Why "100% Efficient" Doesn't Mean Cheap to Run

Efficiency tells you how much of the energy you buy becomes useful heat. Cost tells you what that energy costs. Those are different questions, and conflating them is the single most common mistake in heating marketing.

Electric resistance heating is close to perfectly efficient in the sense the word is usually used: essentially all the electrical energy drawn ends up as heat in the room. There is no waste flue, no unburnt fuel, nothing lost to a chimney. And that fact, entirely true, tells you nothing whatsoever about whether the heater is cheap to run.

Why the efficiency claim is true and useless

Push electricity through a resistive element and it heats up. The energy has nowhere else to go — some leaves as light, some as sound, but light and sound absorbed by the room’s surfaces become heat too. In a sealed room, essentially 100% of the electrical energy drawn ends up as heat in that room.

This is true of every resistive electric heater. A panel heater, an oil-filled radiator, a fan heater, a quartz infrared bar, the heating element in an electric fireplace: all of them, at equal power draw, put the same amount of heat into the space. There is no efficient brand and no inefficient brand.

Which means “100% efficient” is not a competitive claim at all. It is a description of the category. When it appears in marketing as though it were a feature, it is telling you the appliance obeys physics.

The useful consequence is a negative one: stop looking for a more efficient electric heater. It doesn’t exist. What differs between electric heaters is where the heat goes first, how well they hold a set temperature, how quietly they do it, and how safely — not how much heat you get per unit of electricity.

Where cost actually comes from

Running cost is rated watts ÷ 1,000 × hours run × your price per kilowatt-hour. The full method, including why “hours run” is the hard term, is in what does a space heater actually cost to run?

Notice what’s in that expression and what isn’t. Efficiency isn’t in it, because for resistive heating it’s effectively fixed at one. The three things you can influence are power, hours, and price. And the price of a kilowatt-hour of electricity is set by your tariff, not by your appliance.

That’s why electric heating can be simultaneously the most efficient way to convert energy to heat and the most expensive way to buy heat. In many markets a kilowatt-hour of electricity costs several times a kilowatt-hour of gas. A gas appliance that loses a meaningful share of its heat up a flue can still deliver usable heat more cheaply, because it started from cheaper energy. Efficiency compares the output to the input; cost compares the output to your bill.

The thing that genuinely beats 100%

There is one class of heating that appears to break the ceiling, and understanding why sharpens the whole distinction.

A heat pump doesn’t convert electricity into heat. It uses electricity to move heat from outside to inside, the same way a refrigerator moves heat from its interior to your kitchen. Because moving heat takes less energy than creating it, a heat pump delivers more heat energy to the room than the electrical energy it consumed. Its performance is described by a coefficient of performance rather than an efficiency, and that coefficient is greater than one — considerably so in mild conditions, less so in severe cold, because there is less ambient heat available to move.

This is not a plug-in appliance decision, and installation is a job for qualified professionals, so it’s outside what this site covers. It’s here because it’s the counterexample that shows what the resistive ceiling really is: resistive heating is not “as good as heating gets”, it is the floor that every electric heater sits on and none can rise above.

What “eco” modes really do

Once you accept that efficiency is fixed, every energy-saving feature has to work through power or hours. That makes them easy to evaluate:

A lower power setting reduces the rate of heat output and reduces cost proportionally. Real, and honest.
A lower thermostat set point reduces the temperature difference the room has to hold, which reduces heat loss, which reduces the time the element runs. Real, and often the largest single saving available.
A timer reduces hours. Real, and the most reliable of the lot because it doesn’t depend on the room.
A thermostat where there was none reduces hours by cycling the element off once the room reaches temperature. Real, and its size depends on how well the room holds heat.
A better element material, a fan, a ceramic core, a “far infrared” claim changes how the heat is delivered and how quickly you feel it. It does not change heat per kilowatt-hour. It can still save you money by letting you run fewer hours or at a lower setting — but that’s a hours-and-power saving, not an efficiency one.

If a claimed saving can’t be traced to power, hours, or set point, it isn’t a saving.

The comparison worth making instead

Because the efficiency question is settled, the real decision is about delivery and duration:

Delivery: does the heat need to reach the whole room or just you? Radiant heat aimed at the occupied zone gets you comfortable at fewer watt-hours than warming the entire air volume, which is the practical case made in infrared vs. convection heaters.

Duration: how long does the element actually have to run? That depends on the room’s heat loss, which is a sizing question, and on draughts, which are usually cheaper to fix than to heat past.

What to actually do

Ignore every efficiency claim on an electric heater. They are all the same and the claim is not informative.
Take the rated wattage and run the cost arithmetic with your own tariff before buying.
Compare against your alternative heat source on cost per unit of heat, not on efficiency — the cheaper fuel can win despite being less efficient.
Attack hours and set point, since they’re the only terms you control.
Draught-proof and insulate before upgrading an appliance. That reduces hours for every heater you will ever own in that room.