What size generator do I need?

The right size depends on what you want to keep running, not on the size of your house. Two numbers decide it: running watts and starting watts. Get those right and you won't buy too small, or pay for far more than you need.

Running watts vs starting watts

Running watts are what an appliance uses once it's going. A fridge might hum along at 400 watts.

Starting watts are the short burst a motor needs to get moving. It lasts a second or two. Fridges, freezers, pumps, furnace fans and air conditioners all have motors, so they all surge. Briggs & Stratton's buying guide says starting watts "may be two to three times" the running watts.

Lights, phone chargers, TVs, space heaters and water heaters have no motor. For those, starting and running watts are the same.

Every generator lists both numbers too. Check both before you buy.

The simple math

1. Add up the running watts of everything you want on at the same time.
2. Find the one appliance with the biggest surge. Its surge is its starting watts minus its running watts.
3. Add that one surge to your running total. That's your starting total.

You only add one surge because motors rarely start in the same second. You'll help by switching big things on one at a time, biggest first. This is the same method the generator makers use in their worksheets.

Then add about 20% headroom to both numbers. A generator run flat out all night is working too hard, and fridges and pumps switch on without asking.

A worked example: the outage essentials

Here's a common list, using the figures in our calculator:

Appliance	Running W	Starting W	Surge (start − run)
Fridge	400	1,200	800
Sump pump (1/2 HP)	1,000	2,150	1,150
Furnace fan (1/3 HP)	700	1,800	1,100
LED lights, a few rooms	100	100	0
Wi-Fi router	15	15	0
Phone / laptop	80	80	0
TV	150	150	0
Total	2,445

• Running total: 2,445 W.
• Biggest surge: the sump pump, at 1,150 W.
• Starting total: 2,445 + 1,150 = 3,595 W.
• With 20% headroom: about 2,950 running and 4,350 starting watts.

So a generator rated about 3,000 running and 4,400 starting watts covers this list. The calculator above is pre-filled with it. Add or remove things and the numbers update.

Rough sizes for common jobs

These come from the same method and the numbers in our calculator. Your own appliances may be higher or lower.

What you want to run	Running W	Starting W	Size to look for
Fridge, lights, Wi-Fi, chargers, TV	745	1,545	About 2,000 W
Fridge + 1/2 HP sump pump	1,400	2,550	About 3,000–3,500 W
The essentials list above	2,445	3,595	About 3,000 running / 4,400 starting
Essentials + an 8,000 BTU window AC	3,195	4,445	About 4,000 running / 5,400 starting
1/2 HP well pump + fridge + lights (240 V)	1,500	3,500	4,000+ W with a 240 V outlet
Electric water heater + fridge + lights (240 V)	5,000	5,800	About 6,000–7,500 W with a 240 V outlet

The well pump line uses 3,000 starting watts for the pump. That's because the motor maker, Franklin Electric, says a 1/2 HP two-wire pump needs a generator rated at least 3 kW. Our well pump guide explains why.

Watch for 240 V appliances

Most plug-in things run on 120 V. A few big ones run on 240 V: most well pumps, electric water heaters and many mini splits. For those you need two things:

• A generator with a 240 V outlet, usually a 4-prong L14-30.
• A transfer switch or interlock installed by a licensed electrician.

Never plug a generator into a wall outlet or a dryer outlet to "backfeed" the house. It can send power down the line and kill a utility worker. It can also start a fire. Our interlock vs transfer switch guide explains both safe options.

How to find your own appliance's watts

Chart figures are averages. Your appliance's own label beats any chart.

• Find the nameplate. It's a sticker or metal plate. On a fridge, look inside near the crisper or behind the kick plate at the bottom. On a pump or furnace, look on the motor. On a window AC, look on the side.
• If it lists watts, use that.
• If it lists amps, multiply by volts. 5 amps × 120 volts = 600 watts. That gives you running watts.
• For starting watts, look in the manual for "starting," "surge" or "LRA" (locked rotor amps). If you can't find any, the makers' rule of thumb is two to three times running for anything with a motor.

When a power station makes more sense

If you only need a fridge, a CPAP and your phone, a portable power station may do the job. It's quiet and safe indoors because it burns no fuel. It's sized in watt-hours rather than just watts. See power station vs generator.

Quick checklist

• Size for what you'll run at once, not everything you own.
• Start the biggest motor first, then add the rest one at a time.
• Use the label on your own appliance where you can.
• Plan for 240 V early if you have a well pump or want a transfer switch.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.