What size generator do I need?

Common generator size scenarios

Quick reference for typical emergency and backup setups. Use the calculator above for your exact appliances.

All figures use typical published wattages. Starting demand = running total + largest single motor starting surplus. Recommended size = next standard generator size at or above starting demand × 1.1.

Standard generator sizes

These are nominal rated (continuous) watt tiers common across residential generator lines. A generator also has a peak/surge wattage rating (typically 10–20 % higher) — the surge rating is the maximum it can sustain briefly; the rated wattage is what it delivers continuously. Always check both ratings before buying.

Want the full explanation? Read the generator sizing guide →

Common questions

What size generator do I need?

Add up the running watts of every appliance you want to power at the same time. Then find the appliance with the highest starting surge (motors briefly draw 2–3× their running watts when they start). Add that starting surplus to the running total: that is your peak demand. Choose a generator rated at least 10% above the peak demand. For example: a refrigerator (150 W running, 600 W starting) + three rooms of LED lights (150 W) + TV (150 W) + phone charger (100 W) = 550 W running. Largest starting surplus = refrigerator's 450 W surge. Peak = 1,000 W. Recommended: 2,000 W generator.

What is the difference between running watts and starting watts?

Running watts (also called rated or continuous watts) is the steady power an appliance draws while operating normally. Starting watts is the brief surge — 2–3 times higher for motor-driven appliances like air conditioners, pumps and refrigerators — that lasts a fraction of a second when the motor first starts. Resistive loads (heaters, LED lights, microwaves) have no starting surge: their starting and running watts are the same. A generator must supply the full starting watts without stalling.

How many watts does a generator need to run a house?

A whole-house generator typically needs 10,000–20,000 watts to power everything including central air conditioning, a well pump, appliances and lights. For essential loads only — refrigerator, lights, TV and device charging — a 2,000–3,500 W generator is usually enough. Add a sump pump or furnace fan and you'll need 5,000–6,500 W. Adding central AC (2-ton) pushes the requirement to around 10,000–12,500 W. Use the calculator above to add your specific appliances.

What size generator do I need for a 200 amp service?

A 200-amp service panel at 240 V can handle up to 48,000 watts in theory, but no generator powers a whole panel at full capacity. The right generator size depends on which circuits you want to back up — not the panel size. Size the generator to the total wattage of the specific appliances you plan to run during an outage, using the running + starting surge method above. Most standby generators for a typical home are 10,000–22,000 W.

Do I need to account for starting watts for a refrigerator?

Yes. A refrigerator's compressor motor draws a starting surge of roughly 3–4× its running wattage when it cycles on — typically 150–200 W running, with a starting surge of 600–700 W. If a refrigerator is the only motor-driven appliance on the generator, the starting surge is what determines the minimum generator size, not the running load. The calculator above uses typical starting-surge figures from generator manufacturer sizing guides.