“Soak up some sun”

By Frank Lanier
Chesapeake Bay Magazine

I first embraced the idea of solar power while up a pole (literally) in the Intracoastal Waterway, replacing dead batteries. It was the early 1980s, and I was maintaining buoys, beacons, and navigational aids for the Coast Guard, replacing massive, non-rechargeable batteries with solar-powered, rechargeable ones. The higher-ups said the solar rechargeable would last six years—twice as long as the one-shot batteries. As the deck-ape who lugged all those batteries up and down the lights, my back and I immediately appreciated the whole “free power from the sun” thing—a concept I continue to embrace to this day.

The strategy behind solar energy onboard is simple: a solar panel converts sunlight into electricity, and the wiring conducts it to your batteries, where it’s stored until needed. The up-front cost might seem high to you, but solar panels easily pay for themselves in money saved and independence. They’re noiseless, they have no moving parts, and they provide free electricity for years with minimal maintenance. Solar panels are also modular, letting you start small and add more as your power requirements increase. Some even carry lifetime warranties—find another piece of gear onboard that can make that claim!

For boats on a mooring or in other situations without shore power, solar panels are often the only way to keep batteries fully charged. They also reduce or eliminate the need to run the engine to maintain battery charge, a wasteful practice that burns fuel while wearing down the costliest piece of equipment onboard. It’s also a plus to be able to recharge a dead battery in an emergency—say, to operate a VHF or navigation gear. Dockside, solar panels keep batteries charged and vital systems (such as bilge pumps) up and running without power hookups.

Solar panels contain photovoltaic cells, small silicon semiconductor devices that convert sunlight into electricity. Each cell generates between 0.45 and 0.5 volts, depending on exposure to direct sunlight. The cell size determines amperage; a 3-inch cell produces roughly 2 amps, 4-inch cells a little over 3 amps, and a 5-inch cell around 5 amps. There are three types of solar panels: monocrystalline, polycrystalline and amorphous (or thin-film) technology. Monocrystalline panels were the first available commercially and remain the most popular. A monocrystalline cell consists of a thin slice cut from a single crystal of silicon. The panels are uniform black or dark gray, and are constructed of cells housed in a rigid, aluminum frame covered with a tempered, shatterproof glass. They’re generally quite rugged, though they can be cracked or broken if subjected to extreme abuse. With a conversion efficiency of around 17 percent, monocrystalline panels are the most efficient, have the highest electrical output per area, and can last as long as 25 years. They’re also the most expensive; prices range from around $650 for a 75-watt panel to almost $2,000 for a 225-watt panel.

Polycrystalline cells are sliced from a cast silicon block and have a shattered glass appearance. Built in much the same way as monocrystalline panels, they’re rectangular, giving the panel itself a tiled look. Their life span is similar to monocrystalline panels, and while their conversion efficiency is lower (14 percent), they’re also less expensive. A 60-watt polycrystalline panel runs about $450, while a 120-watt panel costs $625.

Amorphous panels are made by placing a thin film of active silicon on a solid or flexible backing (such as stainless or aluminum sheeting) depending on whether the panel is to be rigid-framed and glass-fronted or flexible. Flexible amorphous panels—in which cells are sandwiched between rubber and polymer covers—are light and tough enough that you can walk on them and even roll them up for storage. They’re cheaper too—a 32-watt panel costs around $200, and a 64-watt panel about $400. They’re also better if shade is an issue—with the crystalline panels, even the thin shadow of a shroud across one cell can reduce or halt output of an entire module. Amorphous panels have “bypass” diodes that essentially turn off shaded cells and provide a current path around them. Some monocrystalline panels also have bypass diodes, but they’re pricey. On the downside, the efficiency for an amorphous panel is about 8 percent—half that of a monocrystalline type—so you’ll need two of them to produce the same output as a similar sized monocrystalline panel. This is somewhat mitigated in newer models, which use three-layer construction; each layer absorbs different colors of the solar spectrum, so the panel will deliver more power longer each day and during low-light conditions than the other two types. Another disadvantage to amorphous panels is a shorter warranty period, since they’re not considered as durable as rigid, glass-fronted models.

The success of any solar panel installation depends on what you expect it to accomplish. Will the panel simply float-charge a battery, power a single piece of gear, or supplement an overall energy plan? Those answers will help determine what kind of panels you choose and how you should install them, but even a basic installation shows the fundamental requirements. Choose your panel based on cost, space, mounting options and output.

Let’s say you want to maintain a 12-volt, 100-amp hour wet-cell battery that powers an automatic anchor light on your moored vessel. Where you mount the panels obviously helps determine which type to choose. They shouldn’t interfere with the boat’s operation, and ideally you should be able to turn them toward the sun periodically throughout the day, which can increase power generation by up to 40 percent. Dual-axis adjustable mounts are best, followed by single-axis and finally fixed, horizontal mounting. Automatic trackers are available, but very expensive. Popular mounting locations include cabin tops, stern rails, davit mounts, radar arches, stanchions, and Bimini tops (some people even sew flexible panels right into the canvas). 

Ambient heat is another thing to consider. It’s a common misconception solar panels need heat to produce electricity; high temperature actually increases resistance and reduces voltage within the silicon cells. Deck-mounted panels should be raised slightly to allow air circulation beneath, and installations in warmer climates may require panels with a higher maximum voltage to compensate for decreased outputs. Let’s say you want to deck-mount the panel forward of the companionway. You choose the durable, higher efficiency monocrystalline panel. Next, determine how much output you’ll need to keep your battery fully charged. The simplest way to learn this is by compiling a daily power consumption estimate.

The self-discharge rate for a wet-cell battery is typically 1 percent per day, so your 100 amp-hour battery requires roughly one amp every 24 hours just to maintain the status quo. Assuming your anchor light is on about 10 hours each night and draws 50 milliamps per hour of operation, multiply current draw (50 milliamps) by hours of daily operation (10) to reach a daily energy expense of .5 amps. So the solar panel will have to meet at least a total daily energy tab of 1.5 amps. (You’d use this same method to estimate more complex power needs).

Solar panels are typically rated in watts with outputs based on efficiency obtained under the best conditions, meaning perfect 90-degree orientation to bright sunlight, no shadowing, optimal temperatures and no load attached. For the real world, assume that a panel will produce roughly half its wattage in amp hours per day when actively aimed at the sun, and around 30 percent when randomly oriented. Since our example is a horizontal, fixed-mount installation, a 10-watt panel should contribute between 3 to 5 amp hours per day. You need at least 13 volts to fully charge your 12-volt battery. Most solar cells generate at least 0.45 volts, so you’ll want a panel with at least 33 cells (count them while you’re shopping), giving us around 14.85 volts. Bear in mind that’s the minimum needed to get the job done, which may not be enough once you throw in a few cloudy days. Most panels are designed to generate between 15 and 20 volts to overcome problems like clouds or inherent electrical resistance within the panel or installation components themselves.

While this higher voltage lets you make up for less electrically productive days, it also means you need to install a controller (voltage regulator) to avoid battery damage due to overcharging. Regulators cost $30 to over $500 and range in complexity from one step controllers that short or bypass a panel when a certain voltage is reached, to “smart” controllers that sense battery voltage and temperature and adjust charge level accordingly. Regulators also eliminate “dark current”—current drain from the battery back to the solar panel at night. Higher-end controllers use blocking circuits that open the circuit if reverse current is detected, and they also give you the option of installing a blocking diode at each panel in multi-panel installations, preventing a fault in one from draining the others. Other controllers employ blocking diodes between battery and panel to prevent dark current. If you buy a solar panel that comes with blocking diodes in the junction box or leads, it’s best not to install a controller that also uses them; the voltage drop caused by the two sets of diodes will significantly decrease panel output, especially in low-light conditions.

After choosing and mounting your panel, it’s time to connect it. If you haven’t already called in a professional, now is a good time to do so, unless you are well versed in the electrical systems on your boat. The first thing you need to determine is the size, or gauge, of the wire to be used. Multiply your panel’s rated amp output by 1.25 (which adds a 25 percent safety factor). Then find the length of the entire wiring run, panel to battery, and multiply by two. Once you have these two numbers, refer to the American Boat and Yacht Council’s (ABYC) 3 percent voltage-drop table for wire size. There’s also a handy wire calculator under “technical information” at www.ancorproducts.com.

Always use good quality marine grade connectors and tinned, multi-stranded copper wire with vinyl sheathing. The wire will run from the solar panel to the charge controller first, then to the battery. Try to keep the wire run as short as possible, and if it transits an external deck or cabinhouse (and it probably will) be sure to use an appropriate weatherproof deck fitting. The controller should be mounted below decks and as close to the battery as possible. Follow all manufacturer’s instructions for connections, but in general you’ll connect the panel’s positive (red) lead to the controller’s positive input terminal and the negative (black) lead to ground, in most cases the negative battery terminal or perhaps a ground buss bar. Connect the controller’s positive (red) output to the battery’s positive terminal using an appropriately sized in-line fuse or circuit breaker; ABYC recommends these be installed within 7 inches of connection to the battery or other point in the DC system. Finally, waterproof all connections and secure any loose wires with wire ties and cable clamps for a neat installation. Then get ready to lean back and soak up some free sun.

Frank Lanier is a marine electronics technician and owner of Capt. F.K. Lanier & Associates, Marine Surveyors and Consultants, in Chesapeake, VA.