Every argument about space heaters ends up in the same place: is it actually cheaper than turning the central heating up? The answer is arithmetic, and it is simpler than the marketing on either side suggests. A heater's running cost is its wattage divided by 1,000, multiplied by the hours you run it, multiplied by your electricity rate per kilowatt-hour. That is the whole formula. Everything else is a detail on top of it.
Wattage is the only number on the box that matters
A 1500W heater running for four hours uses six kilowatt-hours. At 17 cents per kilowatt-hour that is a little over a dollar a day, or roughly thirty dollars a month if you run it every evening. An 800W heater doing the same four hours uses 3.2 kilowatt-hours — about 54 cents. Over a winter that gap is not trivial, and it is the single biggest reason we rank lower-wattage units that still heat a room quickly above brute-force 1500W boxes.
There is a catch, and honest reviews should say it: a lower-wattage heater cannot brute-force a large, draughty space. It works when the room is small to medium, reasonably sealed, and you are in it. That describes a bedroom, a home office, a nursery or a reading corner. It does not describe an uninsulated garage in January.
Why heating one room beats heating eight
Central heating is efficient per unit of heat delivered, but it delivers that heat everywhere — including the rooms nobody is standing in. Heating accounts for a large share of a typical household energy bill, and a good chunk of that is spent warming hallways, spare bedrooms and the space above your head. Zone heating flips the logic: you set the thermostat lower for the whole house and add local heat exactly where a person is sitting.
In practice most households that make real savings do two things at once. They drop the central thermostat by a few degrees, and they put a small heater in the one or two rooms in constant use. Doing only the second thing adds cost. Doing both is where the winter bill actually moves.
The hidden costs nobody quotes
Cheap heaters fail in predictable ways. A thermostat that overshoots means the unit runs longer than it should. A fan that roars means you turn it off at night and wake up cold. No timer means it runs while you sleep. Each of these adds kilowatt-hours or subtracts comfort, and neither shows up in the price comparison.
The features worth paying for are the ones that shorten runtime: an accurate thermostat, a timer, an ECO mode that cycles rather than blasting, and enough output to hit your target temperature fast so the heater spends most of its time idling instead of working.
A quick sanity check before you buy
Multiply the wattage by the hours you honestly expect to run it. If that number is bigger than the reduction you will make on your central heating, the heater will cost you money rather than save it. If it is smaller — and for a single occupied room it usually is — the maths works, and the only remaining question is which unit heats that room fastest for the fewest watts.