Radiator Power Calculator – Calculate Heating Requirements
Calculate required radiator heating power for any room
Table of Contents
How to Use
- Select your unit system (meters or feet)
- Enter room dimensions (length, width, height)
- Select insulation quality of your building
- Enter total window area in the room
- Specify desired temperature increase (typically 20°C or 68°F)
- Click calculate to see required radiator power
Understanding Heat Loss
Heat loss from a room occurs through walls, windows, floors, ceilings, and air infiltration. The rate of heat loss depends on the temperature difference between inside and outside, the surface area, and the thermal insulation quality of building materials.
Windows are the weakest point in building insulation, losing 5-10 times more heat than well-insulated walls. Double or triple-glazed windows significantly reduce heat loss compared to single-pane windows.
Insulation Quality Impact
| Insulation Level | Heat Loss Factor | Typical Building Type |
|---|---|---|
| Poor | 50 W/m³·K | Pre-1970s buildings, single glazing, minimal wall insulation |
| Average | 35 W/m³·K | 1970s-2000s construction, some insulation, double glazing |
| Good | 25 W/m³·K | Modern buildings, good insulation, double/triple glazing |
| Excellent | 15 W/m³·K | Passive house standard, superior insulation, triple glazing |
Radiator Sizing Guidelines
Once you know the required power in Watts, you can select appropriate radiators. Radiators are rated in Watts at specific conditions (usually ΔT50: 50°C temperature difference between radiator and room).
- Single Panel Radiators: 400-600W per meter length (compact, lower output)
- Double Panel Radiators: 800-1200W per meter length (most common)
- Triple Panel Radiators: 1200-1800W per meter length (high output, bulkier)
- Designer Radiators: Variable output depending on design
- Underfloor Heating: 80-100W per square meter of floor area
Typical Temperature Increases
| Climate | Outdoor Temp | Indoor Target | Temp Increase |
|---|---|---|---|
| Mild Winter | 0°C (32°F) | 20°C (68°F) | 20°C (36°F) |
| Cold Winter | -10°C (14°F) | 20°C (68°F) | 30°C (54°F) |
| Severe Winter | -20°C (-4°F) | 20°C (68°F) | 40°C (72°F) |
| Bathroom/Warm Room | -10°C (14°F) | 24°C (75°F) | 34°C (61°F) |
Frequently Asked Questions
- What temperature increase should I use?
- Use the difference between your desired indoor temperature (typically 20-22°C or 68-72°F) and the coldest expected outdoor temperature for your location. For most moderate climates, 20°C (36°F) increase is standard. Colder climates may need 30-40°C (54-72°F).
- How much more heat do windows lose compared to walls?
- Single-pane windows lose about 5-6 W/m²·K, while well-insulated walls lose only 0.3-0.5 W/m²·K—approximately 10-15 times less. Double-glazed windows reduce this to about 2.8 W/m²·K, and triple-glazed to 1.5-2.0 W/m²·K.
- Can I use one large radiator instead of multiple smaller ones?
- While the total power output would be the same, multiple smaller radiators distributed around the room provide more even heat distribution and better comfort. This is especially important in large rooms or rooms with multiple outside walls.
- Should I size radiators for the coldest day of the year?
- Yes, radiators should be sized for the coldest expected conditions to ensure comfort during peak demand. Modern thermostatic radiator valves (TRVs) allow radiators to operate at lower output during milder weather, preventing overheating and saving energy.
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