Estimating Solar Panel Size for Boats


The starting point for a successful solar panel installation is quantifying your requirements. We present here a simple analysis based on the test boat used for our recent report on choosing and installing a solar panel. Some values are from experience, and others are accepted rules of thumb. For more details on choosing and installing a solar panel, see the March 2018 issue of Practical Sailor online.

Energy Balance

Look up the current draw of each piece of equipment (confirm with panel ammeter if available) and estimate the number of hours operated. Record the number and capacity of your batteries, recognizing that you cannot draw below 50% charge without shortening their life, and that you will seldom charge past 85% while away from the dock-as a result, only 35% of nameplate capacity is really useable. Finally, total your charging sources, including engine, wind, and solar. For solar, take the rated wattage x 5 hours/12 = amp-hours while on passage and wattage x 7 hours/12 = amp-hours while at anchor (sails do not shade and the boom can be rigged out to the side). This is far below the rated capacity-sailors in the tropics will do better, and sailors farther north or sailing in the winter more poorly-but this is an accepted starting point.

Estimating Panel Output

Full Sun-Panel square to the sun 100%
Full Sun-Panel at 45 angle to sun 71%
Light overcast 20-70%
Heavy overcast 10-20 %

How many days can you manage with poor generation? Are you willing to economize during a long cloudy stretch? Will you recharge at a marina or by running the engine periodically? Long-term cruisers appriciate an abundance of power, while the occasional cruisers may be satisfied with less.

Saving Power

Every AH (amp-hour) consumed has a real cost in weight, panels, and dollars. If you can reduce consumption by 50 AH/day you will save a battery (the useable capacity), a 120 watt panel, and perhaps a mounting arch. The cost savings might be $500 and 150 pounds for just a few bulbs.

  • Lighting. Switch from incandescent to LED and fluorescent lighting, starting with the lights you use most. We use LEDs and fluorescent for the anchor, salon, and cockpit lights, but since we seldom run at night, we left the running and steaming lights alone. Likewise, the deck light and many task lights remain halogen or incandescent; they are not used enough to matter.
  • Go to bed at night and get up with the sun. Big savings in juice and more time to play.
  • The gas solenoid is a big user for us; it runs the propane fridge and cabin heater, so it is on for long hours, but we can turn it off at night or go without refrigeration now and then.
  • Fans. Run them on low speed and watch the hours. A wind scoop doesnt use power.
  • Instruments. Do you actually need GPS and other instruments full-time on passage? Twenty years ago they didn't even exist. Balance the sails to minimize the load on the autopilot.

Power Usage Table

Drew Frye is a frequent contributor to Practical Sailor. He blogs at

Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


Please enter your comment!
Please enter your name here