The internet is awash with adverts for various portable heaters, with claims that they will heat your house for pennies. Some are marketed as the “Tesla of the heating industry” (despite being nothing to do with Elon Musk’s carmaker), while others claim they can “heat up a house in three minutes”.

It’s an appealing message, particularly during cold snaps when energy bills are high and many households are looking for quick fixes. But are any of these claims remotely true?

The short answer is no. And the reasons why are rooted in some very basic physics.

One key detail often missing from these adverts: almost all electric heaters are already close to 100% efficient. That doesn’t mean they are cheap to run – only that nearly all the electricity they use ends up as heat.

Electric heaters work by passing a current through a wire, which then heats up. That heat is then transferred to the room either by warming the air (distributed with a fan) or by radiating heat directly from a hot surface. Even things that sound like “losses”, such as friction in a fan motor, still end up as heat inside the room.

This means there is no clever design or secret component that can make one plug-in heater fundamentally more efficient than another. Electricity use is almost entirely converted into heat. So when a product claims to heat more while using less electricity, alarm bells should ring.

Some adverts go further, claiming a small portable device can heat an entire house in just a few minutes. This is where the numbers really matter.

Heating a typical home means warming hundreds of cubic........

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