How to work it out for your own house
You need three numbers and none of them are secret.
First, the power the unit draws. It's on the spec sheet, in kilowatts, and it isn't the same as the cooling output. A 2.5kW cooling unit doesn't pull 2.5kW of electricity. It's usually well under a third of that, because the thing moves heat around instead of creating it.
Second, your electricity unit rate in pence per kilowatt-hour. That's on your bill.
Third, how long it runs at full power. This is where people go wrong. An inverter system pulls hard for the first twenty minutes to bring the room down, then throttles right back to hold it. Average draw across an evening is nothing like the peak.
Multiply the three and you've got your figure. Ask any installer for that first number on the exact unit they're quoting. If they won't give it to you, that tells you something.
Why UK running costs are lower than people expect
Three reasons, all of them specific to this country.
Our summers are short and spiky. You're not cooling a house from May to September. You're cooling a bedroom for a handful of genuinely rotten weeks, and often only overnight.
Our houses are built to hold heat in. That's a curse in July, but it also means a well-insulated room stays down once it's got there, on very little input.
And the kit moved on. Inverter compressors vary their speed instead of clunking on and off at full tilt. The gap between a modern split system and the portable unit somebody lent you in 2018 is not small.
Where the money really goes wrong
Running cost is rarely the problem. Sizing is.
An oversized unit cools the air fast, satisfies its thermostat, shuts down, and leaves the room clammy. Twenty minutes later it does it again. That short-cycling is both uncomfortable and wasteful.
An undersized unit runs flat out and never gets there, which is worse.
Getting the size right for the room, its glazing and which way it faces is most of the job. It's also the part you can't do from a drop-down menu without asking about the actual room.
Most systems we fit run in reverse too. In October and April that'll warm a room for a good deal less than firing the whole central heating system for one cold bedroom.
Common questions
Is it cheaper than the central heating in spring?
For one room, very often yes. Warming a single bedroom with a reverse-cycle split usually beats bringing a whole gas system up to temperature to take the chill off it.
Will it push my electricity bill up a lot?
Across a UK summer, for one or two rooms used mainly at night, most households don't notice it the way they expected to. We'll give you the draw figure for the specific unit so you can do the sum yourself instead of taking our word for it.