Air con
What size air conditioning do I need for a room?
3 min read · Reviewed for accuracy before publishing
Air conditioning is sized in kilowatts to the room's heat load, and that's driven by more than floor area — glazing, which way the room faces, insulation, how many people and how much kit (a home office with a computer runs hotter). As a rough guide a typical bedroom suits a 2.0–2.5kW unit and a bright living room 3.5kW+, but a south-facing glass-heavy room needs more than its floor area suggests. Undersize it and it runs flat-out, never quite cools, and uses 20–30% more energy; oversize it and it short-cycles inefficiently. Correct sizing at the survey is the single biggest running-cost decision.
Why floor area isn't enough
Two rooms the same size can need very different units. A north-facing bedroom with modest windows is easy; a south or west-facing living room with big glazing takes a lot of afternoon solar gain and needs more capacity. A conservatory is the extreme case — mostly glass, so heat load is high. We size to the glazing and aspect, not a square-metre rule of thumb.
The cost of getting it wrong
An undersized unit is the classic false economy: it looks cheaper on the quote, then runs at full tilt on the hottest days (exactly when you need it), never reaches temperature, and burns 20–30% more electricity doing it. An oversized one cools too fast and short-cycles, which is inefficient and wears the unit. Right-sized runs gently and cheaply.
How we size it
At the free survey we look at the room's dimensions, glazing and orientation, insulation and how you use the space, then spec the unit that holds temperature efficiently. It's ten minutes that saves you money every summer the system runs — and it's part of the fixed quote, not an add-on.
Get it sized properly — free survey →
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and North London. Guides are written from real jobs, not brochures.