Heating · Sizing
What Size Electric Heater Do I Need?
Too small and the heater runs flat out and never warms the room; too big and you waste money. Here's a simple way to size an electric heater to a room, and what changes the answer.
As a rough guide for a reasonably insulated Irish room, allow about 100 watts of electric heating per square metre of floor. So a small 6m² room needs roughly 0.6-0.75kW, a 12m² room about 1.2-1.5kW, and a 20m² living room about 2kW. Add capacity for poor insulation, high ceilings, lots of glazing or an exposed/north-facing room; reduce it for well-insulated modern rooms.
Once you've got the wattage, check the running cost and pick the heater type. Then browse the electric heating range.
Well-insulated room, ~100W/m²
Match the room size to a suggested heater wattage. These figures assume a reasonably well-insulated Irish room — see below for when to size up.
| Room size | Approx. area | Suggested heater |
|---|---|---|
| Small (e.g. small bedroom, study) | 6-8 m² | 0.6-0.8 kW |
| Medium (bedroom, kitchen) | 10-14 m² | 1.0-1.5 kW |
| Large (living room) | 16-22 m² | 1.5-2.0 kW |
| Very large / open plan | 24 m²+ | 2.0 kW+ or two heaters |
This is a guide — when in doubt, size up slightly and rely on the thermostat.
What changes the answer
Add roughly 20-50% on top of the quick sizing table for any of the following. A modern, well-insulated room may need less.
- Poor or no insulation. Older walls, floors or attics without upgrades lose heat faster.
- Single glazing or lots of windows. Glass loses heat much faster than a wall.
- High ceilings. More air volume to heat than the floor area suggests.
- North-facing or exposed rooms. Less natural warmth and more wind exposure.
- Rooms above garages or passages. Cold space below pulls heat away.
Wattage sorted?
With a target wattage in hand, compare the heater types to find the best fit for the room, and check likely running costs before you buy.
Frequently asked
How many watts do I need to heat a room?
Is a 2kW heater enough for a living room?
What happens if the heater is too small?
Find your right size
Brennan's Electrical is an Arklow-based electrical and appliance retailer. We stock electric heating, cooling and large kitchen appliances — with local delivery, installation and old-appliance removal that the big chains don't match. Browse the ranges online or talk to us for advice on the right model for your room and budget.