300gpd or above it quite broad mate...Whats your tap tdsAre you on a water metreWhats your water pressureHow much water do you actually need to produce a day (on average)I'd of thought most will say get a 40/40 as then you'll never need to upgrade, but i get the feeling you're on a budget?https://collinswaterproducts.co.uk/index.php?cPath=111&osCsid=03e271ca650ca809bf4ee459d1bc942c - decent, smaller r.o's at decent pricesAnd if your water pressure is less than 60 psi, i'd recommend you fit one of these (http://www.purefreedom.co.uk/24v-aquatec-reverse-osmosis-booster-pump-power-supply-p-274.html?page=2) before the membranes on your r.o