Work out how much cooling your room needs before you buy anything

Most cooling advice starts with the product. This is backwards, and it is why so many people end up with a machine that either cannot cope or was never needed.

Start with the room. How much heat is going into it, in watts. Then decide.

Where the heat actually comes from

Four sources, and they are not equal.

The glass. In a British summer this dominates almost everything else. Sun falling through an unshaded south or west facing window puts several hundred watts into a room, continuously, for hours. A single large west facing window on a July afternoon can outweigh every other source in the room combined.

The fabric. Walls, floor and ceiling conducting heat inwards. Solid brick is worse than filled cavity. A top floor room under an uninsulated roof is worse than both, which is why loft conversions are the classic British overheating story.

People. Roughly a hundred watts each, sitting still, more if moving. Two people in a small bedroom is a small heater running all night.

Equipment. A desktop computer, a television, an old fridge in the corner. Whatever it draws in electricity, it puts into the room as heat, near enough all of it.

The room cooling calculator takes all four, in your dimensions, and gives you a figure in watts and in BTU per hour. It also tells you which of the four is dominant, which is the more useful output, because that is the one worth attacking first.

Attack the biggest number first

If the glass is your dominant term, and for a great many British rooms it is, then a machine is the expensive way to solve it.

Stopping sunlight before it reaches the glass beats removing the heat afterwards, every time and by a wide margin. External shutters, external blinds, an awning, even a temporary external screen. External, because once the energy is through the window it is already in the room; an internal blind absorbs it and re-radiates most of it inside.

This is the least fashionable advice on this subject and the most effective. It is also the same measure that reduces heat loss in winter, which is not something a portable air conditioner will ever do for you.

When a machine is the right answer

When the load is genuinely high, when the room cannot be ventilated at night, or when somebody in the house is vulnerable to heat and comfort is not optional.

At that point the number matters. Buy under it and the unit runs flat out and never gets there. Buy far over it and you have paid more for something that short cycles. The calculator adds a small allowance for a hot day and gives you a band.

Then run the figure through the running cost calculator, because a unit that costs two hundred pounds and runs eight hours a night for three weeks is not a two hundred pound decision.

How many hot days you are actually buying for

This is the part almost every buying guide skips. A machine that earns its keep in Seville is a different proposition in Sheffield.

From this site’s own store of daily readings, Sheffield averages twelve days a year at or above 25 degrees this decade. Newcastle upon Tyne averages under six. London averages twenty four.

Twenty four days is three and a half weeks. Six days is less than a week, and for most people that is a fan, a shaded window and an open door at midnight, not a machine and an outdoor unit.

Look up your own town on the heat data pages before you spend anything. It is the same question as “how much cooling do I need”, asked one level up.