The short answer
For a furnace blower fan, refrigerator, chest freezer, tv, wi-fi router, lights, and sump pump, you need a 3,500 W portable generator: 1,740 running watts, 2,940 starting watts, and 10% headroom on both. Add central air conditioning and you need a 12,000 W portable.
Running and starting watts
Typical running and starting watts for each appliance. Starting watts is the short surge when a motor or compressor kicks on; heating and electronics start at their running watts.
| Appliance | Running | Starting |
|---|---|---|
| Heating & cooling | ||
| Space heater | 1,500 W | 1,500 W |
| Window air conditioner | 900 W | 2,200 W |
| Portable air conditioner | 1,200 W | 3,000 W |
| Central air conditioner (3 ton) | 3,500 W | 9,500 W |
| Ceiling fan | 50 W | 100 W |
| Box or window fan | 100 W | 200 W |
| Dehumidifier | 500 W | 900 W |
| Electric blanket | 100 W | 100 W |
| Furnace blower fan | 500 W | 1,500 W |
| Kitchen | ||
| Refrigerator | 150 W | 1,200 W |
| Chest freezer | 80 W | 1,000 W |
| Microwave | 1,100 W | 1,100 W |
| Electric oven | 2,400 W | 2,400 W |
| Electric stove burner | 1,500 W | 1,500 W |
| Induction cooktop | 1,800 W | 1,800 W |
| Air fryer | 1,500 W | 1,500 W |
| Slow cooker | 200 W | 200 W |
| Electric pressure cooker | 1,000 W | 1,000 W |
| Coffee maker | 1,000 W | 1,000 W |
| Electric kettle | 1,500 W | 1,500 W |
| Toaster | 1,100 W | 1,100 W |
| Toaster oven | 1,200 W | 1,200 W |
| Dishwasher | 1,200 W | 1,500 W |
| Laundry & cleaning | ||
| Clothes washer | 500 W | 1,200 W |
| Electric clothes dryer | 3,000 W | 3,000 W |
| Clothes iron | 1,400 W | 1,400 W |
| Vacuum cleaner | 1,000 W | 1,400 W |
| Hair dryer | 1,500 W | 1,500 W |
| Electronics | ||
| TV (55 inch LED) | 100 W | 100 W |
| Game console | 150 W | 150 W |
| Desktop computer | 150 W | 150 W |
| Laptop | 50 W | 50 W |
| Computer monitor | 30 W | 30 W |
| Wi-Fi router | 10 W | 10 W |
| Lighting | ||
| LED bulb (60 W equivalent) | 9 W | 9 W |
| Incandescent bulb | 60 W | 60 W |
| Lights (about 10 LED bulbs) | 100 W | 100 W |
| Water & outdoor | ||
| Electric water heater | 4,500 W | 4,500 W |
| Pool pump | 1,500 W | 3,000 W |
| Hot tub | 4,000 W | 4,000 W |
| Well pump | 750 W | 3,000 W |
| Sump pump (1/3 to 1/2 hp) | 800 W | 2,000 W |
| Garage door opener | 550 W | 1,100 W |
Source: U.S. Department of Energy Energy Saver and generator wattage guides from Honda, Generac and Champion. Check your appliance's label for its real watts.
How it works
The formula
We add up what your appliances draw while running, add the biggest motor start, give both some headroom, then pick the smallest standard generator that covers them.
starting = running + the largest (starting − running) of any one appliance
need = running × 1.1 and starting × 1.1
portable = smallest of 2,200, 3,500, 5,000, 7,500, 9,500, 12,000 W with running ≥ need and 1.25 × running ≥ starting need
standby = smallest of 10, 14, 18, 22, 26 kW at 90% of its rating
Assumptions
- Running watts are typical figures from the U.S. Department of Energy Energy Saver list; starting watts come from the wattage guides of generator makers such as Honda, Generac and Champion. Your appliances may differ, so check the labels.
- Only one motor starts at a time, so we add just the single biggest start, not all of them.
- We add 10% headroom to both totals, so the generator isn't run at its limit. Running a generator near its limit shortens its life.
- A portable generator's surge rating is about 1.25 × its running rating. We check that too.
- Home standby generators on propane or natural gas make about 10% less than their nameplate (which is for gasoline-like fuel on some models), so we count 90%.
- Soft starts and variable-speed motors cut an air conditioner's starting watts by half or more, which can drop you a size.
- Never plug a generator into a wall outlet. Feed the house through a transfer switch or interlock installed by an electrician, or use extension cords. Run the generator outdoors, at least 20 ft from windows, doors and vents, and use CO alarms indoors.
Worked example: the outage defaults
Furnace blower fan, refrigerator, chest freezer, tv, wi-fi router, lights, and sump pump.
- Running: 500 + 150 + 80 + 100 + 10 + 100 + 800 = 1,740 W.
- Biggest start: sump pump starts at 2,000 W and runs at 800 W, so it adds 1,200 W.
- Starting: 1,740 + 1,200 = 2,940 W.
- Headroom: 1,740 × 1.1 = 1,914 W running; 2,940 × 1.1 = 3,234 W starting.
- Portable sizes: 2,200 W surges to 2,750 W, and 3,500 W surges to 4,375 W. The first one that covers both is 3,500 W.
Too big or too small
- Too big: wasted fuel and money. A generator running at a small share of its rating burns fuel poorly and costs more to buy.
- Too big: wet stacking. A diesel or large engine run lightly for hours can build up unburned fuel and soot.
- Too small: it overloads. The breaker trips or the engine bogs down, and motors can stall when they start.
- Too small: damage. Running at the limit for hours overheats the engine and the alternator, and a brownout can harm electronics and motors.
- Between sizes? Take the bigger one, or run fewer things at once. A soft start on the air conditioner often lets you drop a size.
Questions people ask
What size generator do I need to run a house?
It depends on what you run at once. Just the essentials (furnace blower fan, refrigerator, chest freezer, tv, wi-fi router, lights, and sump pump) needs a 3,500 W portable. Add central air conditioning and it needs a 12,000 W portable. Add a well pump, electric water heater, oven, dryer and the rest of the kitchen and laundry, and it needs a 26 kW home standby.
What can a 5,000 watt generator run?
About 4,545 watts of running load with 10% headroom, plus a motor starting on top: its surge rating is about 6,250 W. That covers a refrigerator, freezer, lights, a sump pump, a furnace blower and electronics. It does not start a central air conditioner without a soft start.
What is the difference between running and starting watts?
Running watts is what an appliance draws once it is going. Starting (surge) watts is the short burst a motor or compressor pulls to get going, often 2 to 4 times more. A refrigerator runs on 150 W but needs about 1,200 W to start. Size the generator for the running total plus the single biggest start, because motors rarely start at the same moment.
Can I plug a generator into a wall outlet?
No. Plugging a generator into a house outlet (backfeeding) can send power out to the utility lines and kill a lineworker, and it can start a fire in your wiring. Use extension cords to the appliances, or have an electrician install a transfer switch or an interlock kit. The NEC requires one of these for a generator that feeds the house wiring.
How far from the house should a generator be?
Run it outdoors, at least 20 feet from windows, doors and vents, with the exhaust pointed away from the house. Never run it in a garage, a basement or near an open window, even with the door open. Carbon monoxide from a generator kills quickly. Install battery-powered CO alarms inside.
Portable or home standby generator?
Portable ones come in sizes from 2,200 to 12,000 W and run on gasoline or sometimes propane. Home standby units, 10 to 26 kW, are wired in, start by themselves, and run on natural gas or propane. On gas they make about 10% less than their rating, so we count them at 90% of nameplate.