How we compare

Seven criteria, and what each is worth

Every machine on this site is scored out of ten against the same seven criteria. The weights are ours and they are arguable; they are printed here so that you can disagree with them knowingly instead of wondering what sits behind a number.

1. Stated coverage in square metres - 25%. The single most useful figure and the one most often absent. A maker or retailer either commits to a room size in print or it does not. Of the five non-Scentech machines we ranked, one does.
2. How long it runs unattended - 20%. One fill, one pod, one charge. This is where the field splits furthest apart: "Up to 45 minutes" at one end and "up to ten weeks" or one to three months at the other. If nobody has to tend the machine, it stays plugged in.
3. Cost per millilitre of what you may put in it - 15%. Price per millilitre of the specific liquid the machine accepts, plus the refill rule itself. A closed pod system and an open essential-oil system both score here, on arithmetic and on the rule, not on how free they leave you.
4. Noise - 15%. A published decibel figure. A machine that hisses beside a bed is unplugged within a week, and a figure nobody publishes cannot earn points.
5. Control - 10%. App, dial, timer, schedules. A timer offering one, three or seven hours earns something; a weekly schedule you set once and never touch again earns more.
6. Upkeep - 10%. Water in and out, limescale, wiping, and how often the machine asks something of you.
7. The public review base - 5%. The average matters less than how many people produced it. A 4.6 from eleven buyers carries less weight than a 4.2 from a hundred and three, so this site prints the count beside every average it quotes.

Criteria one and two carry nearly half the weight between them, and that is the whole reason a pod machine finishes above five essential oil diffusers on a page about essential oil diffusers. It is also the reason someone who wants to buy a bottle of lavender on a Saturday should read the ranking from number three downwards: freedom to choose your own oil is a real advantage and it is not in these weights at all.

Where the facts come from

Manufacturer specification tables (getscentech.com in a United Kingdom shop session, tisserand.com), retailer product pages at John Lewis & Partners, Holland & Barrett for essential oil prices, yankeecandle.co.uk and John Lewis for candle prices, and public review platforms: Trustpilot for Scentech, John Lewis's own reviews, and the Yotpo widget on Tisserand's shop. Everything was read on 9 September 2026, and that date travels with each price and each rating.

When we work something out from published figures - a cost per millilitre, a cost per month - the assumption behind it sits next to the answer, normally twelve hours a day or whatever life the maker states. A calculation is never dressed up as a measurement.

What we did not do

No laboratory work, no measurements of our own, and none of the competing machines in the house. We do not call this site or this ranking independent, because it is not; see About this site.

Four limits are worth naming plainly. We could not get reliable pound prices out of Amazon.co.uk from where we were working, which leaves out the Amazon own-label machines that a great many British buyers actually end up owning. Argos, Boots, The Range and Dunelm blocked our requests or returned empty categories, which narrows the retail picture to John Lewis and two brand shops. The HoMedics Ellia Gather is left out of the ranking although we have its price and its stated ten-hour runtime, because no rating is published on the retailer's page and one of our seven criteria would have had nothing to work with. And no maker of a water-tank diffuser publishes oil consumption per hour, so the monthly oil cost of five of the seven ranked machines cannot be calculated at all - we say so rather than estimating it.

We also found no published test of essential oil diffusers from Which?, Stiftung Warentest or the Dutch Consumentenbond as of September 2026. That is a statement about what we could find, not a claim that nothing exists.

How this site earns

There is a tracking code on every link to getscentech.com. An order placed after following one is credited to this site, and Scentech - the maker of the products and the publisher of these pages - keeps the sale. Your price is unaffected either way. There is no commission from John Lewis, Tisserand or Holland & Barrett; those exist so that you can put our figures next to the source, and nothing of ours rides along with them.

Since every recommendation on the front page ends in a link of the first kind, the sensible reading is that the article as a whole is advertising. So it is labelled that way above the headline, and the disclosure is repeated in the footer.

Update log

• 9 September 2026 - first version. Seven machines ranked: two Scentech pod machines and five water-based essential oil diffusers sold in the United Kingdom. Scentech figures read from the manufacturer's English product pages in a GB shop session; competitor prices and ratings read from John Lewis and tisserand.com the same day; oil prices from Holland & Barrett. At the time of writing the Scentech checkout returned no shipping option for a British delivery address, which is stated in the FAQ on the home page rather than buried here, and this entry will be updated when it changes.