Routes & Assessment 12 min read August 4, 2026

Energy and water aboard a sailboat: the extended-cruising balance

At anchor, a sailboat lives like a small camper van. Underway, everything changes: the autopilot steers 24/7, the fridge fights 28 °C seawater, the AIS watch never switches off — and the engine isn't running. Orders of magnitude for sizing energy and water on an extended cruise.

100–180

Ah/day: typical balance underway (12 V)

24/7

hydrogenerator output while sailing

3–5 L

vital water per day per person

-25%

solar yield under rig shading

The electrical balance underway: count what never switches off

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.
• Comms and computers — 10 to 20 Ah/day with a satellite communicator and routing.

That's 100 to 180 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.

The charging mix at sea: no single source is enough

• 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, 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.

Fresh water: tanks, watermaker, rain

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 — 5 to 30 L/h for 12 V units depending on power. 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.

Key takeaways

Underway, a 40-footer's typical balance reaches 100 to 180 Ah/day at 12 V — dominated by the autopilot.
The hydrogenerator is the only source correlated with the route: around 100 W from 5 knots, day and night.
Count on 20 to 30% less solar yield than onshore: rig shading and heel.
Vital water: 3 to 5 L per day per person, with a safety reserve separated from the main circuit.
Watermaker: test in real conditions before departure, consumables on board.

FAQ

Can you switch to LiFePO4 on an existing sailboat?

Yes, provided you treat the whole chain: external alternator regulator or DC-DC charger, compatible shore charger, accessible BMS and an identified emergency disconnect. The gain in usable capacity at equal weight is major — and weight matters on a sailboat.

Is a watermaker essential?

Not on an Atlantic circuit with regular stops: tanks plus jerrycans suffice with strict management. It becomes structural in the Pacific and arid zones, where reliable water stops are weeks apart — provided it's maintained; a neglected membrane kills it.

How much solar for an ocean crossing?

A well-installed 300 to 500 Wp covers refrigeration and instruments; the autopilot remains the job of the hydrogenerator or alternator. Aiming for 100% solar underway leads to unrealistic areas on a cruising sailboat — it's the mix that makes the balance robust.