How to Size a Generator by Wattage: Running Watts vs Surge Watts Explained

Buying a generator by “how big it looks” is how people end up with a unit that trips the moment the fridge compressor kicks on. Generators are rated in watts, and the number that matters is not one figure but two: running watts and starting (surge) watts. Get those wrong and you either overspend on capacity you never use or burn out a unit that keeps stalling.

The Two Wattage Numbers on Every Label

Running watts is the steady power a device draws while operating. Starting watts (also called surge or peak watts) is the brief spike required to start anything with a motor — typically 2 to 3 times the running draw, lasting only a second or two. A generator must handle the running load plus the largest single surge in your lineup, because appliances rarely all surge at once.

Typical Appliance Wattage

These figures come from common nameplate ratings and Energy Star estimates. Treat them as planning numbers, not gospel — always check the actual label.

Appliance	Running W	Starting W
Refrigerator	700	2,200
Furnace fan (gas)	800	2,350
Window AC (10,000 BTU)	1,200	1,800
Sump pump (1/2 hp)	800	2,100
Well pump (1 hp)	1,000	3,000
Microwave	1,000	1,000
LED lights (10 bulbs)	100	100

The Sizing Formula

Add all running watts first. Then find the single largest starting watt figure in the set and add only that one on top, because in practice two big motors almost never start in the same instant.

Example: refrigerator (700) + furnace fan (800) + sump pump (800) + lights (100) = 2,400 running watts. The biggest surge is the sump pump at 2,100. Total requirement = 2,400 + (2,100 − 800) = 3,700 starting watts.

A 4,000-watt peak generator covers this. A 3,000-watt unit does not, even though its running rating looks close.

Running Watts vs Surge: Why the Gap Matters

Many portable generators advertise something like “3,500 running / 4,375 starting.” The starting figure is only sustainable for a few seconds. If your sump pump and fridge compressor cycle on together, a unit running near its continuous limit will bog down and drop voltage — which is what damages electronics. Keeping your continuous load under about 80% of the running rating leaves headroom for surges.

If you are comparing how much power different appliances really use over time, converting watts to kilowatt-hours is the next step — a general calculator handles that unit math without breaking out a spreadsheet.

Whole-House vs Portable

Type	Typical size	Best for
Portable	2,000–8,000 W	Refrigerator, lights, sump pump, phone charging
Inverter portable	2,000–4,000 W	Sensitive electronics (laptops, medical devices)
Whole-house standby	10,000–22,000 W	Full home coverage, central AC, well pump

FAQ

How much generator do I need for a refrigerator only? A 2,000-watt peak unit is enough — the fridge chains around 2,200 starting watts but many nameplates list less; check yours.

Can I run a generator indoors? Never. Carbon monoxide kills silently. Place units at least 20 feet from doors and windows.

What size for a whole house? Add your two largest loads (usually central AC and well pump) plus everything else running, and add the single biggest surge on top.

The Takeaway

Size for surge, not just running watts. Add all running loads, then add only the largest starting figure once, and keep continuous draw under 80% of the running rating. That single rule prevents the two most common generator mistakes: buying too small and stalling, or buying far bigger than the job requires.