Running Watts vs Starting Watts

Every motor-driven appliance has two power numbers, not one. The first is the steady draw. The second is a short burst at the moment it starts. Backup power has to cope with both.

The two terms

Running watts are the power a device draws continuously once it's going. One maker's guide calls them "rated wattage" and describes them as the power a device draws continuously¹. If a label says 150 W and nothing else, that's usually the running figure, though the label is the place to check.

Starting watts are different. The same guide describes them as the extra watts an appliance needs for two or three seconds to start up¹. You'll also see "surge watts" and "peak watts". They all point at the same idea: a brief spike.

Watt is just a unit of power. Think of it as how hard the supply is working at that instant. A tap running gently and a tap thrown fully open fill the same bucket, but only the open tap strains the pipe.

What it looks like in practice

At the moment of switching on, you may notice a few things. A fridge hums up with a heavier thump than usual. The lights on the same circuit may flicker. A small inverter may beep.

What each one tells you:

• A heavy thump or a slow first turn: the motor is working hard to get moving.
• A brief light dip: something on the supply drew far more than its running figure.
• A beep or a shutdown: the supply's limit was reached for that instant.

None of that means the appliance is faulty. It's how many motors behave. A reference page on inrush current says alternating-current motors may draw several times their normal full-load current when first energized, for a few cycles of the input waveform². A "cycle" is one up-and-down swing of the alternating current. So the spike is very short.

Why both numbers matter

Your backup supply has to pass two separate tests.

First test: can it carry the total running watts of everything switched on together? That's the long-haul question. It decides how many things you can leave running.

Second test: can it carry the starting burst of whatever is about to start, on top of what's already running? That's the short-haul question. Fail it and the supply may shut off for a moment, even though the running total was fine.

A common way to size this, from the same maker's guide, is to add up the running watts of everything you plan to use. Then find the device with the highest starting wattage and add that number on top¹. That treats one motor starting at a time, which is a sensible habit to keep. If two motors start together, their bursts stack. The maker's advice on this is to make sure the total load and combined startup surges don't exceed the rated output³.

Where to find the numbers

Running figures are usually on the appliance label or in its manual. Starting figures are often missing. Some makers list them, many don't. Where a number isn't given, don't guess and don't treat a rule of thumb as fact. Check the manual for your own model, or ask the maker's support team.

Resistive things such as a kettle or a toaster don't have much of a surge story. Motors and compressors do. The page on why motors surge goes into the cause, and reading an appliance label shows where the printed numbers live.

A habit worth building

Write both numbers down for the few things you'd want running in an outage. Put the running figure in one column and the starting figure in another, with a blank where the starting figure is unknown. A blank is honest. A made-up number isn't.

Any change to the wiring of your house, such as connecting a battery system to the main circuits, is work for a licensed electrician. Plugging a single appliance into a unit's own outlet is a different matter, and the unit's manual covers it.