I currently have an ro 200, so i take it that this means litres. im just looking for something that will give me between 500 and 700 litres a day.
In spite of metrication, RO numbers usually refer to GPD (gallons per day). Also, they are US gallons rather than UK gallons. US gallons are about 10% smaller. So let's say that each GPD refers to 4 litres.
A 300 GPD RO should give 1,200 litres per day in theory and if everything is perfect (right water temperature, excellent pressure, hardly used membranes etc). However, you won't get 1,200 per day from a 300 GPD. I've heard of people getting around 800 litres a day from one. I've never had more than around 720 from mine. It has dipped as low as 420 litres but seems to be producing at about 500 litres per day right now. This should be fine for a sole trader. Bear in mind that if you are not working over a weekend, you can always store a load of water into canisters on the Friday and let the tank fill over the weekend. This could be handy storage if you get a very large dirty first clean and need a load of water of if things go wrong with your RO in future.