How Much Solar Do I Need on My Boat?

The most common sizing mistake we see is starting from the panel. Someone buys 200 W because it fits the bimini, then wonders why the batteries are flat by Sunday. Start from what the boat uses each day, then work out how much panel it takes to replace that, in the worst month you plan to be aboard.

Step 1: Do a Power Audit

List every DC load, its current draw in amps, and how many hours a day it runs. Multiply to get amp-hours (Ah) per day. Loads that cycle, like a refrigerator, need a duty cycle: the share of the day the compressor actually runs. The example below is a typical cruising sailboat at anchor. Your numbers will differ, so measure or look up your own.

The refrigerator is more than half the total. That is normal, and it is the reason most people add solar in the first place. Watch for the loads that are easy to forget: satellite internet left on all day, an autopilot on a passage, or an inverter idling with nothing plugged in can each add as much as the rest of the list combined.

Step 2: Know What 100 W of Panel Really Makes

A panel's rating assumes ideal lab conditions. On a boat you lose output to heat, wiring, the controller and a sky that is rarely perfect. We use a 25 percent allowance and the peak sun hours for southern New England: about 5 in summer, 4 as a season average, and 3 in fall and winter.

Daily Ah per 100 W = 100 W / 12 V x peak sun hours x 0.75

That works out to roughly 31 Ah in summer, 25 Ah on an average day and 19 Ah in the fall from each 100 W of panel.

Step 3: Size the Array

Watts needed = daily Ah / Ah per 100 W x 100

For the 76 Ah boat above:

• Summer: 76 / 31 x 100 = about 250 W
• Season average: 76 / 25 x 100 = about 300 W
• Fall: 76 / 19 x 100 = about 400 W

Same boat, same daily use, and a 60 percent difference in the array you need. Size for the last month you plan to be aboard, not for July. In midsummer, long days make up for a lower sun angle almost anywhere on the East Coast. That ends in the fall, and output drops faster the further north you are. A system that holds 13.3 V with the refrigerator running in July can be struggling to stay above 12 V by mid September.

Step 4: Add Margin

Three cloudy days at anchor will drain a bank that solar only just keeps up with. Extra panel wattage is what recovers it once the sun returns. Two rules of thumb we use:

• Size the array to the worst month, then add more if you have the space. On a boat, more solar is almost always better.
• As a floor, carry at least 100 W of panel for every 100 Ah of battery, and more for a lithium bank, which can absorb a strong charge without tapering off the way lead-acid does.

The battery bank should carry two to three days of use on its own. If you are sizing the bank too, see how to size a boat battery bank and selecting batteries for your boat.

What Changes the Numbers

• Shade. A boom or mast shadow can cost far more than its size suggests. The Marine Solar Design Guide covers shading and bypass diodes.
• Panel temperature. Flexible panels bedded on a deck run hot and lose output; see understanding solar panel specifications for the temperature math.
• The controller. A PWM controller can give up 20 percent or more of the array. The figures here assume MPPT; see MPPT vs PWM.
• Where it fits. The array you need has to fit somewhere sensible. See where to mount solar on a boat.

Boat Solar Sizing FAQ

How many amp-hours does a 100 W solar panel make on a boat?

In southern New England, about 31 Ah on a good summer day, 25 Ah on an average day and 19 Ah in the fall, after allowing 25 percent for heat, wiring and controller losses. Sunnier latitudes do better in winter.

Is 200 W of solar enough for a boat refrigerator?

Rarely. A 12 V compressor fridge typically uses 40 Ah or more a day on its own, and 200 W makes about 50 Ah on an average day. Add phones, electronics and lights and most boats with refrigeration need 300 to 400 W.

Should I size solar for summer or fall?

For the last month you plan to be aboard. The same boat in our example needs about 250 W in July and about 400 W in the fall.

Want Us to Run the Numbers?

Sea & Land Yacht Works designs and installs marine solar systems to ABYC standards from our shop in South Kingstown, Rhode Island. We can do the power audit with you and size the array, bank and controller together. Call 401-753-7479 or request a consultation. Browse solar panels and MPPT controllers and battery monitors to measure what you actually use.