The space heater buying guide I wish customers read first
Seven decisions, in the order they matter. Get the first two right and the rest is detail.
Marcus Ellery
NATE-certified HVAC technician · 14 years in residential heating · Updated August 2026
1. Start with the room, not the heater
Measure the floor area and be honest about the draughts. A small, sealed bedroom of 100–150 square feet is served comfortably by 600–1000 watts. A large living room, a room with the door open to a stairwell, or anywhere single-glazed and unlined needs either 1500 watts or twenty minutes of draught-proofing first — and the draught-proofing is cheaper.
The rough rule technicians use is ten watts per square foot for a reasonably insulated room. Treat it as a starting point, not a law: ceiling height, window area and how sealed the door is all move the number.
2. Wattage is your running cost
Running cost is wattage ÷ 1000 × hours × your rate per kilowatt-hour. An 800W heater for four hours costs about 54 cents at 17c/kWh. A 1500W heater for the same four hours costs a little over a dollar. Over a five-month winter that difference is real money, and it buys you nothing if the smaller unit already holds your target temperature.
Buy the lowest wattage that comfortably heats your actual room. Over-specifying does not make the room warmer once the thermostat is satisfied — it just gets there slightly sooner and costs more every hour after that.
3. Certification is non-negotiable
Look for an ETL or UL mark. It means the unit was tested against a recognised US safety standard for sustained current draw, housing temperature and fault behaviour. An unmarked heater from a marketplace listing is not a bargain; it is an untested appliance you plug into a wall for eight hours at a time.
Above that bar, the features that matter are automatic shut-off on overheat, a tip-over cut-off for anything that stands on the floor, and flame-retardant housing.
4. Thermostat quality decides how much you pay
A cheap thermostat measures the air leaving the vent, overshoots your setting by several degrees, and cycles late. A good one reads room temperature and holds within a degree. That difference shows up directly on the bill, because an overshooting heater runs longer than it needs to for the same perceived comfort.
A digital display with a set-point you can read in the dark, plus a timer, is worth more in practice than an extra 500 watts of output.
5. Cord or no cord
The most common real-world heater accident is not a fire — it is a trip. A cord across a doorway, a pet that snags it, a unit knocked off a table. Wall-plug designs remove that entire category by having nothing on the floor. If you buy a floor unit, route the cord along a wall and insist on a tip-over cut-off.
Whatever you buy: plug it directly into a wall outlet. Not a power strip, not an extension lead, not a multi-way adapter shared with a kettle. This is the single most important safety rule in this guide.
6. Placement and clearance
Keep the manufacturer's clearance — usually around three feet — between the heater and bedding, curtains, upholstery and paper. Measure to the fabric, not to the wall. Do not put a heater behind furniture or under a shelf; blocked airflow trips the overheat cut-out and slows the room down.
Position it where the cold comes from, normally the window wall, so its output meets descending cold air instead of fighting it across the room.
7. Make the saving real
Zone heating only saves money if the whole-house thermostat comes down at the same time. A local heater added on top of unchanged central heating is a pure extra cost. Drop the central setting a few degrees, let the occupied room take up the slack, and the net bill falls.
Run the heater on high for the first ten to fifteen minutes to bring the room up, then hold on a lower setting or ECO mode. Holding is far cheaper than climbing.
Ready to pick one?
The full ranking applies every rule on this page to 27 heaters and names the five worth buying.