Boat power: solar, batteries and energy autonomy at sea
A sailing boat or motorboat at anchor is an off-grid system with unique constraints — variable sun angles, salt air, limited space, multiple charging sources and loads that run 24 hours. This guide covers everything from a weekend daysailer to a full-time liveaboard circumnavigation.
Why marine energy planning is different from everything else
- You cannot pull over and plug in. At anchor or at sea, your battery bank is the only source. Autonomy is not a convenience — it is a safety requirement. Running out of power at sea means no navigation lights, no VHF, no chart plotter and potentially no engine start.
- The load profile is 24 hours. Anchor lights run all night. The bilge pump activates automatically. The fridge never stops. AIS and GPS transmit continuously. Daily Wh consumption is the number that determines whether you stay at anchor safely or must motor to a marina.
- Salt air corrodes everything. Connections, terminals, panel frames and cable insulation degrade faster than in any land installation. Marine-grade components, tinned copper cable, sealed connectors and regular inspection are non-negotiable.
- Panel angles change constantly. On a moored or anchored boat, the hull swings with wind and tide. A panel fixed flat on the coachroof loses angle efficiency every time the boat swings off south. Portable panels that can be repositioned or a combination of panels on different aspects compensates for this.
- Multiple charging sources must coexist. Shore power, engine alternator, solar and wind generator all feed the same battery bank. A well-designed system manages these sources without overcharging, undercharging or interference between them.
- Weight and space are critical constraints. Every kilogram of battery and every square metre of panel space competes with sails, safety equipment, stores and living space. Lithium (LFP) batteries offer 2–3× the usable energy per kilogram compared to lead-acid — a decisive advantage on a sailing boat.
Navigation electronics: real consumption figures
These are the loads that can never go off. They run continuously and define your minimum daily energy budget regardless of comfort loads.
| Equipment | Typical watts | Daily hours | Daily Wh | Notes |
|---|---|---|---|---|
| Chart plotter (10–12") | 15–40W | 8–24h | 120–960Wh | At anchor: standby or off. At sea: continuous. Night mode reduces brightness. |
| VHF radio (receive mode) | 4–8W | 24h | 96–192Wh | Transmitting draws 25–55W but only for seconds. Receive is the continuous load. |
| AIS transponder (class B) | 4–8W | 24h | 96–192Wh | Transmits every 30s at sea. Receive-only uses less. Essential for collision avoidance. |
| Autopilot (linear drive) | 30–120W avg | 8–24h | 240–2,880Wh | Highly variable — depends on sea state and course corrections. Biggest single load at sea. |
| Autopilot (hydraulic) | 80–300W avg | 8–24h | 640–7,200Wh | Hydraulic systems draw more. Blue-water passage planning must account for autopilot as dominant load. |
| Wind instruments / sensors | 2–8W | 24h | 48–192Wh | Masthead unit + display. Negligible individual draw. |
| Depth / speed instruments | 2–5W | 24h | 48–120Wh | Very low draw. Usually powered from instrument bus. |
| Navigation lights (LED) | 8–15W | 12h (night) | 96–180Wh | LED upgrade from incandescent reduces this by 80%. Always upgrade to LED. |
| Anchor light (LED) | 1–3W | 12h (night) | 12–36Wh | Negligible. If still incandescent (10–25W), upgrade immediately. |
| Bilge pump (automatic) | 3–8W avg | 24h (duty cycle) | 72–192Wh | Average draw across duty cycle. Increases in wet conditions or with hull leaks. |
| Engine instruments / ignition | 5–15W | When motoring | Variable | Negligible when motor running; alternator charges at same time. |
Comfort and living loads
| Load | Watts | Typical daily hours | Daily Wh | Notes |
|---|---|---|---|---|
| 12V compressor fridge (50–80L) | 20–40W avg | 24h cycle | 480–960Wh | Most efficient marine fridge option. Dometic, Isotherm, Vitrifrigo all use Secop/Danfoss compressors. |
| Holding plate (eutectic) fridge | 150–300W for 2–4h | 2–4h cycling | 300–1,200Wh | Freezes holding plates; maintains temperature without power between cycles. Good for passage making. |
| LED cabin lighting | 15–40W total | 4–6h | 60–240Wh | Full LED conversion of a typical 40ft yacht: ~25W total for all saloon and cabin lights. |
| Laptop / work | 45–100W | 4–8h | 180–800Wh | Critical for liveaboards and remote workers. |
| Phone and tablet charging | 20–40W | 2–3h | 40–120Wh | Low individual draw; adds up with crew. |
| Watermaker (12V, 12–15L/h) | 60–120W | 1–3h | 60–360Wh | 12V watermakers are efficient. 230V models draw 350–500W but produce more. Essential for offshore passages. |
| Watermaker (230V, 40–60L/h) | 350–500W | 0.5–1h | 175–500Wh | Requires inverter. More production per session — run when solar is at peak. |
| Inverter (standby loss) | 15–40W | 24h if left on | 360–960Wh | Switch off the inverter when not needed. Standby draw is a significant hidden load over 24h. |
| Entertainment (TV / tablet via 230V) | 60–120W | 2–3h | 120–360Wh | Use 12V screens where possible to avoid inverter losses. |
| Electric windlass | 800–2,000W peak | 2–5 min | 25–170Wh | Very high peak draw but short duration. Requires large cable and fuse. Total daily Wh is moderate. |
| Electric outboard (small) | 500–2,000W | Variable | Variable | Dinghy tender. Run from portable station or dedicated bank. Do not share with house bank. |
Daily energy budget by boat profile
| Profile | Key loads | Daily Wh at anchor | Daily Wh at sea | Recommended bank |
|---|---|---|---|---|
| Weekend daysailer (30–35ft) | VHF, nav lights, fridge, phones, lights | 600–900Wh | 900–1,400Wh | 100–150Ah LFP (1,280–1,920Wh) |
| Coastal cruiser (35–42ft) | Chart plotter, AIS, autopilot (short), fridge, lighting, watermaker | 1,200–1,800Wh | 2,000–3,500Wh | 200Ah LFP (2,560Wh) + solar |
| Blue-water passage maker (40–50ft) | All nav electronics, autopilot continuous, fridge, watermaker, comms, crew devices | 1,500–2,500Wh | 4,000–8,000Wh | 400Ah+ LFP + 400W+ solar + wind gen |
| Liveaboard (marina-based) | Fridge, laptop, TV, heating/cooling, watermaker, full domestic loads | 2,500–5,000Wh | N/A (rarely passages) | Shore power primary + 200Ah LFP backup |
| Liveaboard (anchor/off-grid) | All domestic + nav + watermaker + work setup | 3,000–6,000Wh | 5,000–10,000Wh | 400–600Ah LFP + 600W+ solar + wind gen |
| Motorboat (weekender) | Chart plotter, VHF, fridge, lighting, TV, watermaker | 1,500–2,500Wh | 2,000–4,000Wh | 200Ah LFP + generator or large alternator |
Charging sources: how they work together on a boat
- Shore power is the fastest and most reliable recharge source. A quality battery charger (Victron, Mastervolt, Sterling) should be sized to charge the bank in 4–8 hours. Rule of thumb: charger output in amps = 20–30% of battery capacity in Ah. A 200Ah bank → 40–60A charger.
- Engine alternator is the traditional offshore charging source. Standard alternators are sized for starting batteries and under-perform on large house banks — they typically deliver 30–70A but may only sustain that for 20–30 minutes before thermal protection reduces output. A high-output alternator (100–200A) with an external regulator (Balmar, Wakespeed) dramatically improves charging efficiency.
- DC-DC charger (B2B) — if you have LFP house batteries and a standard lead-acid start battery, a DC-DC charger (Sterling, Victron Orion) properly isolates and charges both banks from the alternator without damaging the LFP cells or draining the start battery.
- Solar panels — the ideal complement to alternator charging. At anchor with the engine off, solar maintains the bank silently. A 200–400W array keeps a coastal cruiser's house bank topped up during a typical Iberian summer anchorage.
- Wind generator — particularly valuable in trade wind passages and anchorages with consistent breeze. A quality unit (Rutland, Superwind, Air Breeze) produces 50–200W in 15–25 knots of wind. At anchor in a bay with 15kt sea breeze, a wind generator works while solar does not (boat swings away from sun).
- Tow generator (hydrogenerator) — for blue-water passages, a hydrogenerator (Watt&Sea, Aquagen) trails behind the boat and produces 100–400W at passage speeds (6–8kt). Eliminates the need for running the engine to charge during long passages.
| Source | Typical output | Best conditions | Limitations |
|---|---|---|---|
| Shore power charger | 40–120A (480–1,440W) | Marina berth | Not available at anchor or offshore |
| Standard alternator | 30–70A continuous | Motoring 1h+ | Thermal limits; poor for LFP; degrades with age |
| High-output alternator + regulator | 80–200A | Motoring 30min+ | Engine must run; fuel cost |
| Solar (200W flat) | 150–900Wh/day | Sunny anchorage | Variable with boat swing; zero at night |
| Solar (400W optimised tilt) | 300–1,800Wh/day | Sunny anchorage, panels adjustable | Deck space; shade from sails/mast |
| Wind generator | 50–200W at 15–25kt | Trade winds, exposed anchorage | Noise; vibration; low in marinas |
| Hydrogenerator | 100–400W at 6–8kt | Blue-water passages | Drag penalty; only works sailing/motoring |
Solar panels on boats: the specific challenges
- Mast and sail shadow: the mast casts a rotating shadow across the coachroof that can reduce a panel's output by 50–100% for part of each day. Panels mounted on stern arch/gantry are less affected. Multiple smaller panels in parallel reduce the impact — a shadow on one panel does not drag down the others.
- Boat swing at anchor: a boat at anchor swings up to 360° depending on wind, tide and current. A panel mounted flat optimises for the sun's position only twice per day. Panels that can be tilted toward the sun, or a combination of panels on the port and starboard sides of the stern arch, compensates for swing.
- Stern arch / davits: the most common mounting for sailing boats. Away from the mast shadow, maximises panel area, easy to tilt and allows the boom to swing freely. Supports up to 400–600W on a typical 40–45ft yacht.
- Coachroof flat mount: available on powerboats and some motorsailers. Flexible panels or low-profile rigid panels. Limited tilt angle but large surface area. Shade from dodger, bimini and mast must be modelled.
- Bimini integration: flexible panels sewn into or mounted on a bimini provide shade and power simultaneously. Output is reduced by the horizontal angle (15–20% less than optimal tilt in southern Europe) but the installation is completely unobtrusive.
- Saltwater cleaning: sea spray deposits salt crystals on panels. In tropical and trade wind sailing, clean panels every 2–3 days with fresh water. A salt layer reduces output by 5–15%. In marinas, weekly cleaning is sufficient.
- Cable routing: all deck penetrations must be properly sealed with marine cable glands. Use tinned copper cable (not standard automotive cable) for all DC wiring. Minimum 4mm² for panel runs up to 5m; 6mm² for longer runs.
LFP vs lead-acid on boats: the honest comparison
| Factor | AGM / Gel lead-acid | LFP lithium | Verdict |
|---|---|---|---|
| Usable capacity | 50% of rated (100Ah bank → 50Ah usable) | 80–90% of rated (100Ah → 85Ah usable) | LFP gives 70% more usable energy per rated Ah |
| Weight per Wh | 35–40Wh/kg | 120–160Wh/kg | LFP is 3–4× lighter for the same usable capacity |
| Cycle life | 300–500 full cycles | 3,000–6,000 full cycles | LFP lasts 6–12× longer at the same depth of discharge |
| Charge acceptance | Slow above 80% SOC | Fast to 100% SOC | LFP charges faster from alternator and solar |
| Self-discharge | 3–5%/month | 1–2%/month | LFP better for boats stored for months |
| Maintenance | Equalisation needed; check water level (flooded) | None | LFP significantly lower maintenance |
| BMS protection | None | Built-in cell balancing, temperature, over/under voltage | LFP safer for unattended operation |
| Purchase cost | Low (€150–300 per 100Ah) | High (€400–800 per 100Ah) | Lead-acid wins upfront; LFP wins over lifetime |
| Lifetime cost per Wh | €0.30–0.60/Wh usable over lifetime | €0.08–0.15/Wh usable over lifetime | LFP is 3–5× cheaper per usable Wh over lifetime |
- The weight argument on sailing boats: replacing 4 × 100Ah AGM batteries (120kg total) with 200Ah LFP (22kg) saves 98kg of ballast weight from the bilge — improving performance, reducing heel and freeing space. This is often the deciding factor for racing-oriented cruisers.
- Alternator compatibility: LFP batteries accept charge much faster than lead-acid. A standard alternator without an external regulator may overheat trying to keep up. Always use a dedicated LFP-compatible alternator regulator (Balmar MC-614, Wakespeed WS500) when charging LFP from an engine.
Portable power station on a boat: when it makes sense
- Supplementing an under-sized house bank: an F2000 adds 2,048Wh of usable LFP capacity to any boat instantly, without rewiring. Connect the station's AC output to shore power for charging and use its 12V or AC output to power loads that exceed the house bank's capacity.
- The charterer's solution: boats on charter often have inadequate house banks for modern electronics and work loads. Bringing an F2000 and a 200W portable panel covers laptop, phone charging, extra lighting and small appliances without touching the boat's electrical system.
- Dinghy and tender power: an SOLIX C1000 in the dinghy powers an electric outboard, lights, phone charging and a small speaker for beach days without draining the mothership's bank.
- Refit and work power: during a haul-out or refit with no shore power, a station powers tools, lighting, laptop and bilge pump without running the engine or a generator.
- Shore power backup: when marina shore power is unreliable or unavailable, the station bridges the gap. An F2000 charged from shore or solar covers overnight fridge, lighting and phone charging for 20–24h.
- Saltwater precaution: keep the station below deck in a dry, ventilated location. LFP is stable but should not be exposed to direct spray, immersion or sustained high humidity. The original storage bag provides adequate protection in a dry cabin.
Liveaboard remote work: the complete energy setup
- The minimum viable work setup: Starlink (75W), router (15W), laptop (65W), monitor (35W) = 190W continuous for 9h = 1,710Wh/day for work alone. Add fridge (25W avg × 24h = 600Wh) + navigation (200Wh) + lighting (100Wh) = ~2,610Wh/day total.
- Solar needed: in Portugal, Spain or Mediterranean in summer (5.5h peak), 2,610Wh ÷ 5.5h ÷ 0.85 efficiency = ~558W of panels. Two 200W portable panels + one 200W stern arch panel = 600W covers the full daily load.
- Starlink on a boat: Starlink Maritime is designed for vessel use with a motorised dish that compensates for movement. Standard Starlink works at anchor and in marina but must be marked as a fixed address — check your subscription terms for nomadic use.
- Internet backup: 4G/5G router as secondary internet when Starlink is offline (weather, obstruction). A dual-WAN router automatically failovers. Crucial for video calls and client work.
- Work schedule and energy management: align high-power work sessions (video calls, large uploads) with peak solar production hours (10:00–15:00). Charge laptops and devices during this window. Run watermaker during peak solar. Reduce loads after sunset.
- Marina vs anchorage productivity: marina berths with shore power simplify energy management but come with cost, noise and limited privacy. A well-designed solar + LFP system allows indefinite working anchorage time in suitable weather — often the most productive environment.
Offshore passage energy planning
- The autopilot dominates everything. On a 5-day Atlantic crossing, autopilot draw (100W average × 24h × 5 days = 12,000Wh) exceeds all other loads combined. Sizing the charging system around the autopilot is the first step in passage planning.
- Passage energy budget template (40ft sloop, 5 days):
- Autopilot: 100W × 120h = 12,000Wh
- Navigation electronics (chart plotter + AIS + instruments): 50W × 120h = 6,000Wh
- Fridge: 25W avg × 120h = 3,000Wh
- Lighting + communication: 30W × 60h (nights) = 1,800Wh
- Crew devices + watermaker: 50W avg × 120h = 6,000Wh
- Total: ~28,800Wh over 5 days = 5,760Wh/day
- Charging available: alternator (1h/day motoring = 600Wh) + solar (400W × 8h × 0.85 = 2,720Wh) + hydrogenerator (200W × 12h sailing = 2,400Wh) = ~5,720Wh/day available. Near-balance with disciplined load management.
- Safety margin: always plan for 20–30% more charging capacity than expected daily consumption. Bad weather reduces solar, motoring burns fuel, and an unexpected equipment failure can change the load profile entirely.
- Emergency reserve: designate 20% of battery capacity as untouchable emergency reserve — for engine start, distress signals and minimum navigation electronics. Never allow the house bank to drop below this level during a passage.
Technical notes before buying
- All DC wiring on a boat must use marine-grade tinned copper cable. Standard automotive or household cable corrodes rapidly in the salt air environment and creates resistance losses and fire risk.
- Fuse every circuit as close to the battery as possible. DC arc faults are a leading cause of boat fires. A 200A fuse at the battery terminal is the minimum for a house bank connection.
- When adding LFP to an existing AGM system, never connect them in parallel. LFP charges to a higher voltage than AGM can tolerate. Run separate banks with a DC-DC charger or battery-to-battery isolator between them.
- Confirm your MPPT controller accepts the Voc of your panel array in cold conditions. Panel Voc increases by ~0.3%/°C below 25°C. A 200W panel with 24V Voc at 25°C may reach 27V at 0°C — ensure the controller can handle the upper limit.
- For liveaboard insurance purposes, document all electrical modifications. Insurers may require a qualified marine electrician's sign-off for lithium battery installations on insured vessels.
Build a real autonomy estimate
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