Solar panels seem like the perfect solution to producing electricity on board a boat. They have no moving parts, make no noise, and the power is free. Like most things in life, however, solar panels represent a series of trade-offs. And even though sunlight is free, the purchase price of solar panels, plus installation costs, can represent a sizable up-front expense that must be factored into any cost equation.
The advantages of solar panels can be demonstrated by recounting the exploits of one voyaging boat. A few years ago my wife and I were anchored in Charlotte Amalie in the U.S. Virgin Islands, having just completed a rough offshore passage from North Carolina. A beautiful double-ended voyaging sailboat tacked into the crowded anchorage and dropped anchor nearby. At first I thought they were showing off their sailing skills, but we soon found out the truth. Their boat was one of the bestspecifically designed for offshore passagemaking, built like a battleship, and equipped with some of the latest cruising goodies. Before leaving Florida the owners had the boat thoroughly checked out by a top boatyard. New refrigeration and a high-output alternator were among some of the equipment installed. A few days into their 1,000-mile trip a horrible noise announced the end of the refrigeration compressor. The bracket that held the compressor fractured, taking out the alternator at the same time. The crew started eating thawing food as fast as they could.
A few days later the batteries went flat; no radio, no running lights, no engine starting. That tacking show in Charlotte Amalie was born of necessity, not valor. Every day the crew headed ashore to rendezvous with a string of refrigeration and electrical specialists, and two months later we found them anchored in the same spot, still repairing their systems.
To avoid a similar scenario I hooked up my first solar panel more than a dozen years ago. I didn’t want to be caught offshore without a backup power source that was utterly reliable. A small 10-watt panel guaranteed a trickle charge, and I no longer needed a battery charger when the boat was left in a marina for extended periods. Eliminating the shore-power connection also meant I didn’t have to worry about inadequate marina or boat wiring leading to electrolysis.
I then added another panel with a peak output of 45 watts. With both panels hooked up (55 watts total), I found I had plenty of charging for living aboard in New England, though I didn’t have electrical refrigeration. When I added a small Norcold portable refrigerator to the load, I found I could just keep up with demand by adding another 40-watt panel, an extra battery, and a small wind generator. With 95 watts of solar output and 12 hours of daylight, I should have been getting 1,140 watts, or 95 amp-hours at 12 volts.
Solar panels tend to be rated by their peak power output in watts. For example, a Solavolt SV-7500 panel is rated at 75 watts peak output. Since most of us are used to thinking in terms of current draw in amps and battery capacity in amp-hours, we’ll have to convert panel output from watts to amps. To convert watts to amps use the classic formula:
amps = watts ÷ volts.
Most solar panels produce a no-load output of around 17 to 18 volts. This peak voltage is reduced under load to approximately 14.5 to 15.5 volts, providing just about the right voltage to charge your batteries. Many catalogs provide a power rating for the panel in watts and a current rating in amps. Unfortunately, some published amperage ratings are based on peak voltage while others are based on voltage under load. What you need to know is the current rating in amps under load, and it may take some searching through the manufacturer’s brochures to find it. The most efficient solar panels have the highest amp output for a given peak wattage rating.
Dollars per amp
When building a solar panel system, you want to purchase the most amps per dollar, and the most amps per square foot. You might be tempted by the flexible panels that can be temporarily mounted almost anywhere. These panels use thin-film silicon cells that are approximately one half as efficient as the rigid panels; therefore, you’ll need twice the panel area for the same charging capability. The price of some flexible panels also looks good until you calculate how much they are costing you per amp of output. Despite these drawbacks, flexible panels may be ideal for some purposes. For example, a large flexible panel could be lashed to the boom when you’re at anchor, even if you don’t have the deck space to permanently mount rigid panels.
At this time, the best buys in power generation are the biggest rigid panels using large (five-inch), monocrystalline cells. The initial cost of a 50-watt model will be less than $400.00, and the average warranty seems to be about 10 years. In my experience, quality panels have lasted at least this long with no perceptible decline in output. The failure points tend to be the entry points for wires or the wires themselves. Use only the best marine wiring, make sure it is properly sized for the length of the run, and protect every circuit with a fuse or circuit breaker. Never wire a solar panel directly to a battery bank without some sort of circuit protection.
A panel that puts out 3.0 amps will be generating about 36 amp-hours of charging in a 12-hour day. Right? Wrong! Unfortunately, many factors must be taken into account when determining solar panel output: air temperature, solar cell temperature, hours of daylight, degree of shading of the panel, cloud cover, angle of the panel to the sun, age of the solar cells, etc. Manufacturers publish elaborate charts and graphs to illustrate the effects of these various factors, but it is very hard to determine what the end result will be in the real world. Add in the difficulty of calculating your boat’s daily power requirements and it becomes almost impossible to design a solar system to meet your needs. To simplify this critical process I’ve broken down a voyaging boat’s charging demands into three basic power modules for easier analysis (see accompanying sidebars).
Not having a fancy amp-hour meter, I’ve occasionally exceeded my average draw, but I always know the solar panels and wind generator will recharge the batteries eventually, and, hopefully, I have excess battery capacity to meet the excess demand. Your boat may have mechanical refrigeration and a wind generator, reducing your needs to one large panel or possibly several smaller panels. The trick is to balance your boat’s charging capacity with its electrical needs.
The rule of thumb when sizing battery systems is to have a total capacity that is three or four times your daily amp-hour demand. The reason for this safety margin is to avoid discharging your batteries below 50% of total capacitybelow the 50% threshold, battery life is drastically shortened. For my daily draw of 103 amps I should have 300 or 400 amp-hours of battery capacity.
I’ve identified 103 amp-hours of demand per day, which can be approximately balanced by the charging output of five, 50-watt solar panels, putting out a total of 17 amps. I’ve chosen 400 amp-hours of battery capacity. Do I need a regulator? With this level of solar charging, the answer is yes. Unless you plan on living aboard constantly, drawing those batteries down every day, there will come a time when you’re away from the boat and that 17 amps will cook your poor battery bank. In general, you should have a regulator whenever your solar charging capacity (in this case, 102 amp-hours) exceeds 1% of your battery capacity (in this case, four amp-hours), because that’s about the daily self-discharge rate of most marine batteries. Regulators should be sized to exceed the maximum output of your charging system in amps.
So-called self-regulating solar panels actually produce less voltage than standard panels and therefore recharge your batteries more slowly. A fully charged battery will have a voltage of about 13.8 volts, but even the self-regulating panels can produce about 14.5 volts, which would result in some overcharging. This degree of overcharging would probably not be dangerous, but it could lead to excess gassing and electrolyte loss. The larger the battery bank to which a self-regulating panel is connected, the less of a problem this slight overcharge will be. A practical test is to leave a self-regulating panel connected for a week or two and see if your batteries require more electrolyte than normal. Since gel-type batteries require a lower charging current, and since there is no way to restore lost electrolyte, self-regulating panels should only be connected to large gel battery banks.
If you have a system with some solar power, a wind generator, and an engine-driven alternator all providing charging, you might prefer to simply put a few switches in the circuits to your panels. Your 400-amp-hour battery bank will be self-discharging around 1% per day, or 4 amp-hours. One 10-watt panel will be just about right to keep up with this normal discharge, so you can keep that panel on-line all the time. On Echo we turn off the wind generator and disconnect all but the 10-watt solar panel when away from the boat for prolonged periods.
A high-efficiency 50-watt panel will be approximately 40 inches long by 18 inches wide. Five of these will take up a lot of space. On many boats the temptation will be to mount panels on stern pulpits to remove them from deck surfaces, but these exposed positions must be analyzed carefully.
The biggest drawback of a stern mount is the panel’s vulnerability to boarding seas. If you never venture offshore, you might not have to worry about this problem, though short, steep coastal seas are often more treacherous than deep ocean waves. Panels back there will certainly add windage, and might make it more difficult to handle drogue lines.
Deck mounts also present problems. One factor often overlooked is the possibility of a dark-colored panel becoming a virtual griddle in the hot tropical sun. Small children must be kept away from these panels, and your own bare feet will inevitably find them during a moment of inattention. If you don’t burn yourself, you’ll definitely stub your toes occasionally. Deck mounted panels must be very securely mounted to prevent breaking seas or high winds from lifting them. A panel that had survived offshore gales aboard Echo nearly became a flying missile when it began to break free during Hurricane Bob.
A critical problem is to place the panels where they won’t be easily walked upon during sail-handling drills. The panels are actually very strong and, short of dropping a winch handle on them, will take normal wear and tear well, but a panel on deck will become as slick as a patch of ice when splashed with waternot something you want underfoot when pulling a reef down.
When you add in the problem of siting the panels to maximize their exposure to the sun, the difficulties of proper placement can be serious. One approach is to mount a series of panels on top of a hard dodger or even on a frame located over a soft Bimini top. Another approach is to site panels in various locations around the boat. For example, a stern mount might work for a couple of smaller panels, and one panel could go on top of a radar arch, another couple on either side of a dodger, and a sixth might fit neatly on the foredeck.
Even small shadows can drastically reduce the output of most panels because the individual solar cells are wired in series to produce the required voltage. Just like the old Christmas tree lights you may remember from your childhood, when one portion of the circuit fails, the whole string is affected. Some boat owners have devised elaborate systems for mounting solar panelsfor example, mounting arms that swing out from the side of the boat or swivel mounts that allow the panel to track the sun. These systems can be effective in port, but their complexity can make them fail offshore in rugged conditions.
Despite their mounting difficulties and cost, solar panels have come of age. Most voyaging boats have at least a small panel to make sure the GPS will work even if the main power system fails. Newer, more efficient cells and designs have arrived on the market, and the dollar-per-amp figure is at an all-time low. After the initial investment you are almost guaranteed 10 or more years of trouble-free, silent battery charging. Install them, then forget them. What other marine product can provide such satisfaction?
John Kettlewell is a freelance boating writer, editor, and photographer in Camden, Maine. He and his wife, Leslie, have authored The Intracoastal Waterway Chartbook and The International Marine Light List and Waypoint Guide.