Running mains devices from your car: how to size a power inverter

A car gives you 12V direct current. A laptop charger, a camera battery charger or a small appliance wants mains alternating current. A power inverter sits between the two and converts one into the other, giving you a mains socket in the vehicle.

Most of the frustration people have with inverters comes from one thing: buying by price rather than by load, and discovering afterwards that the number on the box was not the number that mattered.

Work out the load first

Start from what you actually intend to run, not from what you might one day run.

Every mains device states its power draw in watts, or states volts and amps, which multiply to give watts. It is on the same small-print label as the voltage. Write down the figure for each thing you plan to plug in, and add them together if they will be running at the same time.

Then add headroom — comfortably more than the total, rather than an inverter rated at exactly your number. An inverter run permanently at its ceiling runs hot, and heat is what shortens its life.

Continuous versus peak: the number that catches people out

Inverters are advertised with two ratings and they mean very different things.

The continuous rating is what the unit can supply indefinitely. This is the real specification and the one to size against.

The peak (or surge) rating is what it can supply for a brief moment. It is often roughly double the continuous figure, and it exists because some devices draw a large gulp of power at the instant they start up before settling down.

Anything with a motor or a compressor behaves this way — a small pump, a fridge, some power tools. A device that runs happily at 150W might momentarily demand several times that when it kicks in. Size against continuous, and treat peak as the margin that lets a motor start rather than as the figure you are buying.

Purely electronic loads — laptop chargers, camera chargers, phone chargers, lights — have very little startup surge, which is why they are the straightforward case.

The 12V accessory socket has its own ceiling

This is the limit people most often run into, and it has nothing to do with the inverter.

A car's accessory socket — the cigarette-lighter socket — is protected by a fuse, and that fuse sets a hard ceiling on what you can draw through it regardless of what the inverter is capable of. Plug a large inverter into a small socket circuit, load it up, and the fuse does its job.

In practice that means socket-powered inverters suit modest loads: charging laptops and camera gear, running a small light, powering a device or two. Larger inverters are wired to the battery directly with appropriately rated cable and their own fuse, which is a different installation and a different conversation.

If you are unsure which category you are in, the honest test is the load figure you wrote down earlier. Modest totals are comfortable through a socket. Large ones are not, whatever the inverter claims.

Don't flatten the battery

An inverter draws from the car battery, and a car battery is designed to start an engine, not to run appliances for hours.

• With the engine running, the alternator is replacing what you draw, and sustained use is reasonable.
• With the engine off, you are spending the charge that starts the car. It goes faster than people expect.

The practical habit: run the engine for anything sustained, and treat engine-off use as short and occasional. If your use is genuinely long stationary sessions, that is a case for a dedicated leisure battery rather than a bigger inverter.

Where you put it matters

Inverters generate heat as a by-product of the conversion, and most have a fan and vents to deal with it.

• Leave the vents clear — not under a bag, not wedged in a footwell.
• Keep it out of direct sun on a dashboard, especially in a UAE summer, where the ambient temperature is already working against it.
• Give it somewhere it will not slide around, since a heavy object loose in a cabin is a hazard entirely separate from electricity.

A short buying checklist

• Add up the watts of everything you will run at once.
• Choose by the continuous rating, with headroom above your total.
• Check whether anything you will run has a motor, and therefore a surge.
• Check your vehicle's accessory socket limit if you are not hard-wiring.
• Plan to have the engine running for anything sustained.
• Give the unit ventilation and somewhere secure to sit.

Where to look next

Our car power inverters list their continuous and peak ratings and their output sockets on each product page, so you can check them against the total you worked out above. Tell us what you are running and we will tell you which one fits — including when a smaller one is the right answer.