Before you price a single solar panel, build your energy budget. It sounds like obvious advice and it is, but boats are full of systems added incrementally by different owners over different decades, and the actual daily draw is almost never what anyone guesses. The number that matters is how many amp-hours your house bank loses between sundown and the time solar production resumes the next morning. That's the hole your renewable system needs to fill.

Work through the list item by item. Refrigeration is typically the largest load: a 12V compressor running a standard live-aboard refrigerator and no freezer draws 40 to 60 Ah per day, depending on ambient temperature, insulation quality, and how often the lid opens. In the tropics, that number climbs. An autopilot on passage draws 20 to 40 Ah per day depending on sea state — a confused chop works the pilot harder than a steady beam reach. Instruments, chartplotter, and VHF in standby add another 8 to 12 Ah. Lighting adds 3 to 8 Ah if you've converted to LED throughout. Add up your specific loads, add 20% for inefficiency and stuff you forgot, and that's your daily deficit to cover.

Solar Panels: What to Buy and Why

The choice between monocrystalline and polycrystalline panels has essentially been settled in favor of mono. Monocrystalline cells — cut from a single silicon crystal — convert sunlight to electricity more efficiently, typically 19–22% vs. 15–17% for poly, and they perform better in low-light and high-temperature conditions. The price premium has largely evaporated. Buy mono.

The flexible vs. rigid debate is more nuanced but the conclusion is similar: rigid panels win on every technical metric. Rigid tempered-glass panels last 20–25 years with minimal degradation. Flexible panels — the kind you stick to a bimini or a curved deck — degrade noticeably within 3 to 5 years, often losing 20–30% of rated output by year three. They're also prone to delamination in marine environments. Flexible panels solve a real installation problem when you have no flat mounting surface, but go in knowing they're a compromise with a shorter service life.

Wiring panels in series increases voltage; wiring in parallel keeps voltage constant and increases amperage. Series wiring works with MPPT controllers to recover more energy, but the critical drawback is shade sensitivity: in a series string, the output of the entire string drops to match the worst-performing cell. One palm frond shadow on one corner of one panel can cut string output by 50% or more. On most cruising boats where shade from the boom, the mast, and standing rigging is unavoidable at certain angles, parallel wiring (or individual panels on separate controllers) often produces better real-world results than the series setup that looks optimal on paper.

Sizing Solar

The frequently cited rule is 100 watts of panel for every 100 Ah of house bank. This is a reasonable starting point for boats in the tropics with adequate solar exposure — roughly 5 to 6 peak sun hours per day — but it breaks down in higher latitudes and cloudy climates where you might see 2 to 3 peak sun hours on a winter day in the Pacific Northwest.

The more precise calculation: estimate your daily Ah deficit from the budget above. Divide by the expected peak sun hours for your cruising area. That gives you the minimum watts of panel needed, before accounting for system losses. Add 25–30% for real-world inefficiency — wiring losses, dirty panels, temperature derating (solar panels produce less in heat, counterintuitively), and the inevitable shade from your rig. A boat with an 80 Ah daily deficit cruising in the Eastern Caribbean should be targeting 150 to 200 watts of panel minimum; 300 watts gives comfortable margin.

More panel than you think you need is almost never a problem. You can divert excess production to a water heater. You can run the watermaker longer. You can charge devices freely. Undersized solar means you're running the engine to charge — which is fine occasionally and tedious as a daily habit.

MPPT vs PWM Charge Controllers

A PWM (Pulse Width Modulation) controller is a simple voltage regulator. It connects your panels to the battery bank and throttles the connection when the battery approaches full charge. Inexpensive, reliable, and adequate for small systems where the panel voltage is closely matched to the battery bank voltage.

An MPPT (Maximum Power Point Tracking) controller actively seeks the voltage and current combination at which the panel array produces maximum power, then converts that power efficiently into the voltage and current the battery bank needs. In practice, this recovers 15 to 30% more energy from the same panel array compared to PWM — the difference is most significant when panel voltage is high relative to battery voltage, which is the case in cold weather, morning conditions, and with series-wired arrays.

The math: on a 200-watt array producing 25 Ah/day with a PWM controller, switching to MPPT might produce 29 to 32 Ah/day. Over a month of cruising, that's 120 to 200 Ah recovered — energy you don't have to make with the engine. MPPT controllers cost $50–$150 more than comparable PWM units. The payback is rapid. Above 200 watts of panels, MPPT is not optional; it's the correct answer.

Victron's SmartSolar MPPT line is the de facto standard in the cruising community. Bluetooth monitoring via the VictronConnect app, integration with the Victron GX monitoring ecosystem, and solid build quality. Renogy and EPever make serviceable alternatives at lower price points for budget-conscious installations.

Wind Generators

Wind generators make sense on boats that spend extended time in cloudy climates, sailors making long passages where solar angles are poor, or any boat where the wind blows reliably at anchor. They produce nothing in calm conditions and peak output in winds above 20 knots — exactly when the boat is heeled, bouncing around, and the solar panels are pointed sideways.

The Primus Air 30 (previously the Air Breeze, rebranded after acquisition) is the unit most commonly installed on cruising boats in the 30-pound and under category. Rated at 400 watts, it realistically produces 30 to 60 Ah per day in 15 to 20 knots of steady wind. The noise is the issue nobody mentions in the product specifications: the Air 30 hums and the blade tips whistle at speed, loudly enough that neighbors in a quiet anchorage will notice. Many cruisers disconnect theirs at night in populated anchorages from consideration for others. This is not a minor social footnote — it's a legitimate operational reality.

Wind generators also require a dump load: a resistor or water heater element that absorbs excess power when the battery bank is full and the wind keeps blowing. Without a dump load, the turbine overspeeds when disconnected from a load, damaging bearings and blades. Most controllers include a dump load output.

Tow Generators

The Watt & Sea is a propeller-driven generator that mounts on the transom and trails in the water on passage. At 7 knots of boat speed it produces approximately 200 watts — 15 to 17 Ah per hour — which over a 24-hour passage day is a meaningful contribution. It deploys in minutes, it's completely silent, and it produces power in any weather condition when the boat is moving.

The trade-off is drag. Expect to lose 0.3 to 0.5 knots of boat speed with the unit deployed. On a 2,000-mile passage at average 6.5 knots, that lost speed adds 20+ hours to your passage time. Whether that tradeoff makes sense depends entirely on your energy situation — boats with robust solar rarely need it; boats making high-latitude passages in grey weather find it invaluable.

The Complete System

A well-designed power system has five layers working together. The generation layer — panels and/or wind turbine — produces raw power. The charge controller manages the flow from generation to the battery bank, preventing overcharge. The battery bank stores energy against periods of low generation. The inverter (if the boat uses AC appliances) converts 12V DC to 120V AC. And the system monitor gives you real-time visibility into what's happening at each layer.

The Victron Color Control GX is the monitoring standard most serious cruisers converge on. It integrates with Victron MPPT controllers, the Multiplus inverter/charger, and the BMV battery monitor to display production, consumption, state of charge, and historical data on a single screen — and remote access via the VRM portal when you have cellular or satellite data. The system isn't cheap but it replaces hours of detective work when something's not right. Knowing your state of charge, your net current flow, and your 7-day production history in one glance is how you manage a boat's power rather than just hoping it works out.