The site and the design
Coachroof space limits the array to about 300 W of flexible
panels, mounted flat (0° tilt). That would be a weak array at
high latitude, but the tropical clearness index stays high
year-round: the bundled PVWatts TMY data for this anchorage runs
from 4.57 kWh/m²/day in December to 6.63 in May, a far
flatter seasonal swing than any mainland site. Steady sun is the
whole story here; a small system in a reliable climate can outdo
a bigger battery in a cloudier place.
- Solar array: 300 W flexible panels, flat on the coachroof
- Battery bank: 200 Ah AGM at 12 V (2,400 Wh)
- Charge controller: MPPT; inverter: modified sine wave, 88%
- Loads: marine fridge (50 W, 12 h/day), LED cabin lights,
autopilot standby, chartplotter and VHF, laptop and phone
charging; about 1,100 Wh/day total
Things to try
Bump panel watts to ~450 if you have a soft bimini you can add a
flexible panel to, or drop the fridge duty cycle if you're under
sail and water-cooled. Then switch the chemistry field from AGM
to LiFePO4 and compare the usable-capacity and battery-duration
columns; the rated amp-hours stay the same while the usable
fraction jumps from about 50 to 80 percent.
Open in calculator →
Frequently asked questions
How much solar does a cruising sailboat need?
It depends on the loads, not the boat. This example runs a marine
fridge, autopilot standby, chartplotter, VHF, LED lights, and
device charging (about 1,100 Wh per day) on 300 W of flexible
panels in the U.S. Virgin Islands, where irradiance stays between
roughly 4.6 and 6.6 kWh/m²/day all year. The same 300 W
array in a cloudier climate would fall short in winter; steady
tropical sun is what makes a small array work.
Can solar keep a boat fridge running overnight?
Yes, if the battery bank is sized for the overnight draw. The
fridge in this example averages 50 W for 12 hours a day; the
200 Ah AGM bank at 12 V stores 2,400 Wh, of which about half is
usable at a 50 percent depth-of-discharge target for lead-acid
chemistry. The overnight loads fit inside that budget with
margin, and the panels refill the bank the next morning.
Is AGM or lithium better for a sailboat house bank?
AGM is sealed, tolerant of the marine charging environment, and
cheaper up front, but you should only plan on using about half
its rated capacity if you want it to last. LiFePO4 gives you
about 80 percent usable capacity and more cycles for the same
rated amp-hours, at a higher purchase price. This example uses
AGM; switch the chemistry field to LiFePO4 in the calculator and
watch the usable capacity and battery-duration columns change.
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How the calculator works