?
Add up the running watts of everything you will power at the same time, then add the single biggest motor-starting surge (this calculator does both). As a rough guide: essentials like a fridge, lights, phone chargers and a laptop total about 1,500 to 2,500 W running, which a 3,500 W peak generator handles. Partial-home backup with a well pump or small window AC usually lands at 5,000 to 7,500 W. Whole-home backup needs a standby unit of 10,000 W or more. Always size from your actual appliances, not rules of thumb.
What is the difference between running watts and starting watts?
Running (rated) watts is the power an appliance draws continuously. Starting (surge or peak) watts is the brief burst — lasting only a few seconds — that electric motors need to get going, often 2 to 3 times the running draw. A refrigerator rated 700 W running can need 2,200 W to start its compressor. Resistive loads like toasters and space heaters have no surge: starting equals running.
Should I add up the starting watts of all my appliances?
No — that is the most common sizing mistake and it makes you buy a generator a size too big. Motors almost never start at the same instant, so surge watts are not summed. The honest math is: total running watts plus only the single biggest (starting minus running) gap. Example: a fridge (700 W run / 2,200 W start) plus a well pump (1,000 W run / 3,200 W start) plus 100 W of lights gives 1,800 W running and 4,000 W of surge — not the 5,500 W you get from summing every starting watt.
How much headroom (safety margin) should I add?
Add 20 to 25 percent on top of your total running watts — 20 percent is the default in this calculator. Generators should not run at 100 percent load for hours on end; aim to stay under about 80 to 85 percent of the rated running watts. Headroom also covers future appliances and the power loss you get at altitude.
Can a 3,500-watt generator run a refrigerator and a microwave?
Usually yes. A typical fridge (700 W running / 2,200 W starting), a 1,000 W microwave and 100 W of lights total 1,800 W running with a 3,300 W surge. With 20 percent headroom you need 2,160 W of continuous capacity, which fits a 3,500 W peak / 2,800 W running generator. Just avoid starting the microwave at the exact moment the fridge compressor kicks in.
How long will my generator run on a tank of fuel?
Divide the energy in the tank by your load: hours = tank gallons times kWh per gallon, divided by load in kW. Example: a 6-gallon tank at 5 kWh per gallon holds 30 kWh; powering a 5,000 W load that is 30 / 5 = 6 hours. At $3.50 per gallon the fuel costs $3.50 per hour, or $21 per tank. Real efficiency varies — check your manual and use the fuel tab above with your numbers.
Gasoline vs propane vs dual-fuel — which generator fuel is better?
Gasoline gives the most power per dollar and is available everywhere, but it goes stale within months unless you add stabilizer. Propane stores indefinitely and burns cleaner, but generators typically make about 10 percent less power on propane than on gasoline. Dual-fuel units let you switch between the two for flexibility. Diesel is the most efficient with the longest engine life, but the units are louder, heavier and pricier.
How loud are generators? What is an inverter generator?
Open-frame conventional generators typically run 68 to 78 dBA — about as loud as a vacuum cleaner. Inverter generators throttle the engine to match the load and typically run 48 to 60 dBA (normal conversation is about 60 dBA). Inverters also output a clean sine wave with low total harmonic distortion, which is safer for laptops, TVs and other sensitive electronics. Figures are typical at quarter load measured 23 feet away.
Can I plug my generator into a wall outlet to power my house?
No — never backfeed a wall outlet. It can energize power lines and electrocute utility workers, and it can destroy your generator when grid power returns. The safe way to power house circuits is a transfer switch installed by a licensed electrician.
Where should I place a portable generator?
Outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors and windows. Never run one in a garage, basement, shed or near open windows — not even with the door open. Carbon monoxide is odorless and can kill within minutes, and it can seep through walls and doorways.
Does altitude affect generator output?
Yes. Engines lose roughly 3 to 3.5 percent of rated power per 1,000 feet of elevation above about 500 feet — at 5,000 feet that is around 15 percent less power. If you live at altitude, size up accordingly and check your manual for the exact derate.
How do I find an appliance's running watts?
Check the nameplate label — usually on the back, bottom or inside the door — which lists watts directly. If it only lists amps, multiply: watts = volts x amps (for example, 120 V x 8.3 A is about 1,000 W). The wattages in this tool's database are typical values from manufacturer sizing worksheets; your exact model may differ, so the nameplate wins.
Can I run sensitive electronics like laptops and TVs on a generator?
Yes, preferably on an inverter generator, which produces a clean sine wave with low distortion (under about 3 percent THD). Conventional open-frame generators can have high harmonic distortion that stresses power supplies and chargers over time.
How often should I service my generator?
Typical guidance: change the oil after the first 20 to 25 hours (break-in), then every 50 to 100 hours of use or at least once a year. Run the generator 15 to 30 minutes monthly under a light load, use fuel stabilizer, and check the air filter and spark plug yearly. Your manual's schedule wins over any generic advice.
What is the difference between a generator, a UPS and a portable power station?
A generator (what this tool sizes) burns fuel and runs for hours, with a big surge capacity for motors. A UPS is a battery box that keeps equipment alive for minutes through blips and brownouts — size one with our watts-to-va calculator, which handles power-factor math. A portable power station is a large silent battery: great for electronics, but its surge capacity is far smaller than a combustion generator's.
Why does the calculator recommend a bigger generator than my total running watts?
Two reasons. First, the heaviest motor's starting surge: a generator must survive the few seconds when, say, a well pump draws 3,200 W even though it only runs at 1,000 W. Second, headroom: the recommendation adds 20 percent (adjustable) so you are not running flat-out for hours. The picked size is the smallest standard tier covering both numbers.
Free tool by NRTechStudio