Running watts vs starting watts: what's the difference?

Generators list two numbers, and so do appliance charts. Mix them up and you'll buy a generator that stalls the moment your fridge kicks on. Here's what each one means and how to add them up.

Running watts: the steady number

Running watts are what an appliance uses once it's up and going. A fridge compressor uses about 400 W while it runs. A space heater uses about 1,500 W on high. This is the number you'll see most of the time.

Generator makers may call it running watts, rated watts or continuous watts. It's how much the generator can supply hour after hour.

Starting watts: the short burst

Anything with a motor needs a big burst of power to get the motor spinning. Champion says a peak or surge rating "would only be good for a few seconds" until the motor reaches speed. After that, it drops back to running watts.

Generator makers call this starting watts, surge watts or peak watts. It's what the generator can push out for a few seconds, not for long.

Which appliances surge?

Motors and compressors surge. Plain heaters and electronics don't. These are the figures our calculator uses, from maker charts (Champion, Honda and Franklin Electric):

Appliance	Running watts	Starting watts	Surges?
Fridge	400	1,200	Yes, compressor
Sump pump (1/2 HP)	1,000	2,150	Yes, motor
Well pump (1/2 HP, 240 V)	1,000	3,000	Yes, big motor
Furnace fan (1/3 HP)	700	1,800	Yes, motor
RV AC (13,500 BTU)	1,500	3,000	Yes, compressor
Circular saw	1,400	2,300	Yes, motor
Space heater	1,500	1,500	No
Microwave	1,000	1,000	No
Electric water heater (240 V)	4,500	4,500	No
Lights, TV, Wi-Fi, chargers	Small	Same	No

Starting watts here are the total at start-up (running plus surge). Some charts list only the extra surge, so read the column headings with care.

How to add it up

You don't add every appliance's starting watts together. Motors rarely all start in the same second. So:

1. Add up the running watts of everything that will be on at once.
2. Find the single biggest surge. For each motor, take starting minus running. Pick the largest one.
3. Starting watts needed = total running + that biggest surge.
4. Add about 20% headroom to both numbers.

That's exactly how our calculator works.

Worked example: fridge, sump pump and lights

• Running: 400 (fridge) + 1,000 (sump pump) + 100 (lights) = 1,500 W
• Surges: fridge 1,200 − 400 = 800 W. Sump pump 2,150 − 1,000 = 1,150 W (the biggest). Lights 0.
• Starting: 1,500 + 1,150 = 2,650 W
• With 20% headroom: about 1,800 running and 3,200 starting watts

A generator rated about 2,000 running and 3,500 starting watts covers this. The calculator above is pre-filled with these three, so you can add your own.

When the simple rule isn't enough

• Two big motors at once. A sump pump and a well pump can kick on together. If you have more than one big motor, size for both surges, or stagger them.
• Well pumps. The motor maker Franklin Electric gives a minimum generator size for each pump, which is higher than most charts. See our well pump guide.
• Old fridges. Older or larger fridges can surge to about 2,200 W, Honda's chart shows. Check your label.

How to shrink a surge

A soft start is a small device fitted to an AC compressor. It eases the motor up to speed, so the start-up spike is lower. It's popular for RV and central AC. See generator for central air.

You can also start the biggest motor first, then switch on everything else.

What happens if the surge is too big

The generator labors, its voltage sags, and it may stall or trip its breaker. Champion notes that air compressors and big saws are common trouble spots. If that's happening, see generator overload tripping.

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