What does air conditioning actually cost to run in the UK?

"Lovely, but what will it cost me to run?" is the most sensible question in air conditioning — and the one most often answered with hand-waving. Let's do actual arithmetic.

The formula (30 seconds, promise)

Running cost = electrical input (kW) × hours × your unit rate (p/kWh).

The trap: an air conditioner's cooling capacity (the advertised kW or BTU) is not its electrical draw. The ratio between them is the efficiency (EER/SEER). Portables typically achieve an effective 1.0–1.5 cooling-out per electricity-in once hose losses are counted; modern splits achieve 3–4+.

Worked examples at ~27p/kWh

(Illustrative unit rate for 2026 — check your own tariff; the method matters more than the pence.)

A 9,000 BTU (2.6 kW cooling) portable draws roughly 1.0–1.1 kW electrical.

• Per hour: ~27–30p
• Hot evening (6 hours): ~£1.70
• Punishing week (10 h/day × 7): ~£20

A 2.5 kW split system draws roughly 0.6–0.8 kW at full tilt, and far less once the room is holding temperature (inverter units modulate down):

• Per hour at full: ~16–22p; holding a cooled room often under 10p
• Same punishing week: roughly £8–£12

The whole summer? A genuinely hot UK summer might demand 250–350 hours of real cooling. Portable: ~£70–£105. Split: ~£30–£55. Both are a fraction of what most people guess — the fear of "American power bills" doesn't survive contact with arithmetic, mainly because UK heat, for all its misery, is measured in weeks not months.

Five ways to cut the bill further

1. Size correctly. Undersized units run flat-out forever (max cost, poor comfort). Correctly sized units cruise. Start with our calculator.
2. Seal the portable's window kit properly. The single biggest portable efficiency leak is hot air re-entering around a badly fitted hose. A sealed kit is worth degrees and pounds.
3. Cool the room before peak, then hold. Pre-chill at 4–6pm (and pre-bed), then let the thermostat hold. Holding is far cheaper than crash-cooling at the hottest hour.
4. Shade first, cool second. Every kWh of sunshine you block with blinds/film is a kWh you don't pay to pump out. Shading can halve the cooling load of a sunny room.
5. Set 24°C, not 18°C. Each degree lower costs roughly 5–10% more energy. 24°C with a whisper of air movement feels excellent when it's 33°C outside.

Efficiency labels, decoded quickly

When you compare units: EER is full-tilt efficiency; SEER is seasonal efficiency (more realistic); bigger is better in both. A SEER 6.1 split beats a SEER 5.1 by ~20% on the bill. For portables, treat advertised EER sceptically and prefer dual-hose or well-reviewed sealed-kit designs — the test bench doesn't include a draughty sash window.

The honest conclusion

Running cost is a weak reason to avoid cooling in the UK and a strong reason to choose equipment carefully. The expensive mistakes are all made at the buying stage — wrong size, wrong type, panic purchase at heatwave prices. Which is, once more, the argument for deciding calmly in advance: reserve priority, see honest final pricing when it's confirmed, and put your £10 toward the right unit instead of a rushed one.