Calculate total cost, not checkout price
Use five buckets: purchase/installation, pre/post filters, membrane, water sent to drain, and electricity/mineral consumables where applicable. Countertop machines can also have proprietary descaling or cleaning consumables.
A simple five-year model
Add the purchase price to all scheduled consumables over five years, plus estimated water and electricity. Divide by expected litres dispensed if you want a cost-per-litre comparison. Keep assumptions visible; otherwise cost-per-litre figures become marketing theatre.
Water cost and efficiency
A less efficient RO system uses more metered water for the same output. Use our RO efficiency calculator to estimate feed and reject volumes, then apply your actual combined water/sewer tariff if relevant.
Filter pricing traps
Check whether replacement intervals are time-based, capacity-based or both; whether a “filter” includes the RO membrane; and whether subscriptions are optional. Proprietary quick-change cartridges are convenient but can lock you into one supplier.
Countertop electricity
Instant-hot countertop systems consume power for pumps, electronics and heating. The real cost depends far more on litres heated and temperature than on a standby headline. Compare like with like.
When RO can still save money
If it replaces frequent bottled-water purchases, the economics can be favourable even with consumables. But compare against the cheapest solution that meets the actual need—not only against premium bottled water.
Consumables can rise in price or be redesigned. Proprietary cartridges also create supplier dependence. When comparing systems, note today's replacement price and whether compatible filters are widely available, but avoid pretending a five-year forecast can be exact to the penny.
Allow for price changes
Suppose a system costs £400 and you expect £90 of filters/minerals each year plus a £70 membrane during year four. Before water and electricity, five-year ownership is already about £920, or £184 per year. If the household produces 8 litres a day, that fixed cost alone is roughly 6.3 pence per litre. This is only an illustration—the useful calculation uses the actual replacement schedule and prices for the model you are considering.
Example ownership calculation
Sources and further reading
For factual claims that matter to health, regulation or product performance, we prefer primary and official sources. Useful references for this guide include:
- US EPA WaterSense — Point-of-use RO efficiency
- NSF — NSF/ANSI 58 reverse osmosis systems
- Drinking Water Inspectorate — Domestic water filters and softeners
Related guides
Last reviewed: 6 October 2026. This guide is written for a UK audience. Always follow current supplier, regulator, manufacturer and healthcare advice where it applies.