How much battery do I need for a watermaker?
For 100 litres a day, between 31 Ah and 164 Ah at 12 volts. Because you should not routinely take a bank below about 50%, that implies roughly 62–328 Ah of nominal capacity set aside for the watermaker alone.
This is capacity for the watermaker on top of the rest of the boat, and it assumes you are replacing it daily from solar or the engine. A watermaker does not need a bigger bank so much as a bank that gets refilled — running one out of stored energy you never replace just moves the problem.
The 50% usable figure is a lead-acid convention and conservative for lithium. If your bank is lithium, the nominal figure comes down accordingly; the amp-hours drawn do not change.
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.