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.
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%|
|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 appreciate an abundance of power, while the occasional cruisers may be satisfied with less.
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 doesn’t 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.
If you’re not sure if it’s time to upgrade or replace your solar panels, the upcoming June 2023 issue offers detailed guidance on evaluating old solar panels and house batteries. If you are upgrading your boat’s electrical system, adding new accessories, or just replacing some wires, our recently updated six-volume ebook Marine Electrical Systems covers everything you need to know about electrical systems–including Batteries, System Installation (including rewiring and lightning protection), Panels, Monitors, Charging, Alternative Energy, and AC Systems.
Power Usage Table
Drew Frye is a frequent contributor to Practical Sailor. He blogs at www.saildelmarva.blogspot.com