Key takeaways
- At 15W running continuously, an air purifier costs about 9p a day, or £34 a year
- Turn it up to a 35W medium setting and that becomes about £80 a year
- Replacement HEPA and carbon filters typically run £25 to £60 a year, often more than the electricity
- Size by CADR: aim for at least five air changes an hour, so room volume in cubic metres times five
- A purifier filters mould spores out of the air but does nothing about the mould, which needs ventilation and drying
Air purifiers by room size
Match the coverage figure to the room. Running one 24/7 in a room it is too small for is where the cost creeps up.
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The short answer
The electricity is not the problem. A HEPA purifier on a low continuous setting draws around 15W, which is 0.36kWh a day, 9.4p at 26.11p per kWh, and about £34 a year. That is less than most people expect.
The filters are the problem. A HEPA and carbon set is typically £25 to £60 and wants replacing every six to twelve months, so the consumables usually cost more than the power. Check the replacement filter price for the exact model before you buy anything, because that number varies wildly and it is the one you will live with.
Electricity cost by fan speed
Running 24 hours a day, 365 days a year, at 26.11p per kWh.
| Setting | Typical power | Per day | Cost per day | Per year |
|---|---|---|---|---|
| Sleep / auto at rest | 8W | 0.19kWh | 5.0p | £18 |
| Low, continuous | 15W | 0.36kWh | 9.4p | £34 |
| Medium | 35W | 0.84kWh | 21.9p | £80 |
| High | 60W | 1.44kWh | 37.6p | £137 |
Worked example for the low setting: 0.015kW × 24 hours = 0.36kWh, and 0.36 × 26.11p = 9.4p a day, which over 365 days is £34.
Auto mode, which ramps the fan when the sensor sees particles and drops it back down, usually lands somewhere between the sleep and low rows in a normal home. In a house with a wood burner or a busy road outside, expect it to spend more time high.
The filter bill
| Cost | Typical annual figure |
|---|---|
| Electricity, low setting continuous | £34 |
| Combined HEPA and carbon filter set | £25–£60 |
| Pre-filter (usually washable) | £0 |
| Total running cost, low setting | £59–£94 |
Two things reduce that. Vacuum the pre-filter monthly, which is free and stops the expensive HEPA layer loading up with hair and dust that never needed to reach it. And run the purifier in one closed room rather than trying to treat the whole house, which halves the air volume it is fighting.
Sizing by CADR, not room area
The room size printed on the box is a marketing figure calculated at a fan speed nobody uses. The number to look for is CADR, the clean air delivery rate, in cubic metres per hour.
Aim for at least five air changes an hour. Work out the room volume, multiply by five, and that is your minimum CADR.
| Room | Volume | Minimum CADR for 5 ACH |
|---|---|---|
| Small bedroom, 3 × 3m | 21.6 m³ | 108 m³/h |
| Double bedroom, 4 × 4m | 38.4 m³ | 192 m³/h |
| Living room, 5 × 4m | 48.0 m³ | 240 m³/h |
| Open plan, 7 × 5m | 84.0 m³ | 420 m³/h |
All at a 2.4m ceiling height. If a spec sheet quotes CFM, multiply by 1.7 to get cubic metres per hour. Buy for a CADR comfortably above your minimum, because that lets you run it on a lower, quieter, cheaper speed and still hit the target, which is exactly what you want in a bedroom.
A purifier does not fix damp or mould
It captures spores drifting through the air. The colony on the wall is fed by moisture landing on a cold surface, and it will keep growing while the machine hums away. What actually stops it is getting relative humidity below about 60%, ventilating properly, and raising surface temperatures with wall insulation. A dehumidifier is the appliance for that job.
Where purifiers genuinely help
- Pollen, in a closed bedroom. A true HEPA filter captures particles in the pollen size range, and a shut room is the only place a purifier can win.
- Traffic particulates. If you are on a busy road, PM2.5 indoors tracks outdoors closely, and filtration is one of the few things that moves it.
- Wood burners and cooking. Both spike indoor particulates sharply. An extractor is better for cooking; a purifier picks up what escapes it.
- Pet dander. Well within HEPA's range, and a pre-filter catches the hair before it reaches the expensive layer.
And where they do not: gases and odours need the activated carbon layer, which saturates fast and needs replacing more often than the HEPA. Carbon monoxide is not filtered at all by anything domestic, so an alarm is the answer there. And nothing filtered helps with a damp problem you have not fixed.
Ventilation is the cheap version
Opening windows for a few minutes twice a day changes the whole air volume for nothing. It also loses heat, which is where controlled ventilation rather than uncontrolled draughts becomes the sensible way to think about it: seal the random gaps, then ventilate deliberately when you choose to.
What to look for
A genuine HEPA filter, described as H13 or HEPA rather than "HEPA-type" or "HEPA-style", which are marketing terms with no standard behind them. A published CADR figure. A published replacement filter price. A washable pre-filter. A quoted noise figure at the speed you would actually run it.
Things to skip: ionisers, UV lamps, and anything described as plasma or active oxygen, all of which range from ineffective to producing ozone. Also app connectivity, unless you genuinely want the sensor history. A quiet fan pulling air through good filters is the whole technology.
Common questions
How much does an air purifier cost to run 24/7?
It depends entirely on the fan speed. On a low or sleep setting at around 15W, running continuously uses 0.36kWh a day, which is 9.4p at the 26.11p per kWh cap rate, or about £34 a year. On a medium setting at 35W it is 0.84kWh a day, 21.9p, or about £80 a year. On maximum at 60W you are looking at £137 a year, which is why almost nobody runs one flat out for long.
Do filters cost more than the electricity?
Usually yes, and it is the cost people forget to check. A combined HEPA and activated carbon filter set typically costs £25 to £60 and manufacturers recommend replacing it every six to twelve months. Against £34 a year of electricity on a low setting, the consumables are the bigger line. Look up the replacement filter price for a specific model before you buy the machine, because it varies far more between brands than the purchase price does.
What size air purifier do I need?
Size by CADR, the clean air delivery rate, rather than the room area printed on the box. Aim for at least five air changes an hour: multiply room length by width by height to get the volume in cubic metres, then multiply by five. A 4m by 4m room with a 2.4m ceiling is 38.4 cubic metres, so you want a CADR of at least 192 cubic metres an hour. If a spec sheet quotes CFM, multiply by 1.7 to convert to cubic metres per hour.
Will an air purifier get rid of mould?
It will capture mould spores that are floating in the air, which can help with symptoms. It will do nothing whatsoever about the mould growing on the wall, because that colony is fed by moisture and a cold surface, not by air movement. The actual fix is lowering humidity and raising the surface temperature: ventilation, a dehumidifier, and insulation on cold external walls. Buying a purifier instead of fixing the cause leaves you running a fan forever.
Should I use the ioniser setting?
Generally no. Ionisers charge particles so they clump and fall out of the air, which makes the air look cleaner on a sensor while depositing the particles on your surfaces. Some ionising and so-called plasma technologies also produce small amounts of ozone, which is a respiratory irritant. A plain HEPA filter and a fan is the well-evidenced approach. If your machine has an ioniser, leave it switched off.
Does an air purifier help with hay fever?
For pollen specifically, a true HEPA filter does capture particles in the relevant size range, and running one in a bedroom with the door and window shut is the setup most likely to help. It works on the air already in the room, so it cannot compete with an open window during a high pollen count. Keep the room closed, run the purifier on a low continuous setting rather than blasts on high, and change out of outdoor clothes before you come in.
Sources and assumptions
Unit rates are the Ofgem default tariff cap for 1 July to 30 September 2026, direct debit, averaged across England, Scotland and Wales including VAT: Ofgem energy price cap. Wattages are the manufacturer ratings printed on the appliance plate or in the product listing; we have not lab-tested anything. Where an appliance cycles rather than drawing full power continuously, we say so and show the duty cycle we used. Check the plate on your own unit before trusting any of these numbers for your house.