What size watermaker do I need for 4 people?
Plan on about 100 litres a day — 4 people at 25 litres each, which is a cruising rate rather than a marina one. Almost anything in the table will make that; the question is how long it takes and what it costs in energy.
Resist sizing for your worst day. A watermaker bought for the one week a year you have guests aboard is oversized and thirstier for the other fifty-one, and tankage is a far cheaper buffer than pump capacity.
A useful rule: pick the system that makes your daily need in one to two hours, then check that the energy for those hours fits inside what your boat can spare. If it does not, the answer is a more efficient system, not a smaller one.
Where this is worked through
Other quick answers
Where these numbers come from. Every figure on this page is computed at build time from the comparison table — rated output and rated draw as the manufacturer publishes them, with watt-hours per litre derived as one divided by the other. Nothing is typed in by hand, so nothing here can drift from the table. Where a figure is an assumption rather than a specification it is labelled as one. Methodology · the table.