What Size Generator Do I Need? Wattage Chart + Examples
To run a refrigerator, a TV, a computer and a few lights you need a generator of at least 3,000 watts. Add a 12,000 BTU window air conditioner and you need about 7,000 watts. What sets the size is not what your appliances draw while running but what they need to start: a refrigerator runs on about 750 watts, but its motor can pull three times that for a few seconds. The chart below lets you do your own math.
How to calculate what size generator you need
Michigan utility DTE Energy breaks it into three steps:
- List the appliances you want running at the same time.
- Multiply each one’s running watts by its starting factor (chart below). Anything with a motor or compressor — a refrigerator, a well or sump pump — has a factor of 3: it needs three times its running watts to start.
- Add it all up. Your generator must be rated for that total or more.
If the label only lists amps: watts = volts × amps. A refrigerator rated at 6 amps on 120 volts draws about 720 watts.
Generator wattage chart by appliance
DTE Energy’s averages. Your appliance may draw more or less — the exact figure is on its label.
| Appliance | Running (W) | Factor | To start (W) |
|---|---|---|---|
| Refrigerator/freezer | 750 | 3 | 2,250 |
| Sump pump, 1/2 HP | 1,000 | 3 | 3,000 |
| Well pump, 3/4 HP | 1,500 | 3 | 4,500 |
| Furnace fan, 1/2 HP | 600 | 3 | 1,800 |
| Washing machine (no hot water) | 1,000 | 2 | 2,000 |
| Microwave | 600–1,500 | 1 | 600–1,500 |
| Coffee maker | 750–1,200 | 1 | 750–1,200 |
| Space heater | 500–1,500 | 1 | 500–1,500 |
| TV | 150–400 | 1 | 150–400 |
| Computer | 200 | 1 | 200 |
| Light bulb | 40–150 | 1 | 40–150 |
| Water heater (50 gal) | 4,500–5,000 | 1 | 4,500–5,000 |
| Central A/C, 3 ton | 6,000 | 3 | 18,000 |
The bulbs in DTE’s chart are incandescent; LEDs use far less (check the wattage printed on them).
For a window AC: a 12,000 BTU unit draws 1,000 to 1,500 watts while running (how that’s calculated). DTE gives central air a starting factor of 3; applying the same factor at 1,200 running watts plan on about 3,600 watts to start. The exact number is in the manufacturer’s specs.
Examples: what size generator for my house?
The essentials (fridge, TV, computer, five bulbs):
- Refrigerator: 750 × 3 = 2,250 W
- TV: 300 W
- Computer: 200 W
- Five 60 W bulbs: 300 W
- Total: 3,050 W → a 3,500-watt generator gives you some headroom.
Add a 12,000 BTU window AC: about 3,600 W more → 6,650 W → a generator of about 7,000 watts.
Add a 1/2 HP sump pump instead: 3,050 + 3,000 = 6,050 W.
A small 1,000–2,000 watt generator handles a TV, computer, router, lights and chargers, but by DTE’s numbers it can’t start an average refrigerator (2,250 W).
Central air needs about 18,000 watts to start by DTE’s chart — a portable generator won’t run it. DTE warns customers not to turn on central A/C or a heat pump on its portable-generator hookup.
What will a 12,000-watt generator run?
By DTE’s chart, all of this at once: refrigerator (2,250 W), 12,000 BTU window AC (3,600 W), washing machine (2,000 W), microwave (1,500 W), TV (300 W) and lights (300 W). That’s 9,950 W, with about 2,000 W to spare. Don’t add a 50-gallon electric water heater (4,500–5,000 W) on top of all that.
How to run a generator safely
- Outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors and windows. Never in a garage, basement, crawlspace, shed or on a porch, even with doors open: carbon monoxide is colorless and odorless and can knock you out before you notice symptoms, the CPSC warns. It estimates about 85 people die each year in the U.S. from CO poisoning caused by portable generators.
- Install battery-backed CO alarms on every level of your home (CPSC, Ready.gov).
- Never connect it to your home’s wiring without a transfer switch installed by a qualified electrician. Backfeeding can energize lines for great distances and electrocute utility workers, OSHA says.
- Plug appliances directly into the generator with heavy-duty, grounded (three-prong) extension cords that are not frayed or cut (OSHA).
- Don’t overload it; it can overheat and start a fire (OSHA).
- Let it cool before refueling — fuel spilled on a hot engine can ignite (Ready.gov).
- Keep it dry. Touching a wet generator can shock you (Ready.gov).
What this page can’t tell you
- Your appliances’ exact draw. The chart shows averages; each label or manual gives the real figure.
- How much fuel your generator will burn. It depends on the model and the load — check the spec sheet.