# What is Boiler and Radiator Capacity?
**Boiler and radiator capacity tells you how much heat your heating system can deliver, measured in watts (W) or kilowatts (kW).** Getting the capacity right is essential for a comfortable home without wasting energy.
## Boiler Capacity
The required boiler output depends on:
– The **total heat loss** of the building (through walls, roof, floor, windows and ventilation)
– The desired **indoor temperature** (typically 20°C / 68°F)
– The **design outdoor temperature** (varies by region)
– The **hot-water demand** for kitchen and bathroom
Rules of thumb for dwellings:
– Well-insulated home: 30-50 W per m²
– Moderately insulated home: 50-80 W per m²
– Poorly insulated home: 80-120 W per m²
A typical mid-terrace house (approx. 120 m²) with reasonable insulation needs a boiler of 15-24 kW.
## Radiator Capacity
Each room has its own heat requirement. The radiator output needed depends on:
– The **room volume** (length × width × height)
– The **insulation value** of the surrounding construction
– The **type of room** (living room, bedroom, bathroom)
– The **glazing area** and type of glazing
An undersized radiator leaves the room cold; an oversized one wastes energy.
## Application
For new builds or renovations, a heat-loss calculation should be carried out to determine the correct radiator size per room and the total boiler capacity.
## Related Terms
– Boiler
– Radiator
– Insulation
– Heat loss
– Energy label
Want to learn more about construction terms? Visit the full [knowledge base at fredsdiyplans.com](https://fredsdiyplans.com).