All three of our reverse osmosis systems use the same core technology and remove the same things from your water. The differences are practical rather than chemical: how much cupboard space they take, how fast they deliver, how much water they send to the drain, and whether they need a plug socket. This page lays those differences out so you can pick on the constraints of your actual kitchen.
| Specification | RO-300 | RO-400 | RO-500 |
|---|---|---|---|
| Filtration stages | 3 (sediment, carbon block, RO membrane) | 4 (sediment, granular carbon, carbon block, RO membrane) | 5 (sediment, carbon, carbon block, RO membrane, remineralisation) |
| Rated output | 300 GPD (approx. 1,100 litres per day) | 400 GPD (approx. 1,500 litres per day) | 500 GPD (approx. 1,900 litres per day) |
| Tap flow rate | approx. 1.0 litre per minute | approx. 1.5 litres per minute | approx. 1.2 litres per minute |
| Storage | Pressurised tank, approx. 1.8 gallons / 7 litres | Pressurised tank, approx. 3.2 gallons / 12 litres | None — tankless, filters on demand |
| Waste-water ratio | approx. 3:1 | approx. 2:1 (2 litres to drain per litre filtered) | approx. 1.5:1 |
| Power required | No — mains water pressure only | No — mains water pressure only | Yes — built-in booster pump, 13amp socket needed |
| Minimum mains pressure | 2.8 bar | 2.8 bar | 1.0 bar (pump-assisted) |
| Under-sink footprint | approx. 34cm W x 42cm H x 18cm D, plus tank | approx. 42cm W x 42cm H x 20cm D, plus tank | approx. 42cm H x 18cm W x 40cm D |
| Membrane life | 2-3 years | 2-3 years | 2-3 years |
| Pre-filter life | 6-12 months | 6-12 months | 6-12 months |
Figures marked with this symbol are provisional and are being confirmed against the manufacturer's datasheet. We would rather show an honest estimate than an unsupported claim.
The RO-300 is the entry point into reverse osmosis. It drops one carbon stage and uses a smaller tank, which brings the footprint down to something that fits under a sink already crowded with a waste trap, a bin and cleaning products.
Choose it if:
Small kitchens and one to two-person households. The cheapest way into real RO filtration.
Main trade-off:
Three stages rather than four means one less carbon stage, so chlorine taste removal is good but not quite as thorough as the RO-400.
The RO-400 is the middle of our three reverse osmosis systems and the one most UK kitchens end up with. Water passes through four stages in sequence: a sediment filter catches grit and rust from the mains, granular carbon takes out the chlorine taste, a dense carbon block picks up the finer organic compounds, and the RO membrane does the actual heavy lifting on dissolved solids.
Choose it if:
Most UK households. Pick this if you want full RO filtration and have normal under-sink space.
Main trade-off:
RO sends water to the drain as part of how it works — expect roughly two litres wasted per litre filtered. If your household is metered, factor that into running cost.
The RO-500 removes the storage tank. Instead of filtering slowly into a reservoir, a booster pump pushes water through the membrane fast enough to serve the tap directly. You get back the cupboard space the tank would have taken, and you never draw down a half-empty tank.
Choose it if:
Tight cupboards, low mains pressure, or if you dislike the flat taste of plain RO water. Requires a socket.
Main trade-off:
Needs a 13amp socket under the sink. If you have no power there, you need the RO-400 instead — this is the single most common reason a tankless unit has to go back.
Every one of these systems uses a reverse osmosis membrane, and that membrane is the stage that does the work. It removes dissolved solids, heavy metals such as lead from older pipework, fluoride, and the calcium and magnesium behind kettle scale. Choosing between the three does not change what comes out of the tap in any meaningful way — it changes how much room the system takes, how quickly it delivers, and what it costs to run.
They also share the same honest drawbacks. Reverse osmosis sends water to the drain as a by-product of how it works, it filters one tap rather than the whole house, and it strips beneficial minerals along with unwanted ones. Only the RO-500 adds them back.
If none of that appeals, reverse osmosis may not be what you need at all. Read reverse osmosis vs carbon filtration before you commit — for a chlorine-taste complaint, a carbon or limescale filter does the job for a good deal less.
All three systems take the same replacement parts, so ongoing cost is effectively identical: a carbon block cartridge every six months and an RO membrane every two to three years. Where they differ is water: the RO-300 has the highest waste ratio and the RO-500 the lowest, which matters if your household is on a meter.
