How many W/m2 are needed? The specification / construction determine the heat loss of the room or dwelling. To maintain equilibrium the energy input must balance the heat lost. Electricity consumption is not governed by the W/m2 rating but by the thermostat and how it is programmed to control the underfloor heating system.
If you have heat loss data, you can simply choose a kit with the power rating that you need and then lay the heating cable at a cable spacing that will enable the whole length of cable to be used evenly across the floor area. If the cable spacing data in the table above doesn't suit your application you can easily calculate the cable spacing required
Don't worry if you don't have heat loss data! You can simply choose a suitable W/m2 rating and read off the appropriate heating mat for your particular requirement from the table above. 150W/m2 is generally suitable for most primary heating requirements but in well or very well insulated dwellings to the latest design specifications 130W/m2 or even less can be suitable. 200W/m2 is generally suitable for most primary heating requirements in bathrooms and areas of higher heat loss such as conservatories and other similar constructions.
If in doubt, use a greater value of W/m2 rating. It is NOT the W/m2 that determines running costs but the thermostat setting and insulation provision. | 
