Running watts vs surge watts: the number that trips a battery before capacity ever matters

Every power station carries two output numbers, and the box shows the one that matters less. Continuous watts is what the inverter can deliver all day. Surge watts is what it can deliver for a moment, and a moment is exactly what a motor needs. A unit can hold 2,000 Wh and still never start the pump. This page is the mechanism, the way to check any battery against any appliance in ten seconds, and the two places the makers’ marketing will mislead you.

What happens in the first half-second

A pump, a compressor, a fan or a well pump is an induction motor. At rest its windings look almost like a short circuit, so the instant it is energised it draws several times its running current until the rotor spins up. That draw is called locked-rotor current, and it lasts a fraction of a second to a few seconds depending on the load on the shaft. An inverter that cannot supply it either sags its voltage until the motor stalls or shuts itself off to protect its transistors. Either way the appliance never starts, and no amount of battery capacity changes that.

How big the spike is depends on the motor

The “three to five times” figure repeated everywhere is a capacitor-start induction motor figure, and it is real: well pump motors, air compressors and older pedestal sump pumps behave that way; the well pump page is built on Franklin Electric’s published 32.2 A locked rotor for a 1/2 HP motor. Modern submersible sump pumps mostly use shaded-pole or permanent-split-capacitor motors, which start far more gently. Liberty Pumps publishes the number for its 1/3 HP 257: 8 A locked rotor against 5.2 A running, a ratio of about 1.5. Most makers publish nothing, which is why the sump pump wattage guide lists the one that does and treats the rest as margin.

Load	Start-up behaviour	What to check
Heater, kettle, incandescent light	No surge; draws full rating from the first instant	Continuous watts only
Refrigerator or freezer compressor	Capacitor-start, 3 to 5 times running for under a second	Surge watts
Submersible sump pump (shaded pole or PSC)	Roughly 1.5 to 2 times running	Continuous watts with margin, surge as insurance
Well pump, air compressor, older pedestal pump	3 to 6 times running, longer under load	Surge watts, then continuous
Microwave, laptop, TV, CPAP	Electronic supplies, small inrush	Continuous watts

The ten-second check

1. Find the appliance’s running watts on its nameplate, or amps times volts.
2. If the plate lists locked-rotor amps, multiply by volts: that is the surge figure. If not, multiply running watts by three for a compressor, by four or five for a well pump, by two for a submersible sump pump.
3. Compare that number to the battery’s surge rating, not its continuous rating. If surge is lower, stop; capacity does not matter.
4. Only then compare running watts to the continuous rating, and only then size the watt-hours. The battery backup for a sump pump page is the worked example, with the check printed above its runtime table.

Two ways the box misleads

Boost modes are not surge. EcoFlow’s X-Boost, Bluetti’s Power Lifting and Anker’s SurgePad let an inverter run a load rated above its continuous output by dropping the voltage it delivers. That works for resistive loads, and the makers are explicit about it: Bluetti’s Elite 200 V2 page limits its 3,900 W Power Lifting mode to “pure resistive loads.” A motor needs full voltage to develop torque, so a boost figure tells you nothing about starting a pump. Read the plain surge line on the spec sheet, and when a maker lists only a boost figure, treat the surge as not stated. That is exactly what the spec strips on this site do.

The retailer listing and the spec sheet disagree. Amazon listings routinely print a “peak” figure the maker’s own page does not. When they conflict, the maker’s sheet is the number that goes on our cards, and the blurb says the two disagree. The usable capacity guide covers the other number the box inflates.

Resistive loads are the opposite problem

A space heater or a kettle has no surge at all, so it starts on anything. It also draws its full rating every second it is on: a 1,500 W heater empties a 2 kWh battery in a little over an hour of outlet time. That is why a battery is the wrong tool for heating a room, as the space heater page shows in full, and the right one for keeping a refrigerator cold, and why every runtime table on this site is built from watts times duty cycle rather than from the sticker.