Short answer
Very little on a low setting, more on top speed, and a lot on heat. Watts × hours ÷ 1,000 gives kWh; multiply that by the unit rate on your bill. The purifiers we track that publish a lowest-setting figure use 7 to 58 kWh for a year of 8-hour nights on it. Then add the filters, from four a year to one every five years depending on the model.
Most running-cost questions come down to one thing: can I leave it on all night? Every power figure here comes from the makers’ manuals, spec sheets and product listings for the models in our air purifier comparison and its sister categories. We stop at kWh, the unit on your bill, because unit rates depend on your tariff and change during the year.
How to work it out
- Find the watts for the setting you use, in the manual or under “Noise and power” on each of our product pages.
- Watts × hours ÷ 1,000 = kWh. Multiply by 365 for a year.
- kWh × your unit rate, the price per kWh on your bill or your supplier’s app, gives the cost.
Take the Winix T500 WiFi. Winix gives 12 W on its lowest setting and 55 W on top speed. Eight hours on low is 12 × 8 ÷ 1,000 = about 0.1 kWh a night, or 35 kWh over a year of nights. Eight hours on top speed is 0.44 kWh a night, about 161 kWh a year.
What our purifiers use
Only five of our models have a published lowest-setting figure, so for the rest the table shows top speed only. Levoit and Shark give rated power, which we treat as top speed.
| Model | Watts (lowest / top) | kWh per 8-hour night (lowest / top) | kWh per year of nights (lowest / top) |
|---|---|---|---|
| Winix T800 WiFi | 2.5 / 45 | 0.02 / 0.36 | 7 / 131 |
| Coway Airmega 150 | 2.9 / 35 | 0.02 / 0.28 | 8 / 102 |
| Coway Airmega Mighty | 4.9 / 82 | 0.04 / 0.66 | 14 / 239 |
| Winix T500 WiFi | 12 / 55 | 0.10 / 0.44 | 35 / 161 |
| IQAir HealthPro 250 | 20 / 135 | 0.16 / 1.08 | 58 / 394 |
| Philips AC0650 | not published / 12 | – / 0.10 | – / 35 |
| Dyson HushJet Compact | not published / 19 | – / 0.15 | – / 55 |
| Blueair Blue Max 3250i | not published / 20 | – / 0.16 | – / 58 |
| Shark HP150UK | not published / 26 | – / 0.21 | – / 76 |
| Philips AC2220 | not published / 28 | – / 0.22 | – / 82 |
| Levoit Core 300 | not published / 33 | – / 0.26 | – / 96 |
| Philips AC3220 | not published / 36 | – / 0.29 | – / 105 |
| Blueair Blue Max 3450i | not published / 38 | – / 0.30 | – / 111 |
| Levoit Core 300S | not published / 39 | – / 0.31 | – / 114 |
| Philips AC3420 | not published / 43 | – / 0.34 | – / 126 |
| Levoit Core 600S | not published / 49 | – / 0.39 | – / 143 |
Five models have no power figure in our records: the Tower T673000 and Dyson’s PC1, HP1, PH2 De-NOx and Big+Quiet. Philips gives none for the AMF870’s purifier and fan modes either. We don’t estimate those.
Why yearly estimates differ so much
Which?’s buying guide gives a yearly energy cost for each model it tests, based on 12 hours a day on the highest setting. That’s a fair like-for-like comparison and a sensible worst case, but it isn’t how most people run a purifier overnight. On the Winix T800, 12 hours a day on top speed comes to about 197 kWh a year; 8 hours a night on its lowest setting is about 7 kWh, under a twentieth as much.
The catch is that the lowest setting also moves far less air. CADR, the clean air delivery rate, is measured on top speed only, so AHAM notes it tells you nothing about cleaning on lower settings. Auto mode sits in between, speeding up when the sensor sees particles: Coway’s Airmega 150 drops back to Silent once the air has stayed clean for a while, and Levoit’s Quiet Auto on the Core 300S never uses top speed. Your real figure lies between the two columns.
Standby: small, but not zero
None of our records has an exact standby figure: the makers that mention it give a ceiling. Levoit says under 0.5 W for the Core 300 and at most 2 W of networked standby for the Core 300S; Philips says under 2 W for the AC3220. At 2 W, 16 hours a day of standby is at most about 12 kWh a year (2 × 16 × 365 ÷ 1,000).
Switching off at the wall removes it, but read the manual before adding a timer or smart plug. Meaco says switching an Arete on and off with a smart plug or plug timer voids its warranty, and Philips says the AMF870 must go straight into a wall socket, with no timer plug, because of the heater’s thermal cut-out.
Filters: the other running cost
Each filter change is a purchase, and the maker’s interval decides how many you buy:
- Four a year: Meaco’s Arete HEPA filters, about every three months (sold in packs of three).
- About two a year: Levoit Core 300 and 300S (six to eight months), Blueair 3250i (six months in its spec table, up to nine on its filter page) and the Tower T673000. The Levoit Core 600S is one or two (six to 12 months).
- One a year: Philips AC0650, AC3220 (a two-filter pack), AC3420 (plus a wick about every six months) and AMF870; Winix T500 and T800; Coway Airmega 150 and Mighty (the Mighty’s carbon filter twice a year).
- Four every three years: Blueair 3450i, up to nine months.
- One every three years or longer: Philips AC2220 (about three years) and the IQAir’s HyperHEPA (three to four years, though its other two filters are due sooner); Shark HP150UK and Dyson’s HushJet Compact (up to or about five years on their own tests).
To compare two purifiers, multiply each filter’s price by the changes a year. A filter that lasts three years can cost three times as much as a yearly one and still come out level. Which costs more, filters or electricity? Do both sums: Which?’s guide covers energy but not filter prices, so the filter sum is yours to do. Part numbers and intervals for every model are in our guide to air purifier replacement filters.
The Philips AC2220 shows the other side of the trade: one FY2200 filter about every three years by Philips’ estimate, and 28 W on top speed.
Heater mode is a different scale
Heat is where the numbers jump. The Philips AMF870, a 3-in-1 purifier, fan and heater, draws 2,200 W when heating: 2.2 kWh in one hour, or 17.6 kWh over eight. That single hour uses as much as the Winix T800 does in 110 nights on its lowest setting. For the Dyson Purifier Hot+Cool HP1, no heater wattage appears in the manual or the UK listings we read; Dyson’s UK description says the heater suits well-insulated spaces or occasional use. Compare both in purifier fans and heaters.
Dehumidifier combos draw more
Meaco’s Arete dehumidifiers with HEPA filters list 105 W (6L), 163 W (10L), 230 W (Arete Two 20L) and 250 W (Arete One 20L) when dehumidifying, so eight hours on the Arete One 20L is 2 kWh. Meaco gives no figure for Air Purification mode, which runs only the fan. Of the 53 Arete One 20L owner reviews we read, one says it costs less to run than the dehumidifier it replaced. See purifier dehumidifiers and humidifiers.
Keeping the cost down
- Run it on low, sleep or auto overnight, and save top speed for when the air needs clearing fast.
- Size it for the room. A purifier too small for the room needs higher speeds to keep up; our rule of thumb is room m² ≈ CADR (m³/h) ÷ 12. See what size air purifier you need.
- Count the filter before you buy, not after. Still choosing? Start with the best air purifiers in the UK.
Running cost questions
How much electricity does an air purifier use overnight?
Very little on its lowest setting. The five models we track with a published lowest-setting figure draw 2.5 to 20 W, which is 0.02 to 0.16 kWh over eight hours. On top speed the plain purifiers we track draw 12 to 135 W, or about 0.1 to 1.1 kWh over the same night. Multiply by the unit rate on your bill.
Can I leave an air purifier on all the time?
You can, and on a frugal model the electricity stays low: the Winix T800 on its lowest setting would use about 22 kWh in a year of round-the-clock running. Allergy UK says running a purifier continually, as the maker instructs, can help reduce airborne allergens. Remember the filter light counts running hours, so filters fall due sooner.
Why are Which?'s running costs higher than my sums?
Which? bases its yearly energy cost on 12 hours a day on the highest setting, a fair like-for-like test and a worst case. Overnight on low, sleep or auto uses a fraction of that: the Winix T800 draws 45 W on top speed but 2.5 W on its lowest setting. Your figure depends on the settings and hours you actually use.
Does standby use much electricity?
Not much. Makers that state it give ceilings rather than exact figures: under 0.5 W for the Levoit Core 300 and under 2 W for the Philips AC3220. At 2 W, 16 hours a day in standby is at most about 12 kWh a year. Check the manual before using a smart plug to cut it: Meaco says that voids the Arete warranty.
Which costs more, the electricity or the filters?
It depends on the model and how you use it, so do both sums. Electricity is watts × hours ÷ 1,000 × your unit rate. Filters are the filter’s price × changes a year, from four a year for Meaco’s HEPA refills to one every three years for the Philips AC2220. Overnight on low, the five models with a published figure use 7 to 58 kWh a year.
How much more does heater mode use?
Far more than purifying. The Philips AMF870 draws 2,200 W on heat, which is 2.2 kWh every hour, against 12 to 135 W on top speed for the plain purifiers we track. Philips gives no figure for its purifier and fan modes, and we found no heater wattage for the Dyson HP1 in its manual or the UK listings we read.
