How to Size a Boat Solar Array

Nobody guesses the size of their boat’s fuel tank, yet plenty of owners pick a solar panel the same way they pick a cooler: by what looks big enough on the shelf. Then they get to the anchorage, run the fridge for two nights, and wake up to a dead house bank and a warm beer. Sizing is not the exciting part of a solar project, but it is the part that decides whether the whole system actually works.

The goal here is simple. You are matching the energy your panels make in a day to the energy your boat burns in a day, with enough cushion for cloudy weather and long nights at anchor. Do that math honestly and the rest of the install is just wire and screws.

This guide walks the whole process in order, with real numbers from real boats, so you can size boat solar array capacity for your own vessel instead of copying a number off a forum.

Step 1: Audit what your boat actually uses in a day

Everything starts with an energy audit, which is just a list of every DC load on the boat and how long each one runs in 24 hours. The unit you care about is the amp-hour (Ah), which is a current of one amp flowing for one hour. A device that pulls 5 amps for 4 hours uses 20 amp-hours.

Walk the boat and write down every load. Fridge, cabin lights, chartplotter, VHF, phone chargers, water pump, autopilot, fans. For each one, note the amps it draws and a realistic number of hours it runs per day.

Be honest about run time. The mistake I see most weekends is someone listing their fridge at “a few hours” when a 12V compressor fridge actually cycles on and off around the clock and averages 30 to 60 amp-hours a day all by itself.

Add it all up. This example lands near 69 amp-hours a day, which is a normal figure for a coastal weekender. Multiply amp-hours by 12 volts if you want the answer in watt-hours instead: 69 Ah times 12 is about 828 watt-hours a day.

Step 2: Turn daily draw into a panel wattage target

Now convert your daily need into panel size. The rough rule most of us use in temperate cruising grounds: one watt of panel makes about 4 to 5 amp-hours per day in decent summer sun. That already accounts for the sun not being overhead all day and for normal system losses.

So take your daily amp-hour number and divide by roughly 4.5. Our 69 Ah boat needs about 69 divided by 4.5, which is around 15 amps of daily production, or put differently, about 155 watts of panel just to break even.

Breaking even is not the target, though. You want the batteries to actually reach full most days, with headroom for a gray, rainy weekend.

Add margin, then round up to real panels

Take that break-even figure and add 25 to 40 percent for cloud, shading, panel aging, and hot-cell losses. On the example boat, 155 watts plus a 30 percent cushion is about 200 watts. In practice you would fit a single 200-watt rigid panel, or two 100-watt panels, and call it sized right.

Panels come in standard sizes, so round up to what you can actually buy and mount. Nobody sells a 178-watt panel. If your math lands between sizes, go bigger. Extra solar on a boat is rarely a problem; too little always is.

Step 3: Match the battery bank to the array

Panels make power during the day. Your battery bank stores it for the night, and if the bank is too small or worn out, the finest array on the market cannot help you. Sizing solar without checking the bank is like buying a bigger hose for a leaky bucket.

A usable rule: your usable battery capacity should cover at least one full day of draw, ideally two, so you can ride out a cloudy day. Remember that lead-acid batteries only give up about half their rated capacity before you damage them, while a lithium boat battery upgrade lets you use most of the rated number.

For our 69 Ah-per-day boat, that means roughly 140 usable amp-hours. In lead-acid terms that is a 280 Ah bank; in lithium, a 150 Ah bank does the same job and weighs a third as much. If your bank is smaller than your daily draw, fix that before you spend a dollar on panels, and our Marine Solar Power Setup Guide covers how the two are sized together.

Step 4: Size the charge controller and the wire

The charge controller sits between the panels and the battery, regulating the voltage so the panels never overcharge the bank. Size it by the maximum current your array can push into the battery.

Take array watts divided by battery voltage to get amps. A 200-watt array on a 12V bank is 200 divided by 12, about 17 amps, so a 20-amp controller gives comfortable headroom. Panels can briefly exceed their rating in cold, bright conditions, which is why you never size a controller with zero margin.

Controller type matters as much as size. A cheap PWM unit (pulse width modulation) simply connects panel to battery and wastes the extra voltage, while an MPPT unit (maximum power point tracking) converts that surplus into usable charging current, gaining 15 to 30 percent on the same panels. For anything over 100 watts I fit MPPT every time; the reasons are laid out in our comparison of MPPT vs PWM Solar Controllers.

Wire the array in tinned marine cable and fuse every positive lead near its source. A 200-watt array wants roughly 10-gauge wire on a short run, stepping to 8-gauge for longer runs to hold voltage drop under 3 percent. The 12V Boat Wiring Guide for Beginners spells out gauge selection if this part is new to you, and the American Boat and Yacht Council publishes the drop standards most reputable installers build to.

Step 5: Account for mounting, angle, and shade

Two identical arrays can perform wildly differently depending on where they sit. A panel flat on a cabin top in a marina surrounded by masts might make half its rated output, while the same panel high and clear on an arch makes nearly all of it.

Shade is brutal on solar. The cells in a panel are wired in series, so shading even one cell, from a boom, a backstay, or a folded bimini, can choke the entire panel. High, clear mounting beats raw wattage almost every time.

A boat I rewired last spring had 300 watts mounted low on the coachroof and could barely keep up with a single fridge, because the boom threw a shadow across it every afternoon. We moved 200 of those watts up onto the bimini frame and the owner suddenly had surplus power with fewer panels in the sun.

A full worked example, start to finish

Put it together on a 32-foot coastal cruiser. The energy audit came to 69 amp-hours a day. Divided by 4.5 gives a 155-watt break-even, and a 30 percent margin pushes that to roughly 200 watts of panel.

That 200-watt array feeds a 20-amp MPPT controller, sized from the 17-amp peak, into a 150 Ah lithium bank that carries the boat through one cloudy day with margin to spare. The run is wired in 10-gauge tinned cable with a 25-amp fuse at the panel and another at the battery.

Parts cost lands somewhere around 450 to 700 dollars for the panel, controller, cable, and fuses, depending on whether you buy rigid or flexible and how good your wiring already is. That is a weekend of work and a system that keeps the fridge cold at anchor indefinitely.

Where owners still get it wrong

The most common miss is sizing to the boat instead of the loads. A 40-footer with modest electronics may need less panel than a 28-footer with a big fridge and a watermaker. Length tells you nothing; the audit tells you everything.

Second is forgetting the margin. A system sized to exactly break even on a perfect July day will fall behind the first time clouds roll in for a weekend. Build in the cushion and you stop babysitting the battery monitor.

Do the audit, add the margin, match the bank and controller, and mount the panels where the sun actually reaches them. Get those four things right and you will spend your weekends sailing instead of rationing amps.