Do I add up starting watts for every appliance?

No. Add up every appliance's running watts, then add only the single largest starting-watt figure. Motors rarely all surge at the same instant, so you size for the total running load plus the biggest one-time spike, not the sum of every surge.

How do I convert amps to watts for an appliance?

Multiply amps by volts. In the US that is your 120-volt supply, so a 6-amp appliance is 6 × 120 = 720 watts. Elsewhere the mains is 230 volts, so 6 amps is 1,380 watts. The label usually lists watts directly, but this covers devices that only show amps.

How much headroom should a generator have?

About 20% above your combined peak. Running near 100% continuously overheats the engine, shortens its life and leaves no room for an extra load. A little headroom keeps the unit efficient and lets it absorb a surge you did not plan for.

Can I plug a generator into a wall socket?

No — never back-feed through a wall outlet. It sends power onto lines that utility workers assume are dead and can start a fire. Plug appliances directly into the generator, or have a qualified electrician install a transfer switch for a safe, permanent home connection.

What Size Generator Do I Need? Add Up Your Running and Starting Watts

Published 4/29/2026 · 4 min read · Real-estate calculators

Marco Bianchi — Home, DIY & motoring writer at Allin

Renovation · Materials

Checked against 2 sources

View profile →

In short

List every appliance you need to run at once and add up their running watts. Then add the highest single starting (surge) watts, since motors draw a brief spike when they kick in. A fridge (700 W run, 2,200 W start), lights (300 W) and a sump pump (800 W run) need about 1,800 running watts but a 3,200-watt surge, so a 4,000-watt generator gives safe headroom.

Size a generator by listing the appliances you must run, adding their running watts, and allowing headroom for the surge when motors start.

List what you actually need to power

Generator sizing starts with an honest list of the appliances you must run at the same time during an outage. Prioritize essentials: a refrigerator, a few lights, a phone charger, maybe a sump pump or a furnace fan. You rarely need to power the whole house at once, and pretending you do buys a generator that is too big and thirsty.

Find each appliance's wattage on its rating label, in the manual, or from the maker's spec. If a device lists amps and volts instead, multiply them for watts: 6 amps at 120 volts is 720 watts. Write down two numbers per motor-driven item — running watts and starting watts — because they can differ sharply.

Running watts versus starting watts

Running watts are what a device draws in steady operation. Starting watts, also called surge watts, are the much larger spike a motor pulls for a fraction of a second as it starts — a fridge compressor, a well pump or an air conditioner can surge to two or three times its running figure. Resistive loads like lights and heaters have no surge.

This is why you cannot just add running watts. The generator must supply the total running load plus the biggest single surge, because motors rarely all start at the same instant. Add up every device's running watts, then add only the largest starting-watt figure among them — that combined peak is what the generator has to cover.

Add headroom and pick the size

Once you have the combined peak, add a safety margin of about 20% and choose a generator rated above it. Running a generator flat out at 100% wears it fast and leaves nothing for an extra device, whereas a bit of headroom keeps it cool, quiet and able to absorb a surge you did not plan for.

One caution on connection: for anything wired into your home rather than plugged in directly, a generator must go through a proper transfer switch fitted by a qualified electrician. Back-feeding through a wall outlet can kill utility workers and start fires. Size the generator yourself, but leave any permanent hookup to a professional.

Worked with our own calculator

Generator size calculator

Given

Appliance wattages (comma-separated): 1500, 700

Result

Total running watts: 2,200
Recommended size (kW): 2.75

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

Do I add up starting watts for every appliance?: No. Add up every appliance's running watts, then add only the single largest starting-watt figure. Motors rarely all surge at the same instant, so you size for the total running load plus the biggest one-time spike, not the sum of every surge.
How do I convert amps to watts for an appliance?: Multiply amps by volts. In the US that is your 120-volt supply, so a 6-amp appliance is 6 × 120 = 720 watts. Elsewhere the mains is 230 volts, so 6 amps is 1,380 watts. The label usually lists watts directly, but this covers devices that only show amps.
How much headroom should a generator have?: About 20% above your combined peak. Running near 100% continuously overheats the engine, shortens its life and leaves no room for an extra load. A little headroom keeps the unit efficient and lets it absorb a surge you did not plan for.
Can I plug a generator into a wall socket?: No — never back-feed through a wall outlet. It sends power onto lines that utility workers assume are dead and can start a fire. Plug appliances directly into the generator, or have a qualified electrician install a transfer switch for a safe, permanent home connection.

Articles you may find interesting

All guides →

How-toWhat Size Heater Do I Need? Sizing BTUs by Room and ClimateSize a heater from room area and your climate: a BTU-per-square-foot rule gives the heat output, with watts and kW for electric units.How-toWhat Size Water Heater Do I Need?Match tank size to household size and peak-hour demand: roughly 40–50 gallons for two people, more as bathrooms and simultaneous use grow.ExplainerHow Many Square Metres Are in an Acre?One acre is about 4,047 square metres; a hectare is exactly 10,000. Learn the conversions and how land-area units relate.How-toWhat TV Size Is Right for My Room?Turn your viewing distance into a recommended screen diagonal in inches, so a 4K TV fills your field of view without straining your eyes.How-toWhat Wire Gauge Do I Need?Pick wire size from the load current, run length, and allowed voltage drop — the amps decide the minimum cross-section, distance forces you thicker.ComparisonInsulate or Replace the Heating: Which Repays First, and Why the Answer InvertsPut both on one axis — euros paid per kilowatt-hour a year of permanent saving — and the comparison becomes decidable. The same loft job costs 0.83 euros per kilowatt-hour a year in a 1965 house and 5.00 in a 2005 one, a factor of six. That, and not the age of the building by itself, is what inverts the verdict.

Sources

Spotted a mistake in this article?