Is a mini-split cheaper to run than a window air conditioner?

Yes. A modern inverter mini-split (SEER 18–22) draws meaningfully less power than a fixed-speed window unit (EER ~10.5) for the same BTU — roughly 830 W vs 1,140 W on a 12,000 BTU unit, or about £0.20/hr vs £0.28/hr in the UK. It's also far quieter and most also heat.

Is a mini-split worth it over a window unit?

If you cool the same room for years, yes — the lower running cost, near-silent operation, heating capability and unblocked window repay the higher purchase and install cost over several seasons. A window unit wins on upfront cost and DIY install.

Can I install a mini-split myself like a window unit?

No. A window unit slots into the opening and plugs in. A mini-split has an outdoor compressor and refrigerant lines that must be installed by an F-Gas (UK/EU) or EPA 608 (US) certified engineer, unless you use a pre-charged DIY kit and still handle the mounting and wiring.

Mini-Split vs Window Air Conditioner: Efficiency, Cost & Which to Buy

Published: May 25, 2026

A window unit and a mini-split are the classic North-American “cool one room” choices — and the decision is a clean trade: the window unit is cheap and self-installed, the mini-split is efficient, silent and permanent. Here’s the gap in numbers.

Side by side (12,000 BTU)

	Window	Mini-split
Typical efficiency	EER ~10.5	SEER 18–22 (effective EER ~13–15)
Power draw	~1,140 W	~800–900 W
Cost/hr — UK @ £0.245	£0.28	£0.20–£0.22
Cost/hr — US @ $0.16	$0.18	$0.13–$0.14
Install	DIY, near £0	Professional, £400–£1,000
Noise	50–60 dB	19–30 dB (indoor head)
Blocks a window?	Yes	No (small wall penetration)
Also heats?	Rarely	Yes — reverse-cycle heat pump
Lifespan	~8–10 yrs	~15–20 yrs

Why the mini-split runs cheaper and quieter

Both reject heat outside, so neither wastes cooled air the way a portable does. The mini-split pulls ahead on two fronts: its inverter compressor modulates output for a much higher SEER, and that compressor sits outside, leaving only a quiet coil indoors. A window unit’s compressor is in the box in your window, which is why it’s noisier and cycles fully on and off.

Over a 90-day summer at 8 hours a day in the UK, the window unit costs about £201 and the mini-split about £148 — roughly £50 a year less, plus the mini-split usually heats in winter too, where a reverse-cycle heat pump (COP 3–4) is far cheaper than electric heating.

Why a window unit can still be the smart buy

Upfront cost — cheap to buy, and you fit it yourself with no engineer.
Renting — nothing permanent, take it with you.
Quick fix — cooling a room this heatwave, not for the next 15 years.

The verdict

If you have a suitable sash window and want cooling now, cheaply, with no installer, a window unit is hard to beat. If you’ll cool the same room for years and want the lowest running cost, near silence, an unblocked window and winter heating, the mini-split is the better long-term machine — and its efficiency and lifespan repay the install.

Next: split vs mini-split · can an AC heat your home? · SEER vs EER explained