Running watts vs starting watts

By Dave, generator and controls engineer · Last reviewed October 31, 2026

Why motors need a burst to start

A motor that's standing still draws far more current than one that's running. That current, called locked-rotor current, stays high while the motor speeds up and only drops once it's close to full speed. For household motors that takes a moment, but the generator has to supply it the whole time.

Think of a loaded wheelbarrow. Getting it rolling takes a hard shove. Keeping it rolling takes a steady push. Running watts are the steady push; starting watts are the shove.

Which appliances surge

Anything with a motor or compressor: refrigerators, freezers, sump and well pumps, furnace blowers, air conditioners, washers and garage door openers. In our appliance list, these typically need two to three times their running watts to start.

Loads that just make heat or light don't surge in any way that matters for sizing: space heaters, electric water heaters, ovens, coffee makers and LED lights. Their starting watts equal their running watts.

Inverter-driven compressors, found in inverter refrigerators and most mini splits, ramp up slowly instead of starting at full speed. Their starting watts are only a little above their running watts, which makes them much easier on a generator.

What it means when you buy

Generators list two ratings: running watts (sometimes called rated watts) and starting watts (sometimes called surge or peak watts). Battery power stations list a continuous output and a surge rating. You need both numbers to cover your load:

• Running rating covers everything that runs at the same time, with room to spare.
• Starting rating covers your running load plus the burst from the motor that's starting.

Motors usually don't start at the same instant, so you add the burst of the biggest motor only, not every motor. If two motors start together, as can happen when power comes back or two appliances that switch themselves on happen to line up, you need room for both bursts. The math section below shows both ways.

If your generator's starting rating is too low, the motor may stall or fail to start, the lights dim hard, or the generator's breaker trips. A motor stuck trying to start runs hot, so this isn't just an annoyance.

A worked example

A refrigerator, a sump pump, some lights and the internet. Typical values from our appliance list:

Running total: 1,880 W. The biggest burst is the refrigerator's 1,500 W, so the starting requirement is 1,880 + 1,500 = 3,380 W.

A generator for this load needs at least 2,350 running watts (so the steady load stays at 80% of its rating) and at least 3,380 starting watts. A unit rated 3,000 starting watts would run everything once it's going, but could struggle to start the refrigerator while the sump pump is running.

Try your own appliances in the calculator.

Where to find your own numbers

The nameplate or sticker on the appliance is the best source. If it lists watts, use that. If it lists volts and amps, multiply them: 120 V × 5 A = 600 W. For a motor, volts × amps slightly overstates the watts it actually draws, which errs on the safe side for sizing (kW vs kVA explains why).

For starting, look for LRA (locked-rotor amps) on air conditioners, heat pumps and some pumps. Volts × LRA gives the starting load in volt-amps, which is the number that actually matters to a generator. See locked-rotor amps and NEMA code letters. Without an LRA, use the typical values in the calculator. They're chosen on the high side.

Running total
  = Σ running W of everything on
  = 700 + 1,050 + 100 + 30
  = 1,880 W

Extra to start (per motor)
  = starting W − running W
  Refrigerator: 2,200 − 700
    = 1,500 W
  Sump pump: 2,150 − 1,050
    = 1,100 W

Starting requirement,
one motor at a time
  = running total + largest extra
  = 1,880 + 1,500
  = 3,380 W

Starting requirement,
all motors at once
  = running total + every extra
  = 1,880 + 1,500 + 1,100
  = 4,480 W

Portable generator
  Rated running W
    ≥ running total ÷ 0.8
    = 1,880 ÷ 0.8 = 2,350 W
  Rated starting W
    ≥ starting requirement
    = 3,380 W

References

• NEMA MG 1, Motors and Generators (locked-rotor kVA code letters and motor nameplate marking).
• IEEE Std 3002.7-2018, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems (motor starting current and voltage drop).
• U.S. Department of Energy, Motor Systems Tip Sheet #6 (locked-rotor current and how it lasts until a motor nears full speed).
• Appliance values: Know Your Watts appliance list, typical values chosen on the high side. Check your own nameplate.

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