Understanding your boat's solar sizing
The direct answer for a typical cruising boat
A cruising sailboat using 1,200Wh a day with 5 peak sun hours and typical boom or rigging shading needs about 350 watts of panel. A powerboat with the same load and minimal shading needs closer to 300 watts, since it doesn't lose output to moving shadows crossing the panels throughout the day. Start with the NREL's PVWatts solar production calculator if you need the official wording rather than a summary.
Why sailboats need extra margin that powerboats don't
A boom, mast, or rigging line casting even a narrow moving shadow across a solar panel can disproportionately reduce that panel's output, since solar cells wired in series are limited by their weakest, most-shaded cell. This effect is why sailboats commonly need 10% to 15% more panel wattage than a powerboat with an unobstructed deck to produce the same daily output. Panels with built-in bypass diodes reduce, but don't eliminate, this penalty.
House bank versus starter battery
Solar panels on a boat charge the house battery bank, which powers cabin lights, refrigeration, navigation electronics, and other onboard systems, not the engine starting battery. Most marine electrical systems keep these two banks separate through a battery isolator or combiner, since draining the starter battery would leave you unable to start the engine. Confirm your solar charge controller is wired to the house bank specifically, not a combined or shared system, and use the marine battery bank sizing calculator to size that bank correctly. Pair this with the battery lifespan estimator to see the full picture before you commit. An off-grid system lives or dies on upkeep. The panel lifespan estimator in our Maintenance & Lifespan section can help you plan cleaning and inspection costs over the years.
Why marine-rated equipment matters
Salt spray and constant humidity accelerate corrosion on standard electrical connectors and panel frames faster than most inland environments. Marine-rated panels, connectors, and wiring use corrosion-resistant materials specifically built for this environment, and skipping this rating in favor of standard land-based equipment risks connection failures and reduced equipment lifespan over time on the water. This is exactly the kind of thing the US Coast Guard's electrical compliance guidelines cover, and they're worth reading before wiring anything on a boat. The real comparison is against a grid connection. The levelized cost of energy calculator in our Savings & Payback section can help you weigh the system cost against what the grid would charge.
What to check once you have a panel wattage target
Once you know your target wattage, check the charge controller sizing tool to match a controller to your array. The sun-hour assumptions used throughout this calculator come from NREL's PVWatts solar production calculator. For a boat you plan to keep for years rather than a single season, the battery lifespan estimator in our maintenance and lifespan tools is worth running against your house bank's expected duty cycle, and the general solar panel calculator in our system sizing tools is useful if you ever want to compare a marine array against a land-based one built to the same daily load.