Continuous watts are what an inverter delivers indefinitely; surge watts are a much higher figure it tolerates briefly while a motor starts. A device can sit well inside the continuous rating and still trip the inverter at the instant it switches on.
The two numbers on the front of an inverter describe different promises, and only one of them is about running your equipment.
What the continuous figure covers
Continuous output is the load the unit can deliver without stopping, at a stated temperature. It is the number to compare against the sum of everything plugged in at once — and appliances add up, so the kettle and the heater together are what matters, not either alone.
What the surge figure covers
A motor draws far more current for the instant it starts turning than it does once it is running. The surge rating says the inverter tolerates that spike, and for how long. Without the duration, the figure is a marketing number: a very short tolerance and a long one are printed identically.
Why the battery matters too
An inverter can only pass on what the battery lets it draw. A large inverter on a small battery, or on thin cable, will trip on the same surge that a smaller inverter handles easily with a bank sized to match. This is the commonest cause of a system that fails at the moment it is asked to do something.
Which loads to measure before sizing
The awkward loads are the motorised ones. Their labels often print running watts only. A clamp meter or a plug-in energy meter on the mains supply shows the real starting behaviour, and it is the only figure worth sizing against.
Where this page and the manufacturer’s documentation differ, the documentation is the one to follow. Every capacity, output, input window, current limit, cycle count and temperature threshold on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any peak figure: a surge rating means something only with the duration it can be held. A very short tolerance and a long one are printed identically. This page quotes the maker’s continuous rating as the figure to size against, reports a surge rating with its stated duration when one is published, and computes no starting current from either.
What people ask before they order
Which appliances need a surge headroom?
Anything with a motor or a compressor: fridges, pumps, power tools, air conditioners. Their starting draw is a multiple of their running draw, and it lasts a fraction of a second.
Is a surge rating useful without a duration?
Much less so. A surge figure means something only with the time it can be held; a maker that publishes the duration is telling you more than one that publishes a bigger number alone.
Last reviewed 16 September 2026