What Size Electric Radiator Do I Need? A Room by Room Guide
John Phelan
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Sizing an electric radiator correctly matters twice over. Undersize it and it runs flat out all evening without ever satisfying its thermostat, which is the most expensive way to run any heater. Oversize it and you paid for capacity you never use. The good news is that sizing is simple arithmetic, and this guide gives you the numbers for typical Irish rooms.
How do I work out what size electric radiator I need?
Allow roughly 100 watts per square metre of floor area. Measure the room's length by width in metres, multiply the two, then multiply by 100 for the wattage.
A 3m by 4m bedroom is 12 square metres, so it needs around 1200W. That rule assumes a reasonably insulated room with a standard ceiling height of about 2.4 metres. For rooms in older houses, rooms with large windows, or rooms with more than one outside wall, go up a size rather than down. Heat rises, so rooms with high ceilings also need more than their floor area suggests.
Electric radiator size chart for Irish rooms
| Room | Typical size | Radiator wattage |
|---|---|---|
| Box room or small office | 6 to 8 m2 | 600W |
| Single bedroom | 8 to 10 m2 | 900W |
| Double bedroom or home office | 10 to 13 m2 | 1200W |
| Large bedroom or small sitting room | 13 to 16 m2 | 1500W |
| Living room | 16 to 20 m2 | 1800W to 2000W |
| Open plan living space | Over 20 m2 | Two radiators, sized per zone |
These bands line up with the five sizes in the Titan Smart WiFi Electric Radiator range, which runs from a 600W four fin model up to an 1800W nine fin model.
Is it better to go bigger or smaller if I am between sizes?
Go bigger. A slightly oversized radiator reaches temperature quickly and settles into economical cycling. An undersized one runs continuously and still leaves the room cold.
This feels backwards to many buyers, because the bigger radiator has a higher maximum draw. But the wattage rating is what the element draws while heating, not what it uses all evening. A 1500W radiator that holds a room at temperature while cycling on for twenty minutes in the hour costs less to run than a 900W unit drawing continuously and never getting there.
Do insulation and glazing change the answer?
Yes. Poor insulation, single glazing, high ceilings and more than one outside wall each push the requirement up. New well insulated builds can size down slightly.
For a room in an older uninsulated house, work off 120 to 130 watts per square metre rather than 100. If the room never held heat well with the old heating, that is the signal to size up. Conversely, a well sealed new build or a small room over a heated space can manage on 80 to 90 watts per square metre.
What about heating an open plan space?
Split the load across two radiators at opposite ends of the space rather than one very large unit in the middle.
A 30 square metre kitchen living space needs about 3000W of capacity, and two 1500W radiators heat it far more evenly than one 2000W unit working beyond its coverage. Two radiators on WiFi control can also run as one zone from the same app schedule.
Where can I get sizing advice before I buy?
Call Shamrock Electrical on 01 401 9907 with your room measurements and we will size it with you before you order. We are based in Rathcoole, Co. Dublin, just off the N7.
Browse the full range in Electric Radiators & Heaters, including wall mounted electric heaters, with free delivery to every county in the Republic of Ireland and no minimum spend. For which type to choose once you know the size, see our guide to the most efficient electric heaters and radiators.