The sticker price is only half the story. The number that actually decides whether a water filter is worth it over five years is the annual filter cost — and it’s the figure most reviews (especially US ones) gloss over or quote in dollars. Here’s what UK buyers really pay, in £.
The short version
A reverse-osmosis system costs roughly £110–£150 a year in replacement filters. That sounds like a lot until you compare it to bottled water — at which point it pays for itself within months. The upfront price matters less than people think; the running cost is what to budget for.
Annual filter cost by system
| Waterdrop G3P600 | Waterdrop G3P800 | AquaTru Classic | |
|---|---|---|---|
| Upfront (UK) | £399.99 | £599 | £449 |
| Annual filters | ~£148 | ~£150 | ~£110 |
| Type | Under-sink RO | Under-sink RO | Countertop RO |
The differences between systems are small — running cost shouldn’t be the thing that decides which RO system you buy. It’s more useful for deciding whether RO is worth it at all versus cheaper alternatives.
What drives the cost
Reverse-osmosis systems use several filters on different schedules:
- Sediment / pre-carbon filters — change roughly every 6–12 months. The cheapest filters, but the most frequent.
- Carbon block — typically every 12 months.
- RO membrane — the priciest part, but lasts longest (often ~24 months).
The annual figure blends all three. In hard-water areas like London, pre-filters tend to need changing toward the shorter end of their stated life, so budget a little above the headline number.
Reverse osmosis vs bottled water
This is where RO clearly wins. If a household currently buys bottled water:
- Even a modest 2 litres a day of bottled water at UK supermarket prices runs to well over £200–£400+ a year — more for branded bottles.
- An RO system’s ~£150/year in filters undercuts that comfortably, gives you unlimited water, and removes the plastic waste.
So beyond the first year (which absorbs the upfront cost), RO is typically cheaper than bottled while filtering far more thoroughly.
How to keep running costs down
- Buy filters in multipacks or on subscription where the brand offers a discount.
- Don’t change filters early — follow the schedule (or your water’s actual condition), not anxiety.
- Match the system to your water. In soft-water areas you may not need RO at all — a cheaper carbon filter has lower running costs if taste is your only concern.
- Check filter prices before you buy the system — a cheap unit with expensive proprietary filters can cost more over five years than a pricier one with affordable filters.
The verdict
Budget around £110–£150 a year for reverse-osmosis filters in the UK, a little more in hard-water areas. It’s a real cost, but a modest one — and almost always cheaper than the bottled water it replaces. When comparing systems, weigh the five-year total (upfront + filters), not just the price tag. Our picks — the Waterdrop G3P600 and AquaTru Classic — are both reasonable on running costs.
Frequently asked questions
How much does a water filter cost to run per year in the UK?
A reverse-osmosis system costs roughly £110–£150 a year in replacement filters, a little more in hard-water areas where pre-filters need changing more often. Simple carbon jug filters cost less but remove far less.
Is a reverse osmosis system cheaper than bottled water?
Beyond the first year, yes. RO filters cost about £150/year, while buying even 2 litres of bottled water a day runs to £200–£400+ annually. RO undercuts bottled water and gives unlimited supply.
Why are reverse osmosis filters expensive in hard-water areas?
Hard water (like London's ~280 mg/L) makes pre-filters work harder, so they need changing toward the shorter end of their stated life. The RO membrane also works harder, nudging annual costs slightly above the headline figure.
How can I reduce water filter running costs?
Buy filters in multipacks or on subscription, follow the replacement schedule rather than changing early, and check filter prices before buying the system — a cheap unit with pricey proprietary filters can cost more over five years.