Choose the wrong size panel heater and you will either be shivering in a room that never quite warms up, or quietly paying for a capacity you never needed in the first place. Getting the wattage right is one of the most practical things you can do before you buy, and it is simpler than most people think once you know what to look out for. 

Start with Floor Area, then Adjust

For a reasonably insulated modern UK room with standard 2.4m ceilings, the table below gives you a solid starting point. The underlying rule is roughly 70W to 100W per square metre of floor area when the panel is the primary heat source. Use the lower end for a new, well insulated room and the upper end for anything older or more exposed. 

Room Size  Typical Use  Starting Point  Example from our range 
Up to 4m² Cloakroom, ensuite, small hall 250W to 300W Dimplex IRP 300W
4 to 6m² Small bedroom, study  400W to 500W Trianco Aztec 500W
6 to 9m² Bedroom, home office  600W to 700W Dimplex IRP 600W
9 to 12m² Double bedroom, small lounge  800W to 1kW Dimplex IRP 850W
12 to 16m² Lounge, large office  1.1kW to 1.25kW Herschel Comfort 1250W
Over 16m² Open plan area  Two panels, not one  Contact us for recommendations

 

Adjust for the Things That Actually Drive Heat Loss

Floor area is only a proxy. These factors matter just as much when sizing a heater correctly. 

  • Ceiling height - above about 2.7mm, add roughly 10% per extra half meter. You are heating a bigger volume and, with convection, losing more of it upwards. 
  • External walls and glazing - a corner room with two external walls and a large window can need 25% to 40% more than the table suggests. A room with one internal wall and a small window needs less
  • Insulation - solid wall Victorian room is a completely different problem from a 2015 build. Where insulation is poor, fix that before sizing a heater, because you will be paying to heat it every winter otherwise
  • Conservatories and garden rooms - treat these as a special case and go up a size at minimum. Large glazed areas lose heat fast and infrared handles them better than convection does.
  • Primary or supplementary? - If the panel is topping up existing heating, you can size well below the table. If it is the only heat in the room, do not.

Two Panels Usually Beat One Big One

For anything over about 16m², or any long or L shaped room, two smaller panels placed apart give noticeably more even comfort than a single large one. This is especially true of infrared, because infrared needs line of sight to what it is warming. Two 600W panels at opposite ends of a 20m² room will feel better than one 1.25kW panel at one end, for a similar running cost.

Bathrooms: Check the Zone, Not Just the Wattage

Bathrooms have electrical zone requirements, and a panel heater must be rated for the zone it is installed in. This is not something to guess at. If you are heating a bathroom, tell us the layout and we will confirm what is suitable before you order.

Low Surface Temperature: Where It Matters Most

In care homes, schools, nurseries and anywhere with vulnerable users, a low surface temperature panel is not optional. Look for panels with casings designed not to burn on contact. If your setting has a duty of care, start there rather than sizing on wattage alone.

What This Guide Does Not Do

It is a starting point, not a heat loss calculation. For a whole house, a commercial space, or anywhere you are replacing a wet system, get the calculation done properly.

  • It assumes the panel is thermostatically controlled. Without a thermostat, sizing barely matters because the heater will simply run.

  • Wattage sets the maximum output, not the running cost. A correctly sized panel on a good thermostat cycles off and costs less than an undersized one struggling at full draw