What a portable air conditioner actually costs to run

A portable air conditioner is not a two hundred pound decision. It is a two hundred pound decision plus whatever it costs to run, for as many summers as it lasts, and the second number is the one nobody puts on the box.

The arithmetic

It is not complicated. A unit’s rated power, in watts, divided by a thousand, gives kilowatts. Multiply by the hours it runs to get kilowatt hours. Multiply that by your rate in pence per kilowatt hour.

A 1000 watt unit running for eight hours uses eight kilowatt hours. What that costs depends entirely on your tariff, which is why the running cost calculator asks for it rather than guessing.

That last point is worth being blunt about. This site ships with no default electricity rate, and the calculator says so on the page, because the current price cap has not yet been checked against Ofgem directly and a figure lifted from a comparison site is not a source. Type your own rate in from your bill and every number on the page becomes real.

Hours are the number people get wrong

The rate is easy to look up. The hours are where estimates fall apart.

People imagine running it on the worst three nights of the year. Then it is warm for a fortnight and the thing runs every night, plus afternoons at the weekend, because once it is in the room it is what you reach for.

So work with the real count rather than the imagined one. From this site’s own store of daily readings, London averages 24.2 days a year at or above 25 degrees this decade. Manchester averages 10.3. Newcastle upon Tyne averages 5.7. Your own town is on the heat data pages.

Twenty four hot days, at eight hours a night, is nearly two hundred hours of running. Six hot days is under fifty. Those are different appliances, financially, even if they are the same box.

Single hose against double hose

This is the biggest efficiency difference in the category and it is barely mentioned in most reviews.

A single hose unit takes air from the room, passes it over the condenser, and blows it out of the window. That air has to be replaced, and it is replaced by air pulled in from everywhere else: the hallway, the gaps around doors, the letterbox. Much of that replacement air is warm. You are cooling the room and, at the same time, actively drawing warm air into it.

A double hose unit draws its condenser air from outside and returns it outside. The room air stays in the room. It costs more, it is bulkier, and it does more cooling for the same electricity.

If you are buying portable, this is the specification to check first, ahead of the BTU figure.

Where the money actually goes

Three things dominate, in this order.

Sizing. An undersized unit runs continuously and never reaches the target, so it costs the maximum and delivers the minimum. Work out the requirement with the room cooling calculator before choosing a capacity.

The window seal. A hose hanging out of a window that is propped open is a hole letting hot air in beside the hose letting hot air out. A proper sealed kit is not optional; it is most of whether the unit works at all.

Hours. Everything above about the count. A shaded window that keeps the room two degrees cooler is hours the machine does not run, and that is the cheapest cooling there is.

The honest summary

For most British houses, a portable unit is the right answer for a small number of rooms, on a small number of nights, and only after the free measures have been done.

If the calculator tells you the load is modest, the money is better spent on external shading than on a machine and the electricity to run it. This site would rather say that than sell you a box.