Portable Generator Sizing: How Many Watts You Actually Need

Most generator sizing mistakes come from confusing two different numbers printed on the same label: running watts (continuous load the generator sustains) and starting watts (the surge a motor needs for 1–2 seconds to spin up). A refrigerator rated at 150 running watts may need 600 starting watts. A generator that can’t supply that surge will fail to start the appliance even if the steady load is well within its capacity.

Typical Appliance Wattage

Appliance	Running watts	Starting watts
Refrigerator	100–400	400–1,200
Window AC (5,000 BTU)	500	1,500
Window AC (10,000 BTU)	900	2,700
Sump pump (1/2 hp)	800	2,000
Chest freezer	100–500	500–1,500
Box fan	50–200	50–200
Phone charger	5–20	5–20
LED light (10 bulbs)	100	100
Microwave (1,000W rated)	1,000	1,000
Electric water heater	4,000	4,000

Calculating Your Load

List everything you need to run simultaneously. Add running watts for all items, then identify the single appliance with the highest starting surge. Your generator needs:

• Running capacity ≥ sum of all running watts
• Starting capacity ≥ running total + largest starting surge

Example: fridge (150 running, 800 starting) + sump pump (800 running, 2,000 starting) + 5 LED lights (50 running) = 1,000 running watts. Starting: 1,000 + 2,000 = 3,000 starting watts. A 3,500W generator (3,500 running / 4,375 starting is a common pairing) handles this comfortably.

Runtime per Tank

Runtime scales inversely with load. Most portable generators spec runtime at 25% or 50% load. At 50% load:

• 1-gallon tank (small 2,000W inverter): approximately 4–6 hours
• 5-gallon tank (3,500–5,500W generator): approximately 8–12 hours
• 6.5-gallon tank (7,500W generator): approximately 8–11 hours

At 25% load, runtime roughly doubles. At full load, it drops 30–40% from the 50% figure.

Inverter vs Conventional

Inverter generators throttle engine speed to match actual load. Conventional generators run at fixed RPM regardless of demand. For most household use during outages, an inverter wins on three counts: fuel efficiency at partial loads (often 20–40% less fuel), lower noise (50–60 dB vs 65–75 dB), and cleaner power that’s safe for electronics and sensitive medical equipment.

The tradeoff is price: a quality 2,000W inverter costs $500–$900 versus $300–$500 for a conventional unit of similar rated output.

Critical Safety Rules

Carbon monoxide kills faster than most people expect. A generator produces 100 times more CO than a car exhaust. Run generators outside, at least 20 feet from any window, door, or vent — and on the downwind side of the house. Never run one in a garage, even with the door open. CO is odorless; get a CO detector if you don’t have one.

• Never backfeed into your home’s wiring — connect appliances directly with heavy-duty extension cords (12-gauge minimum for runs over 25 feet) or use a transfer switch installed by an electrician.
• Store no more than 30 days’ worth of stabilized fuel indoors. Ethanol-blended gas degrades in 30–60 days; add fuel stabilizer and rotate stock.
• Let the generator cool 15–20 minutes before refueling.

A 3,500–5,500W portable generator handles the essentials for most households. Know your starting surge numbers before buying; that single figure eliminates more sizing mistakes than anything else.