How to Calculate the Watts Needed for Solar Panels

••• solar panels image by Yali Shi from Fotolia.com

If you're planning to go green on your boat, solar panels to charge your deep-cycle battery probably occupy a place on your "to do list." Sizing the solar panel for your boat's needs requires that you calculate the wattage that the solar panel must produce to gently charge the battery--a trickle charge. This means knowing the sources of electricity on your boat and how much electricity is used for each appliance or device.

Make an inventory, using pencil and paper, of everything on your boat that uses electricity, from the gauges in the wheelhouse to the bilge pump. As you make the list, record the number of watts each item uses. The wattage will be located on the manufacturer's plate where the model and serial number for the device is shown and is sometimes indicated by the letter "W," as in "100W." If no wattage is shown, record the amps, sometimes indicated by the letter "A," as in "1 A." If a device or appliance lists only the amps, record the number of volts the device uses, also listed on the manufacturer's plate.

Convert amps to watts by multiplying, with a calculator, the number of amps of current the device uses by the number of volts. If a 12-volt device shows it uses 2 amps, then: 12 by 2 = 24 watts.

Estimate how many hours each of these devices will run (draw on your battery) in a typical day. If you have a stereo that uses 24 watts of electricity (12 volts by 2 amps = 24 watts), and you run it 3 hours a day, that's 72 watt-hours of electricity.

Add up all the watt-hours of electricity the devices and appliances on your boat uses. If you have a light that uses 60 watt-hours, a stereo that uses 72 watt-hours and your instruments use 10 watt-hours, the total is 142 watt-hours in a typical day.

Divide the total watt-hours your boat uses by 5. If you use solar panels, this is the number of hours per day you have to recharge your batteries to maintain a full charge. In this example, 142 / 5 = 28.4 watts, so a 30-watt panel would be sufficient.


  • Allow for expansion. You may buy a bigger stereo or add other devices or appliances to your boat's electrical inventory.


  • Your batteries won't overcharge, because your panel will connect to the batteries through a charging controller that prevents this.


About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits