Chris, I have been using-, building-, and selling RO systems for about 25 years.

Unless the vendor in question is doing something odd, like reintroducing calcium through that last stage, the number of stages (more on that in a minute) should have little-, to no effect on the pH of the output water.

For one, pure water is VERY difficult to get an accurate measure of its pH, due to the closely equal number of H+ and OH- charges. Secondly, pure water in a container open to the air will absorb carbon dioxide from the air, forming carbonic acid, which in today's atmosphere, will have an equilibrium pH of about 5.3.

Stages of a basic system: Stage one is a sediment filter to remove suspended solids in the incoming water. Stage 2 is a carbon filter to remove organics. Stage three is the membrane itself, and stage 4 is a carbon "polishing" filter. If you have chlorinated water, a second carbon filter is typically added before the membrane, making the system a 5-stage.

Unless you go with a very large, very costly system, RO systems are not "on demand" water supplies. In a typical residential system, there is typically a 3-gallon bladder tank to provide the needed water, and the system refills it as needed. Don't forget that a 100 gallon-per-day system, working at its design capacity (more to follow) is only producing about 260 ml/min, and nobody is watering anything at that rate.

Systems are rated at 65 psi and 77F water at the membrane. Lower pressure and/or cooler temperatures lower the output.

Standard residential systems produce 3 gallons of flush water for each gallon of pure water produced. Most of mine were on the order of 1.5 - 2:1; recently I have seen 1:1, but if your water cost is high, the extra cash for the improved membrane would be worth it. I was on a well in PA, so just dumped the wastewater on the floor of the greenhouse for humidity, but here in NC, we have very pricey water, but luckily for me, it is very low in dissolved minerals.

If you want to know more, please email me.