What Does a Space Heater Actually Cost to Run?

Rated watts ÷ 1000 × hours run × your price per kilowatt-hour. That’s the whole calculation, and you can do it on your phone in ten seconds. The only input we can’t give you is the tariff — that varies by region, supplier, and time of day, so take it from your own bill rather than from any website, including this one.

The complication isn’t the formula. It’s that “hours run” is almost never the same as “hours switched on.”

The formula, properly

A heater’s rated power is printed on the unit and in the manual, in watts. Divide by 1,000 to get kilowatts. Multiply by the hours it actually runs to get kilowatt-hours. Multiply that by your per-kWh price.

Two things worth knowing about the rating:

Resistive heaters draw essentially their rated power whenever the element is on. There’s no efficiency spread between models to hunt for. A heater at a given wattage produces heat at that rate, whatever the marketing says.

Multiple heat settings are usually just lower wattages, often with a second element that stays off. A “low” setting genuinely costs proportionally less, because it draws proportionally less.

Look up the rated wattage rather than any “equivalent” or “effective” figure in the marketing.

Why “hours run” is the hard part

A heater with no thermostat runs continuously while switched on. Hours on equals hours run, and the cost is the full calculation above.

A heater with a thermostat cycles: it heats until the room reaches the set point, then the element switches off until the temperature falls back. Over an evening, the element might be on for a fraction of the time the heater is switched on — and that fraction is what you pay for.

This is why thermostatic control is the feature that most affects running cost, and why comparing two heaters purely on wattage is misleading if one cycles and the other doesn’t. The duty cycle depends on the room, though, not the heater: a well-insulated room reaches its set point and stays there, so the element is off most of the time. A draughty one never quite gets there, so the element runs continuously and you’re back to the full cost.

Practical version: in a well-sealed room, a thermostatic heater costs substantially less to run than its wattage suggests. In a leaky one, it costs close to the full amount. Insulation and draught-proofing change your heating bill more than any appliance choice.

Timers, eco modes, and what they actually do

Timers reduce hours, which reduces cost proportionally. Straightforward and genuinely useful, particularly the “off after N hours” kind that catches the case where you fall asleep.
“Eco” modes are usually a lower wattage setting, a lower thermostat set point, or both. Real, but not magic — it’s the same arithmetic with smaller numbers.
Oscillation and fans consume a small amount extra and can make a room feel warmer sooner by distributing heat. Usually a net positive for comfort per unit spent.
Smart plugs and app control save money only insofar as they reduce hours. A smart plug on a non-thermostatic heater is a reasonable way to add scheduling. A caution: high-draw appliances exceed the rating of many smart plugs — check the plug’s rated maximum load against the heater’s draw before using one.

The comparison that matters

The useful question isn’t “is this heater cheap to run” but “is heating this one room with electricity cheaper than heating the whole house with whatever the central system uses?”

There’s no universal answer, and it turns on the price of electricity relative to your central heating fuel in your area. But the structure of the comparison is consistent:

Electricity typically costs more per unit of energy than gas in many markets, so a kilowatt-hour of electric heat is often more expensive than a kilowatt-hour of gas heat.
But a central system heats the whole house. If you’re occupying one room, a space heater delivers heat only where you are, and the total energy used can be much lower even at a higher unit price.

So: space heaters usually win when you’re heating one room for part of the day, and lose when you’re heating most of the house for most of the day. Compute both with your own tariffs before assuming either way.

Safety, which matters more than the cost

Running-cost optimisation is pointless if the setup is unsafe. The non-negotiables:

Plug the heater directly into a wall socket. Not an extension lead, not a multi-way adaptor, not a travel adaptor. High-draw appliances on extension leads are a well-established fire cause.
One heater per socket circuit. If a socket, plug, or cable gets more than mildly warm, stop using it and have the circuit checked by a qualified electrician. Don’t diagnose it yourself.
Respect the clearance distances in the manual on all sides, including above. Curtains, bedding, furniture, and drying laundry are the usual offenders.
Use the tip-over and overheat cutoffs — don’t defeat them, and don’t use a heater whose cutoff you know is faulty.
Never leave an unattended heater running overnight unless the manual explicitly supports it, and even then think about it.
Anything involving fixed wiring or a dedicated circuit is an electrician’s job. Not a DIY project.

What to actually do

Find the rated wattage on the unit.
Find your per-kWh price on your bill, including any standing or time-of-use variation.
Estimate hours honestly — and if the heater is thermostatic, recognise that the real figure is lower in a sealed room and close to the full figure in a draughty one.
Compare that against the cost of running your central heating for the same period, on your own tariffs.
Before buying anything: draught-proof the room. It changes the answer more than any appliance will.

If you’re deciding between heater types rather than sizing one, infrared vs. convection covers how the same wattage can feel very different.