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Marine Solar Power Calculator RoundUP

Solar Power Calculator for Off Grid Mariners

Attention all marine enthusiasts!!  You know you love the water. You know you hate a dead battery. Warm drinks out on the bay spoils the fun. Use this handy calculator to estimate how much solar power you need to stay charged out on the water until the wee hours of the day are over.

The Amps listed below are estimates for typical electronic gear and appliances. You may alter the amp number to suit your actual appliances and equipment. Then, enter the number of hours, or fraction thereof as a decimal, that you estimate using those items during a typical day out on the water. Scroll down to "calculate" your estimated solar panel watt requirements.

The default below is set to 5 hours of direct peak sunlight per day, and you may also edit this data field as needed. You may get more or less direct sunlight, depending on your geographic location.

Marine Solar Panel Size Calculator

Appliance Amps (at 12VDC) Daily Use (hrs/day)
Bilge Pumps
VHF (receive)
VHF (transmit)
Medium Fridge
Anchor Light
Laptop Computer
Total AmpHours per Day: 0


AmpHours used per day


Hours of Direct Sunlight per day
Minimum Watts Required


Recommended Panel Size (add 20%)



To identify the number of peak sun hours you can expect, determine the solar irradiance figure (kWh/m2/day) for your geographic location by using the interactive photovoltaic map provided by National Renewable Energy Laboratory (NREL), US Department of Energy. This will give you specific peak hours for a given month in your geographic location. Or, if annual average peak sun hours is sufficient for your estimates, review the NREL national photovaltaic map.

Now, cross-check your panel size estimate from the calculator vis-a-vie your marine on-board battery equipment. Calculate how many total watts of energy per day that you can get from your panels. Multiple your solar irradiance figure (kWh/m2/day) by 75% to account for inefficiencies during the charge process, and then multiply that figure by the total wattage of your solar panel.

Let’s look at one example with 180W of solar power in Miami, FL.

5.3kWh/m2/day x .75 x 180W = 715.5W 

In this case, from your 180W panels, you can expect to realize approximately 715.5 watts per day of solar power to charge your on-board batteries, and/or run other equipment directly from solar during the day.

As an aside, LearningDirect provides a clear and simple YouTube video on the issue of calculating peak sun hours.