How to size a wall-split AC for your office

The single biggest mistake we see on commercial AC installs is the wrong-sized unit. Too small and it never reaches setpoint; too big and it short-cycles, wears out compressors and costs you more in electricity. Here's the sizing logic our engineers actually use.

Step 1, Start with the room area

Measure the floor area in square metres. For a standard UK office with 2.4m ceilings, you need roughly 150 watts of cooling capacity per square metre as a starting point. A 30m² office? Start at 4.5kW.

Step 2, Add for heat sources

Now adjust for what's actually in the room:

• +150W per person in normal office work
• +300W per desktop computer
• +1000-2000W for a server rack
• +30% if west or south facing with single-glazing
• +50% for kitchens or print rooms

Step 3, Pick the unit one size up

Once you've added everything, round UP to the next standard kW size (2.5, 3.5, 5.0, 7.1, 10kW etc). Never round down, you'll regret it on the hot August afternoons.

Four common mistakes

1. Sizing for today, not the worst day. Your unit needs to cope with 32°C peak summer, not the 22°C April day you tested.
2. Ignoring the pipe run. Anything over 15m needs extra refrigerant and capacity loss accounting.
3. One unit, two rooms. Walls block airflow. Two rooms means two units or a multi-split, not one big one.
4. Forgetting heat-pump (heating) mode. A unit sized for cooling at 4.5kW may only deliver 3kW of heating in January. Check both numbers.

What to do next

If you'd rather not work this out yourself, ring us on 0203 488 1889 with the room dimensions and what's in it. An engineer will spec the right unit while you're on the call. No call-out fee for the sizing conversation.