Sailboat energy and water budget for extended cruising

The at-anchor balance always reassures; the underway balance is what sizes the system. For a 40-foot cruiser offshore, typical loads at 12 V:

• • Autopilot — 50 to 120 Ah/day depending on sea state, tuning and sail balance. The boat's biggest consumer, by far.
• • Refrigeration — 30 to 60 Ah/day in warm waters, where the compressor runs twice as much as in northern seas.
• • Instruments, AIS, nav lights — 20 to 40 Ah/day, incompressible: this is the watch that protects the crew. Beyond 60 nm from a shelter, French rules (division 240) require a fixed VHF, a waterproof handheld VHF and an EPIRB: the fixed VHF, linked to the GPS, is part of this watch.
• • Comms and computers — 10 to 20 Ah/day with a satellite communicator and routing.

That's 110 to 240 Ah/day. The sizing rule: a usable bank covering 2 to 3 days without production — 300 to 400 usable Ah in LiFePO4, double that in nominal lead-acid.

• • Solar — a realistic 300 to 500 Wp on arch and biminis. Deduct 20 to 30% from onshore yield: the rig's shadow sweeps the panels on every point of sail, and heel degrades the angle.
• • Hydrogenerator — the only source correlated with the route: around 100 W from 5 knots (the figure Watt&Sea gives for its cruising models), day and night. It's what pays for the autopilot on an ocean crossing; its drag costs about a tenth of a knot.
• • Alternator (or genset) — top-up and backup. On a lithium bank, an external regulator or DC-DC charger is imperative to avoid cooking the alternator under prolonged full load.
• • Wind generator — valuable at anchor in the trades, marginal downwind: apparent wind drops with boat speed.

The baseline for frugal cruising: 3 to 5 L per day per person for drinking and cooking, saltwater showers with a freshwater rinse. Three rules structure the rest:

• • Never 100% in one container — a safety reserve in separate jerrycans, isolated from the pressurised circuit: one leak or one polluted tank must not take out all the boat's water.
• • Watermaker tested before, not during — up to 30 L/h for a 12 V unit: the Schenker Zen 30 datasheet gives 30 L/h for 110 W, about 9 A. Run it in real conditions before departure; carry a spare membrane, filters and pickling biocide.
• • Rain is rigged in advance — a catchment awning fills tens of litres per tropical squall, provided gutters and fittings are ready before the squall arrives.