Hours to make a tank top-up or daily need: gallons ÷ production GPH. Optional DC power: amp-hours ≈ (watts ÷ volts) × hours. User rates only — not an OEM sizing chart.
Use this when you know how many gallons you need to make (tank top-up or daily budget) and the watermaker’s production rate in GPH. The calculator returns runtime hours. Optional watts and system volts add a simple amp-hour budget: (W ÷ V) × hours.
Nameplate GPD assumes continuous 24-hour production — for a few hours of run, use GPH (or GPD÷24). Cold water, fouled membranes, and low feed pressure make less than the brochure; enter a measured GPH when you have one.
This is runtime and an optional DC budget check — not a watermaker sizing survey, not membrane chemistry, and not a generator load study.
Runtime hours equal gallons needed divided by production GPH. If both watts and volts are positive, amp-hours are approximated as (watts ÷ volts) × hours.
hours = gallons ÷ GPH
Ah ≈ (W ÷ V) × hours (optional)
The Ah path assumes steady draw at the watts you enter. Real systems surge at start and may share a bus with other loads — treat Ah as a planning ceiling, then confirm with a clamp meter or the maker’s current draw.
| Gallons | GPH | Watts | Volts | Hours | Ah |
|---|---|---|---|---|---|
| 40 | 8 | 600 | 12 | 5.00 | 250 |
| 20 | 10 | — | — | 2.00 | — |
| 60 | 6 | 800 | 24 | 10.00 | 333 |
| 15 | 5 | 400 | 12 | 3.00 | 100 |
| 80 | 12 | — | — | 6.67 | — |
Rows were computed independently (hours = gal/GPH, Ah = (W/V)×hours) and match this page. Default example: 40 gal ÷ 8 GPH → 5.00 h · optional 600 W at 12 V → 250 Ah.
Units: Gallons and GPH (US). Watts and volts for the optional Ah path.
Where the formula fails: OEM derate tables for temperature and salinity, product-water divert/flush cycles that steal runtime, and AC compressor draws that are not a steady DC watt figure. This page does not invent derate curves.
Not a substitute: planning aid only — not the watermaker manual, not a survey of tank hygiene, and not electrical design advice.
Nameplate GPD assumes 24 h continuous. For a few hours of run, use GPH (or GPD÷24) and multiply by planned hours.
Many skippers run the watermaker with the generator or while charging. The Ah estimate is a simple (W/V)×t budget check against the house bank.
Enter measured GPH if you have it. This page does not invent OEM temperature or salinity derate tables.
Enter the freshwater gallons you need in the tank — not feed seawater volume. Recovery ratio is outside this model.
Use the bus voltage that actually feeds the unit (or its inverter DC input). Mixing a 24 V draw with a 12 V field understates Ah.