How to Size a Home Generator: Watts You Actually Need

The right generator size comes down to one number: the total watts your home pulls at its busiest moment. That's the sum of the running watts of everything you want on at the same time, plus the single largest starting surge among your motor-driven appliances. Get that number, add a safety margin, and you buy a generator rated above it. Undersize and the unit stalls or trips the moment your AC or well pump kicks on; oversize and you waste thousands of dollars and burn extra fuel running a big engine at a fraction of its capacity.

There are three questions to answer before you shop: what do you actually need to back up (essentials or the whole house), what's the biggest startup surge on your list (usually central AC, a heat pump, or a well pump), and does your electrical panel have room for the circuits you want to power. This guide walks all three, gives typical appliance wattages, and explains running vs. starting watts so you don't fall into the most expensive trap in generator buying: sizing on running watts alone.

Running watts vs. starting watts

This is the single most important concept, so it comes first.

• Running watts (also called rated watts) is the steady power a device draws once it's running. A refrigerator might run at 150–400 watts.
• Starting watts (also called surge watts or inrush) is the brief spike a motor pulls at the instant it starts — typically 2 to 3 times its running watts, for a second or two. That same fridge can surge to 300–800 watts when the compressor kicks on.

Anything with a motor or compressor has a surge: refrigerators, freezers, well pumps, sump pumps, AC compressors, heat pumps, furnace blowers. Resistive loads have no surge — electric heaters, incandescent bulbs, water heaters, and electric ranges draw the same watts starting as running.

The sizing rule that falls out of this:

Total your running watts, then add the single largest starting surge — not every surge added together. Motors don't all start at the same instant, so you size for the biggest one hitting while everything else is already running.

If your generator can't absorb that surge, the motor won't start, the engine may stall, voltage and frequency sag, and sensitive electronics can be damaged — or the generator's own breaker trips and everything shuts off.

Typical appliance wattage

Use these as planning ranges. Your actual numbers are on the appliance nameplate or in the manual — check them for big motor loads. Wattages below are stable typical values for US 120V/240V homes.

Note the pattern: the big running number (water heater, dryer, range) is rarely your sizing problem — you can shed those during an outage. The big starting number (central AC, heat pump, well pump) is what actually sets the generator size.

Decide what to back up

Generator sizing follows from this choice, so make it deliberately.

Essentials only

The cheapest, simplest path. You power a handful of critical circuits and leave the rest dark:

• Refrigerator and freezer
• Furnace blower or a heat pump's air handler (heat in cold climates)
• Well pump and/or sump pump (water and a dry basement)
• Internet, a few lights, phone charging

Add the running watts of these (commonly 2,000–4,000 W), then add the largest surge — usually the well pump or furnace blower (another ~2,000–3,000 W). Most essentials setups land around 5,000–8,500 starting watts, squarely in portable generator territory paired with a manual transfer switch or interlock.

Whole-home (or near-whole-home)

You run most or all circuits, including central AC or a heat pump and the kitchen. Here the central AC/heat pump surge dominates: a 3-ton heat pump can spike 9,000–16,000 W for a couple of seconds. Add that to a realistic running total (with the water heater, dryer, and range managed, not all on at once) and most homes need a standby generator in the 14–22 kW class.

The key trick at this tier is load management (also called load shedding): a smart module or managed circuits automatically drops the water heater, dryer, or AC for a few seconds during a surge so a smaller generator can carry a bigger home. With managed loads, a 16–18 kW unit often does the work that a naive calculation says needs 22 kW+.

How to pick the kW

Convert your worked total into a generator rating:

1. Sum running watts of everything on simultaneously.
2. Add the single largest starting surge (your biggest motor — typically AC, heat pump, or well pump).
3. Add a 20–25% margin so two motors cycling near the same time, or a cold-start surge, don't max you out.
4. Round up to the next standard size. Generators are rated in watts or kW (1 kW = 1,000 W).

Worked example — a standard home on essentials-plus-comfort:

• Fridge + freezer: 600 W running, 1,700 W combined surge
• Furnace blower: 800 W running, 2,000 W surge
• Well pump: 1,500 W running, 3,500 W surge (the largest surge)
• Lights + internet + TV: 700 W running
• A few outlets / small loads: 500 W running

Running total: ~4,100 W. Largest surge: 3,500 W (well pump). Peak: 4,100 + 3,500 = 7,600 W. Add 25% margin → ~9,500 W. A 9,000–10,000 W portable (or a small standby) fits with headroom.

For whole-home with central AC, the same method applies — you just add a 10,000–14,000 W surge instead, which is what pushes you into standby territory.

Don't forget the electrical panel

Your panel sets a ceiling regardless of your watt math. A standby generator wires into your panel through an automatic transfer switch, and the total load it carries can't exceed what the panel and service are built for. Older 100A services limit your options and often push you toward an essentials setup or a service upgrade; 200A services (common in newer homes) comfortably support 14–22 kW standby units. Have an electrician do a proper load calculation before you commit to a size — it's the step DIY estimates skip and then regret.

Two ways to tame a big surge

If one appliance's startup surge is forcing you up a generator size, two add-ons can pull it back down:

• Soft starter on the AC or heat pump compressor: reduces the startup surge by roughly 50–70%, often letting a smaller generator run a load it otherwise couldn't.
• Load management / load shedding: automatically disconnects the water heater, dryer, or other big loads during surges so the generator never sees the worst-case peak.

Both can drop you a full generator size, which usually pays for itself.

Common sizing mistakes

• Sizing on running watts only. The classic error: you total the steady draws, buy a 12 kW unit, and discover it can't start the central AC's 12,000 W surge. Always add the largest surge.
• Adding every starting surge together. The opposite error oversizes wildly. Motors don't all start at once — add only the single biggest surge.
• Ignoring the panel. Buying a 22 kW unit for a 100A service that can't carry it. Get the load calculation first.
• Undersizing to save money. The gap between a unit that barely works and one with comfortable margin is usually small relative to total install cost — and margin is what keeps the lights on when two motors cycle together.
• Forgetting future loads. Adding an EV charger, a heat pump, or central AC later can leave a just-right generator undersized. If big loads are coming in the next few years, size with headroom now.

The bottom line

Size on the peak, not the average: running watts of everything on at once, plus the single largest starting surge, plus a 20–25% margin. Essentials usually fit a 5,000–8,500 W portable; whole-home with central AC or a heat pump usually needs a 14–22 kW standby, often trimmed with a soft starter or load management. Decide what you're backing up first, check your panel, and buy above your worked number — not from a square-footage rule of thumb.

For more on choosing equipment, see portable vs. standby generators and generator fuel types. To estimate what running a generator or your home actually costs, check current electricity rates, browse the rest of our home-energy guides, or see all guides.