Running watts versus starting watts explained simply, with examples for a fridge and a sump pump, so your backup power does not trip.
- Running watts are steady use, starting watts are a short burst.
- Motors and compressors have the biggest surge.
- A power station needs both a continuous and a surge rating that fit.
You find a power station that says it can supply a certain number of watts. You check your fridge, and the label shows a lower number. It should work, but it trips off the moment the compressor kicks on. The missing piece is surge power, and it is one of the most common causes of backup power disappointment.
Running watts
Running watts, also called rated or continuous watts, are what an appliance draws once it is up and going. A fridge compressor at steady state, a fan at speed or a light turned on all draw their running watts. Your power source needs a continuous output rating at or above the total running watts of everything you plug in at once.
Starting watts
Starting watts, also called surge, peak or locked rotor watts, are the short burst an appliance needs to get going. Anything with a motor or compressor has this. The motor is stationary and needs extra current to start turning. That burst lasts a moment, often a fraction of a second to a couple of seconds, and can be several times the running figure.
Appliances that usually have a surge:
- Refrigerators and freezers (compressor)
- Sump pumps and well pumps (motor)
- Air conditioners and heat pumps (compressor)
- Furnace blowers
- Power tools and garage door openers
- Washing machines
Appliances that usually do not: lights, phone chargers, laptops, routers, TVs, and most heating elements like a toaster or kettle. Those have a steady draw. They can still be too big for a small unit, but the issue is running watts, not surge.
Worked example: a sump pump
Say your pump label lists a running figure of a few hundred watts. The start-up may need two to three times that for a moment. If your power station has a continuous rating just above the running figure but a surge rating below the start-up figure, it may shut down each time the pump starts. You need both ratings to cover the pump. See can a power station run a sump pump.
Worked example: a fridge
A fridge draws a modest amount when the compressor is running and almost nothing between cycles, but each start is a surge. A power station that handles the surge may still struggle if you plug a second motor load into it at the same moment. See how long a power station will run a fridge and how many watts a chest freezer uses.
How to check your equipment
- Find the nameplate on the appliance. It may list running amps and sometimes locked rotor amps.
- Multiply amps by volts to get a rough watt figure.
- Look at the power station spec for both continuous output and surge or peak output.
- Make sure total running watts stay below the continuous rating.
- Make sure the biggest start-up demand is below the surge rating.
- Start the biggest load first, then add smaller ones.
Common mistakes
- Adding up only the running watts and forgetting the biggest surge.
- Trusting a marketing number without reading the spec sheet for continuous and surge output separately.
- Plugging a motor into the same unit as several other heavy loads, then starting everything at once.
- Using a long thin extension cord, which wastes power and can overheat.
Soft start and other tricks
Some appliances, including some inverter-driven air conditioners and heat pumps, use soft-start designs that cut the surge. Aftermarket soft-start kits also exist for some air conditioners, and an HVAC technician can advise. Staggering your loads also helps: do not let the fridge, pump and freezer all try to start together.
Generators and surge
Generators have both running and starting ratings too. The same rules apply. Our guides what size generator do I need and inverter vs conventional generators go deeper. Run generators outdoors only.
To tally your whole list of loads, the outage planner and what size power station do I need are useful. Then see the power station hub for sizing.
Quick answers
What is the difference between running and starting watts?
Running watts are what an appliance draws steadily. Starting watts are the extra burst needed for a moment to start a motor or compressor.
Why does my power station trip when the fridge starts?
The compressor surge may exceed the unit's surge rating, or another load is pushing the total over the limit.
Do LED lights have a surge?
No, lights, chargers and routers have a steady draw, so only running watts matter.