What Size Generator Do I Need? Wattage Chart and Worked Examples (2026)

What size generator do I need? For a fridge, a sump pump, a furnace fan, some lights and a TV, the honest answer for most houses is about 4,000 starting watts and 2,600 running watts, which puts you in the 3,500–5,000 W class. Add a 10,000 BTU window AC or a microwave you actually plan to use, and you’re shopping for 5,000–7,500 W. Want central air or an electric water heater? That’s 9,000 W and up, with a 240 V outlet. The math behind those numbers takes five minutes and saves you from the two classic mistakes: buying a generator that stalls when the well pump kicks on, or hauling around twice the engine you need.

I’ll be straight about where these numbers come from. I haven’t loaded up a generator with a houseful of appliances and logged the readings. What I have done is read a lot of wattage charts, because when you’ve spent years somewhere the power goes out without much warning, the question isn’t whether you’ll need backup but how much. So below I’ve put the manufacturer figures side by side and flagged where they disagree.

Generator wattage chart: 15 common household loads

Running watts are what an appliance draws once it’s going. Starting watts are the short spike when a motor or compressor starts. Honda lists starting watts as the total draw during that spike; Champion lists ranges. Anything with a motor (fridge, pumps, furnace blower, AC, garage door) has a big gap between the two. Resistive loads like a coffee maker or space heater don’t.

Notice the fridge. A modern ENERGY STAR unit runs on under 200 W, while Westinghouse’s manual uses 700 W running and 2,200 W starting, which is closer to an older side-by-side or a garage fridge. Furnace blowers vary just as much: Honda lists a 1/2 HP fan at 875 W running, Champion gives 300–800 W. The label on the appliance (or its amps multiplied by 120) beats any chart.

How to work out what size generator do I need, step by step

1. List only what has to run at the same time. Not everything in the house. The fridge, the sump pump in a wet basement, the furnace blower in winter, a few lights, phones. The microwave and coffee maker can take turns.

2. Add the running watts. That’s the continuous load the generator has to carry.

3. Add the biggest single starting surge on top. Only one motor usually starts at a time, so you add the largest spike, not all of them. With Honda’s figures, which are totals, add the difference (starting minus running) for that one appliance. Champion’s own flyer example lands at 3,880 running watts plus a 2,200 W fridge start, for 6,080 W.

4. Leave headroom. Champion just says to pick a unit that beats your number “by a safe margin.” Running a generator flat out for hours is hard on it and leaves no room for a surprise surge, so I’d keep the continuous load to roughly 80% of the running rating. That 80% figure is common practice, not a manufacturer rule.

Worked example: essentials for a basement house

Using Honda’s numbers: ENERGY STAR fridge 192 W, 1/2 HP sump pump 1,050 W, 1/2 HP furnace fan 875 W, five LED bulbs about 50 W, a 46″ TV 190 W and a laptop 250 W. Running total: 2,607 W. The biggest start is the furnace fan, which spikes to 2,350 W, so add its extra 1,475 W. Peak: about 4,100 W.

A generator rated around 3,500 W running and 4,500 W starting covers that with a little room, as long as the sump pump and furnace don’t start in the same second. Honda makes the same point in its own worked example: stagger the starts and you can get away with a smaller unit. Champion’s 4500 W inverter (3,500 running gas) sits right in this slot; see our Champion 4500W inverter generator review.

Now add a 1,000 W microwave (1,500 W draw). Running jumps to 4,107 W, and with the furnace surge you’re near 5,600 W. That’s the 5,000–7,500 W class, where a dual-fuel open-frame unit like the Westinghouse WGen7500DF lives. Our best dual fuel generators for home backup compares that tier.

Generator size classes and what each runs

What size generator do I need for a whole house?

Once 240 V loads come in, the outlet matters as much as the wattage. A 3-ton central AC (3,000–3,500 W running, 5,000–6,000 W starting), a 1 HP well pump or a 4,500 W water heater all need 240 V. On a portable generator that means an L14-30R (30 A) or a 14-50R (50 A) outlet feeding a transfer switch or interlock. By simple math, 240 V at 30 A is 7,200 W and 240 V at 50 A is 12,000 W; those are calculated ceilings, not ratings printed by the manufacturers.

Realistically, a 10,000–13,000 W portable can run a whole house only if you don’t ask everything to run at once. The water heater alone eats 4,500 W. Most people skip it during an outage. For per-appliance detail, see generator size for central AC, generator for a well pump and generator for a sump pump. Connecting to house circuits safely is covered in our generator transfer switch guide.

Things that shrink a generator’s real output

Altitude. Westinghouse’s WGen12000 manual says maximum power drops about 3.5% for every 1,000 feet above sea level. At 5,000 feet, a 7,500 W generator is closer to 6,200 W.

Heat. The same manual says output falls about 1% for every 10°F above 60°F. A summer outage at 100°F costs roughly 4%.

Fuel. Champion says propane gives about 10% less output than gasoline, and natural gas 10–15% less. Check the propane rating on the spec sheet, not the gas number on the box. Our propane vs gas generator guide goes deeper.

Sensitive electronics. If computers, a CPAP or a furnace control board are on the list, an inverter generator’s cleaner output is the safer bet. See inverter generator vs conventional generator.

Whatever size you land on, read the CPSC’s portable generator safety alert before the first outage: outdoors only, far from windows and doors, and never plugged into a wall outlet. Our carbon monoxide safety guide covers placement, and the portable generator buying guide walks through fuel, outlets and noise once you know your wattage.

Frequently asked questions

Last updated: October 17, 2026. What size generator do I need figures are manufacturer wattage-chart values, not WildJoule measurements. We do not display live prices.