An air purifier left on its lowest fan speed costs about £18 a year in electricity.
The same machine run flat out, day and night, costs about £99.
Almost every household lands between the two, and much nearer the bottom.
Quick facts
- Unit rate assumed: 25p per kWh — check your own bill
- Sleep mode, 8 W, 24 hours a day: about £18 a year
- Auto mode, 15 W average: about £33 a year
- Top speed, 45 W, 24 hours a day: about £99 a year
- Replacement filter: £30–£60, every 6 to 12 months
The sum is watts, times hours, divided by 1,000, times your unit rate
Every purifier carries a rated wattage on the plate underneath it or on the back panel.
That figure is the maximum, drawn at the highest fan speed, and it is the number the marketing never quotes.
A 45 W machine running for 24 hours uses 1.08 kWh, which is about 27p at 25p a unit.
Over a year that comes to 394 kWh, or roughly £99.
Drop it to a realistic 15 W average and the year costs 131 kWh, about £33.
On the lowest, quietest setting most units draw 5 to 8 W — less than a bedside lamp.
We use 25p per kWh throughout because the capped electricity rate has sat in the mid-twenties of pence for some time now.
Your own rate is printed on your bill, and swapping it into the sum takes ten seconds.
Annual electricity for a mid-size purifier running continuously, at 25p per kWh.
Auto mode is the cheap setting, not the expensive one
A purifier on auto spends most of the day at its lowest speed.
It lifts the fan only when the sensor sees something — frying, hoovering, a window open on a still evening.
Averaged across a week, 15 W is a fair estimate for a mid-size unit in an ordinary flat.
That is about 9p a day, or roughly what a kettle costs in three boils.
The meter is not what makes an air purifier expensive. The filter is.
Filters cost two to three times the electricity
A combined HEPA and carbon filter for a mid-size unit runs to £30 to £60 in the UK.
Manufacturers quote 6 to 12 months, and busy households reach the lower end of that.
Set against £18 to £33 of electricity, the consumable is plainly the running cost.
Machines with a washable pre-filter push the main filter further, because most of what shortens its life is hair and dust rather than fine particles.
That is also the number worth checking before you buy, and it is the fifth of the five figures in our Blueair Blue Pure 311i Max review.
Third-party filters undercut the originals by half and their filtration grade is rarely verified independently.
Standby is trivial, and switching off costs you air
A connected purifier on standby draws 1 to 2 W to keep its Wi-Fi alive, which is about £3 a year.
Switching it off at the wall saves that and nothing more.
Turning the machine off while you are out saves pennies and gives you an unfiltered room to come back to.
Overnight sleep mode is the better economy: the fan is at its quietest, the draw is at its lowest, and the bedroom is the room you spend eight hours in.
If the purifier sits in a room nobody uses, the saving is not the fan speed. It is moving the machine.
Check the current capped unit rateOfgem, the energy regulator for Great Britain →
