🔌What size generator do you need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Generator size

Everything running at once

Starting surge of the biggest motor

7,000 W generator ≈ 7 kW

Running load 5,000 W, peak with the surge 6,800 W

29% of it still free while everything runs

Running watts alone would need: 5,000 W
Current at 240 V: 20.8 A
Current at 120 V: 41.7 A
Running load in other units: 5.00 kW · 6.7 HP

Only the largest surge counts — two compressors almost never start in the same instant.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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What size generator do you need?

The AmpSizer generator result for 5,000 running watts with an 1,800 watt starting surge is a 7,000 watt generator. Peak demand is 6,800 W, and 29% of the machine is still free while everything is running.

The surge is what catches people. A refrigerator compressor or a well pump draws two to three times its running current for the first second, and a generator that is exactly the size of the running load will stall or trip when the fridge cycles.

Headroom is measured against the running load rather than the surge, because a surge lasts a second and every generator is built to swallow one. What wears an alternator out is running at its ceiling all night.

The loads people actually back up, with the two numbers each one needs.

Load	Running watts	Starting surge
Refrigerator or freezer	150–800 W	1,200–2,200 W
Furnace blower, 1/2 HP	600–900 W	1,800–2,400 W
Well pump, 1/2 HP	800–1,000 W	2,000–3,000 W
Sump pump	800–1,050 W	1,500–2,900 W
Window air conditioner, 10k BTU	900–1,200 W	2,000–3,000 W
Microwave	1,000–1,500 W	none
Lighting, whole floor in LED	100–300 W	none
Television and router	100–200 W	none
Coffee maker or kettle	1,000–1,500 W	none
Electric water heater	4,500 W	none
Electric range, one element	2,000–3,000 W	none
Electric dryer	5,400 W	none

The top half of that table is what a portable generator is for. The bottom half is why an outage still feels like an outage.

Sizing in practice

Add the running watts of everything you genuinely intend to have on at once, then find the largest surge in that list and add only that. A typical result for a house without electric heat is 4,000 to 6,000 running watts and a surge of 2,000 to 3,000 — which is a 7,000 to 9,000 watt machine.

Going one size up from that is usually worth it, not for capacity but for load factor: a generator loafing at 60% burns less fuel per kilowatt-hour, runs quieter, and lasts longer than one working at 90% for three days.

Frequently asked questions

What is the difference between running watts and starting watts?

Running watts are what the load draws continuously; starting watts are the brief inrush when a motor spins up, typically two to three times higher and lasting under a second. Generators are advertised with both, and the running figure is the honest one — a 7,000 W machine with a 9,000 W surge rating cannot deliver 9,000 W for any useful length of time.

Do I add the surge of everything?

No, only the largest one. Motors start at random moments and the chance of two large compressors starting in the same second is small enough to ignore, which is how the industry sizes. Add every surge together and you will buy roughly twice the generator you need.

Can I run my whole house on a portable generator?

Selectively, yes. A 7,500 to 10,000 W portable through a transfer switch runs a fridge, a freezer, a furnace fan, some lighting and a few receptacles. What it will not run is electric heat, an electric range or a water heater, which are each 4 to 9 kW on their own. Whole-house standby generators start around 18 kW for exactly this reason.

Why does my generator trip when the well pump starts?

Because the pump’s inrush exceeds what the alternator can deliver momentarily. A soft starter on the pump fixes it, as does a larger generator, as does running less at the same time. This is the single most common generator complaint and it is almost always a surge problem rather than a running-watts one.

How many amps does a 7,000 watt generator give?

About 29 A at 240 V, or 58 A at 120 V — but not both at once, and often not the full 120 V figure on one leg. Many portables split their output between two 120 V legs, so a single 120 V receptacle on the panel may be limited to half the machine. The plate on the generator lists the per-outlet limits.

How we calculate this

Peak demand is the total running load plus the starting surge of the single largest motor, on the standard industry assumption that large motors do not start simultaneously. The recommended machine is the smallest common portable rating that covers that peak. Headroom is expressed against the continuous running load rather than the peak, because surge capacity is a momentary rating every generator has and continuous loading is what determines service life and fuel consumption — the usual guidance is to stay below about 80% continuous. Current at 120 and 240 volts is the running load divided by the voltage; the actual current available on any one outlet or leg is limited by the generator’s own nameplate and is often lower.

Sources:

The numbers to gather before you shop

Nameplates, not guesses. Fridges, freezers, furnaces and pumps all carry their running watts or amps. Guessing overshoots on lighting and undershoots badly on motors.

Heat is the deal-breaker. Anything resistive — range, dryer, water heater, electric baseboards — is thousands of watts on its own and is normally left off a portable generator entirely.

The transfer switch is not optional. Backfeeding a panel through a receptacle energises the utility side and is both illegal and lethal to line workers. A transfer switch or interlock is part of the cost of the generator, not an accessory to it.