Ask a group of offshore cruisers what they'd prioritize in a boat refit and solar almost always comes up in the first sentence. The reasons are straightforward: solar is silent, it requires almost no maintenance, it has no fuel cost, and it works continuously at anchor — which is where cruising boats spend most of their time. An adequately sized solar system means the engine runs for propulsion, not to charge batteries. That single change improves the quality of life aboard more than most other upgrades.
The challenge is that solar is frequently sized on enthusiasm rather than math. People install 400W of panels, watch their battery monitor, and conclude that solar "works" — without knowing whether they're charging twice what they need or half. Getting the sizing right requires understanding your actual daily consumption, how much output you can realistically expect from panels in your sailing area, and what the gaps are between the two.
Step One: Calculate Your Daily Amp-Hour Load
Solar sizing starts with consumption, not with panels. Before buying anything, you need to know how many amp-hours per day your boat actually uses. For a 12V DC system — which describes almost every cruising sailboat — the math is straightforward: watts ÷ 12 = amps, and amps × hours of use = amp-hours.
The major consumers on a typical cruising boat break down as follows. Refrigeration is usually the single largest load: a 12V marine refrigerator running at anchor in moderate temperatures draws 40–60Ah per day. In the tropics, the compressor runs harder and that can rise to 70–80Ah. Autopilot is significant on passage but minimal at anchor: a typical below-decks autopilot like a Simrad AP44 or Garmin GHP 20 draws 1–4 amps depending on conditions, amounting to 30–50Ah over a 12-hour day on the helm. At anchor with the autopilot off, this is zero. Instruments, AIS, and navigation electronics together typically draw around 10Ah/day — VHF radio, GPS chartplotter, depth sounder, AIS transponder. Phone and tablet charging adds roughly 5Ah/day for two people. LED lighting throughout the boat is a genuinely small load: an LED cabin light draws 0.4–0.8A, and a well-lit boat at anchor might use 5–8 bulbs for a few hours — call it 3Ah/day total.
Adding that up: a coastal cruiser at anchor in moderate temperatures (refrigeration running, instruments on, evening lighting, phone charging, autopilot off) runs approximately 60–80Ah/day. A more generous setup with a watermaker, a larger fridge, a second chartplotter at the helm, and heavier charging needs arrives at 100–150Ah/day. An offshore passage-maker with the autopilot engaged continuously, a watermaker, a satellite communication device, and a radar on approach runs 200Ah/day or more.
Do this calculation for your boat specifically. Don't use someone else's numbers. Use an amp-hour meter (a Victron BMV-712 or Victron SmartShunt with Bluetooth app is the best tool for this — it tracks actual consumption over days and weeks, not theoretical estimates) to establish a baseline before sizing solar.
How Much Power Panels Actually Produce
A panel rated at 200W produces 200W only under Standard Test Conditions: 25°C cell temperature, 1,000 W/m² irradiance, optimum angle perpendicular to the sun. None of those conditions exist on a moving sailboat — or even on a stationary one in a real marina.
The real-world derating factors stack up quickly. Angle losses: solar panels lose output rapidly as they move off perpendicular to the sun. A panel lying flat on a boat deck is rarely at the optimum angle unless you're near the equator at solar noon. Expect 10–20% loss from suboptimal angle throughout the day. Shading: even partial shading of a single cell in a panel can drop output dramatically on panels without bypass diodes or optimizers. A boom shadow across the corner of a panel for two hours in the morning is a meaningful loss. Heat: solar panels lose roughly 0.4–0.5% of output per degree Celsius above 25°C. A panel on a hot tropical deck may be running at 55–65°C cell temperature — a loss of 12–20% before any other factor. Wiring and controller losses: another 5–10%.
Add these together and a realistic efficiency figure is 60–70% of rated output under good conditions — meaning that 200W panel produces effective output equivalent to 120–140W on a good day. Over a full day (accounting for morning and evening angles, clouds, and the fact that peak irradiance exists for perhaps 4–6 hours), a 200W panel produces roughly 70–100Ah per day in favorable conditions (the tropics, summer, low latitude). In New England in October or Scotland in June, cut that number significantly.
The rule of thumb that works for initial sizing: one amp-hour of daily production per watt of installed panel in average conditions. A 400W array produces approximately 400Ah/day in the tropics and considerably less in higher latitudes or overcast seasons. Use this as a starting point and refine based on where you actually sail.
System Size by Use Case
Coastal weekender with moderate consumption (100–150Ah/day): 200–400W of panels is typically adequate. Two 100W panels on a stern arch or bimini top, feeding a Victron SmartSolar MPPT 100/30 controller, charging a 200–300Ah AGM or lithium house bank. This system covers weekends at anchor without running the engine and provides a reasonable charge buffer for the week between trips. It won't sustain a liveaboard or a passage-maker, but it's not designed to.
Extended cruiser or liveaboard running 150–250Ah/day: 600–900W is the target range. Three to four 200W rigid panels on a solid stern arch or hard bimini, with a Victron SmartSolar MPPT 150/60 or 150/85 controller (the number after the slash is the maximum charge current in amps — size this to your expected peak input). This system runs refrigeration, instruments, and moderate cruising loads at anchor indefinitely in the tropics without engine charging. At higher latitudes or on passage with heavy autopilot use, engine charging supplements solar.
Bluewater liveaboard or ocean crosser running 200Ah/day and up: 1,000–1,400W or more, usually split across multiple arrays — a hard bimini, a stern arch, and possibly panels on davits. Multiple controllers or a large MPPT unit (Victron SmartSolar MPPT 250/100 handles up to 100A output). At this scale, shading management becomes critical: two separate arrays on separate controllers prevents a shaded forward panel from dragging down an unshaded aft array. This is also where panel layout and wiring configuration (series vs. parallel strings) become technical decisions worth working through with someone who has done it before.
Rigid vs. Flexible Panels
The fundamental trade-off is simple: rigid panels perform better and last longer; flexible panels fit surfaces that can't support a rigid frame.
Rigid glass-laminate panels — the standard monocrystalline or polycrystalline panel in an aluminum frame — last 25 years with minimal degradation. Most manufacturers warrant 80% output at 25 years. They're heavier (a 200W rigid panel weighs 12–15kg), they require a mounting structure that can carry their weight and handle offshore loads, and they can't conform to curved surfaces. On a proper stern arch or hard bimini, they're the right answer for any serious cruiser. The additional weight aloft is a real consideration on a racing or performance boat; it's less significant on a typical cruising boat.
Flexible (thin-film or flexible monocrystalline) panels — these can be bonded or adhered to curved surfaces like a coachroof or dodger, weigh significantly less, and don't require the same mounting infrastructure. They're genuinely useful where a rigid panel won't fit. The downsides: they degrade faster, with a realistic service life of 8–12 years before output drops meaningfully. They're more sensitive to heat because they're installed flat against a deck surface with no air gap for cooling. They're also more expensive per watt than equivalent rigid panels. For most cruisers, flexible panels make sense as a supplement to a main rigid array — on a curved dodger or coachroof where there's no room for a framed panel — rather than as a primary system.
One practical note on flexible panels: adhesive installation is irreversible and eventually fails, leaving you with a panel you can't remove without damaging the substrate. Budget for replacement when selecting this route.
MPPT vs. PWM Controllers — This Matters More Than Panel Brand
The charge controller sits between your panels and your battery bank and manages the charging process. There are two technologies: PWM (pulse width modulation) and MPPT (maximum power point tracking).
PWM controllers are cheap and simple. They work by connecting the panels directly to the battery when charging is needed and disconnecting when the battery is full. The problem is that a PWM controller forces the panel to operate at battery voltage — which is almost never the panel's optimal voltage. A 200W panel with a Vmp (maximum power point voltage) of 18V connected to a 12V battery through a PWM controller is being forced to operate at 12V. Output at 12V rather than 18V is roughly proportional to the voltage ratio — you're getting about 67% of what the panel is capable of producing.
MPPT controllers continuously track the panel's actual maximum power point — the combination of voltage and current at which the panel produces peak power — and convert that power to the voltage the battery needs. The result is 20–30% more power extracted from the same panels compared to PWM, consistently, across all conditions. The controller costs more (a Victron SmartSolar MPPT 100/20 runs $130–160 versus $30–50 for a comparable PWM unit) but the payback in additional charging capacity is measured in weeks, not years.
Use MPPT. For any system over 100W, the math is unambiguous.
Brand Recommendations
Controllers: Victron SmartSolar MPPT — this is the industry standard, and it deserves that status. The SmartSolar series integrates with Victron's VE.Direct and VE.Smart networking, so your controller, battery monitor, and charger share data and coordinate charging. The Bluetooth app (VictronConnect) gives you real-time data and historical logging without additional hardware. Build quality is excellent, warranty support is responsive, and the product has a track record across thousands of offshore boats. The 100/20 handles up to 20A charge current (suitable for 260W of panels into a 12V bank); the 100/30 handles 30A (400W); the 150/60 and 150/85 cover larger systems. Buy the size appropriate for your array and add buffer — sizing a controller to 80% of its maximum input is good practice.
Panels: Renogy — for most cruisers, Renogy's rigid monocrystalline panels offer genuine value at $0.60–0.90 per watt. They perform as specified, carry a reasonable warranty (10-year product, 25-year power output), and are available through West Marine and online. They're not the highest efficiency panels available — SunPower cells achieve 22%+ cell efficiency versus Renogy's 19–20% — but the difference in output is proportional to the price difference, and for most boats, the extra cost per watt of SunPower panels isn't justified unless you're extremely space-constrained and need maximum output from a small area.
Flexible panels: SunPower cells — if you're committed to flexible panels, buy ones built with SunPower cells (Maxeon technology). SunPower-celled flexible panels have better heat tolerance, slower degradation, and higher efficiency per square foot than panels built with conventional flexible cells. They cost more, but the premium is worth it if you're relying on them as part of your primary array.