Getting the carbon film heater sizing per square metre right ensures the room heats properly without paying for unnecessary material or high bills. In this guide we calculate the net laying area, suitable power density, total installed power and thermostat capacity step by step, using a worked example.
Step 1: Find the Net Laying Area
Film is laid over the living area not covered by furniture, not the whole room. Areas under wardrobes, fixed cabinets, kitchen units and closed-base sofas are deducted, and a 10–20 cm gap is left along the walls.
Rule of thumb: the net film area is usually 60–80% of the room area.
Step 2: Choose the Power Density
| Space | Recommended Film Power |
|---|---|
| Well-insulated home, under carpet/laminate | 150 – 180 W/m² |
| Moderately insulated home, main heating | 180 – 220 W/m² |
| Poor insulation, conservatory, enclosed balcony | 220 – 240 W/m² |
| Mosque, under carpet | 150 – 220 W/m² |
We explain what the wattage means in our 180 W vs 240 W comparison.
Step 3: Calculate Total Installed Power
Installed power (W) = Net film area (m²) × Film power (W/m²)
Example
- Room: 4 m × 5 m = 20 m²
- Net film area (70%): 14 m²
- Film power: 180 W/m²
- Installed power: 14 × 180 = 2,520 W
- Current drawn: 2,520 ÷ 230 ≈ 11 A
Step 4: Determine Film Length and Width
Film is sold in rolls 50, 60, 65, 80 or 100 cm wide. The total length required is:
Film length (m) = Net area (m²) ÷ Film width (m)
In the example, with 80 cm film: 14 ÷ 0.8 = 17.5 m of film. Laying the strips parallel to the long side of the room reduces the number of connections. See our carbon heating film products for options.
Step 5: Check Thermostat and Circuit Capacity
- The thermostat’s contact rating should be at least 1.25 times the current drawn. For 11 A in the example, a 16 A or 18 A thermostat is suitable.
- For larger loads, the thermostat should drive a contactor.
- The circuit must be protected by a suitable circuit breaker and a 30 mA RCD.
Details: choosing a thermostat for carbon film heaters.
Step 6: Estimate Consumption
Installed power is not consumption – the thermostat runs the film intermittently. Use our electricity consumption guide to estimate your monthly bill.
Common Calculation Mistakes
- Using the entire room area
- Laying film under furniture
- Choosing low-power film for a poorly insulated space
- Not checking the thermostat’s current rating
Frequently Asked Questions
How many watts does 1 m² of carbon film draw?
It varies by product but is usually between 110 and 240 W; the value on the product label applies.
Is laying more film better?
Enough power to cover the heat loss is sufficient. Excess power increases the initial investment and circuit load without significantly reducing consumption.
Conclusion
Sizing a carbon film heater means considering net area, suitable power density, total installed power and thermostat capacity together. Contact us for a free calculation for your space.
