Generator size calculator: what size generator do I need?
List what you want to run at the same time. Enter running watts for each, and starting watts for anything with a motor. The generator needs to cover the running total, and the running total plus the biggest single start.
Enter your measurements
More options: Headroom
- Running watts
- 2,813 W
- Starting (peak) watts
- 3,350 W
- Running watts, all together
- 2,250 W
- Biggest single starting surge (extra)
- 1,100 W
- Running current at 120 V
- 18.8 A
Running total 2,250 W × (1 + 25% headroom) = 2,813 W running
Running total 2,250 W + biggest start-up extra 1,100 W = 3,350 W → 3,350 W starting
- Start the biggest motor first, then add the other loads one at a time — the generator only has to cover one start at once.
Quick answers for common sizes
| Example | Answer |
|---|---|
| What size generator to run a refrigerator | 1,200 W starting — a 2,000 W-class generator |
| What size generator to run a well pump | 2,500 W running / 4,000 W starting |
| What size generator to run a house | 4,550 W running / 4,740 W starting |
Generator size with other headroom
| Headroom on running watts | Running watts | Starting (peak) watts |
|---|---|---|
| 0% | 2,250 W | 3,350 W |
| 10% | 2,475 W | 3,350 W |
| 25% | 2,813 W | 3,350 W |
| 50% | 3,375 W | 3,375 W |
Worked examples: what size generator to run a refrigerator, what size generator to run a well pump and what size generator to run a house. Camping? The RV generator calculator is set up for 30 A and 50 A rigs.
Running watts and starting watts
running watts needed = sum of running watts × (1 + headroom) starting watts needed = sum of running watts + the largest single (starting − running)
Generators are rated twice: running (what they can supply continuously) and starting or peak (a short burst). Champion’s 3500 W dual-fuel model, for example, is rated 3,500 running and 4,000 starting watts on gasoline. Motors and compressors need a burst to start — Champion’s chart lists a refrigerator at 150–400 W running but 800–1,200 W starting, and a 1 hp well pump at 1,000–2,000 running and 2,000–4,000 starting. Resistive loads — heaters, kettles, toasters, light bulbs — don’t need any extra.
Typical generator loads
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 150–400 | 800–1,200 |
| Chest freezer | 100–500 | 500–1,000 |
| Furnace fan, ½ hp | 300–800 | 800–1,600 |
| Sump pump, ½ hp | 800–1,050 | 1,300–2,150 |
| Well pump, 1 hp | 1,000–2,000 | 2,000–4,000 |
| Window AC, 10,000 BTU | 900–1,200 | 1,800–2,400 |
| Central AC, 3 ton | 3,000–3,500 | 5,000–6,000 |
| Electric water heater | 3,000–4,500 | no extra |
| Microwave | 600–1,200 | no extra |
| Space heater | 750–1,500 | no extra |
The full list, including RV and job-site tools, is in the appliance wattage chart.
Connecting a generator to a house — safely
- Never backfeed. CPSC warns that plugging a generator into a wall outlet to power house wiring is extremely dangerous: it can electrocute utility workers and neighbors on the same transformer and bypasses household circuit protection.
- Use a transfer switch. OSHA says never attach a generator to a building’s electrical system unless a qualified electrician has installed it with a transfer switch.
- Or run cords. Plug appliances straight into the generator with outdoor-rated, three-prong cords rated above the load.
- Outdoors only. CPSC: never run a generator inside a home, garage, crawlspace or shed, even with doors and windows open; put it outside, at least 20 feet from your home (CPSC), away from doors, windows and vents, and use battery-operated CO alarms. See the generator safety guide.
What you’ll need
- Dual-fuel portable generatorPremium price
Runs on gasoline or propane. Propane stores for years without going stale; expect somewhat fewer watts on propane.
See options on Amazon (opens in a new tab) - Inverter generator (about 2,000–3,500 W)Premium price
Quieter and more fuel-thrifty at light loads than an open-frame generator, with clean power for electronics. Many have a 30 A RV outlet.
See options on Amazon (opens in a new tab) - Battery-operated carbon monoxide alarmBudget price
CPSC advises battery or battery-backup CO alarms whenever a generator is in use. Look for UL 2034 certification and test monthly.
See options on Amazon (opens in a new tab) - Outdoor generator extension cordMid-range price
Lets the generator sit well away from the house. CPSC says to use outdoor-rated cords whose rating exceeds the total load, with all three prongs.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
What size generator do I need to run a house?
For essentials — fridge, freezer, furnace fan, sump pump, lights, TV, router and a microwave — a typical total is about 3,640 running watts with a biggest start of about 1,100 W extra, so roughly 4,550 running and 4,740 starting watts. Central air or an electric water heater adds thousands more. See what size generator to run a house.
What is the difference between starting watts and running watts?
Running watts are what an appliance draws continuously; starting watts are the brief extra a motor needs to get going. A generator must cover the running total and, for a moment, the running total plus the biggest start.
What size generator to run a refrigerator?
A fridge needs only 150–400 running watts but 800–1,200 to start (Champion). A generator with at least 1,200–1,500 starting watts runs it; a 2,000 W-class inverter generator runs it with room for lights and phones. See what size generator to run a refrigerator.
Can I plug my generator into a wall outlet?
No. CPSC calls this “backfeeding” and warns it is extremely dangerous: it can electrocute utility workers and neighbors. Use a transfer switch installed by a licensed electrician, or run cords to individual appliances.
Related guides
Sources
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.
- ENERGY STAR — Certified Televisions data set (on-mode power) (opens in a new tab) — Certified models, on-mode watts, retrieved October 2026: 32 in about 27–36 W; 43 in about 45–78 W; 50 in about 61–90 W; 55 in about 64–114 W; 65 in about 85–135 W; 75 in about 120–176 W.
- U.S. Consumer Product Safety Commission — Portable Generator Hazards (Safety Alert 5123, 2021) (opens in a new tab) — Never run a generator indoors or in a garage, even with doors open; place it outside far from doors, windows and vents; CO alarms; never “backfeed” by plugging into a wall outlet.
- U.S. Consumer Product Safety Commission — CPSC Warns Consumers in Path of Storms to Take Precautions When Using Portable Generators (July 24, 2020) (opens in a new tab) — “Use your generator OUTSIDE your house ONLY, at least 20 feet from your home”; make sure you have working CO alarms.
- OSHA Fact Sheet — Using Portable Generators Safely (opens in a new tab) — Never attach a generator to a building’s wiring unless a qualified electrician has installed a transfer switch; 3–4 ft of clear space around the unit.
- Champion Power Equipment — 3500-Watt Dual Fuel Generator (model 201213) specifications (opens in a new tab) — 3,500 running / 4,000 starting watts on gasoline; 14 h at 50% load on 4.7 gal; 10.5 h at 50% load on a 20 lb propane tank; 120 V 30 A TT-30R RV outlet.
- Champion Power Equipment — 6500-Watt Dual Fuel Generator (model 201077) specifications (opens in a new tab) — 6,500 running / 8,125 starting watts on gasoline (5,850 / 7,325 on propane); 11 h at 50% load on 7.7 gal; 6.5 h at 50% load on a 20 lb propane tank.
- Champion Power Equipment — 2000-Watt Dual Fuel Inverter (model 201230) specifications (opens in a new tab) — 1,700 running / 2,000 starting watts on gasoline (1,530 / 2,000 on propane); 11.5 h at 25% load on 1.05 gal; 34 h at 25% load on a 20 lb propane tank.