When we bought Sea Spell, our 38-foot sloop, we realized she needed a major power upgrade. The existing electrical system was adequate for a boat kept in a slip, plugged into shore power and used for occasional weekend trips, but we intended to live aboard and cruise to distant tropical islands.

Far from land, there is no grid to plug into. Instead, we now generate power with a wind generator, solar panels, and a high-output alternator run by our 50-horsepower diesel engine. This system powers our refrigerator, lights, radios, navigational equipment, water pumps, and other essential appliances. We still plug in on those rare nights we’re at a dock, but the only things that shore power allows us to do that we can’t do with our DC systems is run our air conditioner, space heater, and toaster.

Becoming Independent

When designing our new DC system, we broke the process into the five major steps described below to make it more manageable.

First determine how much power you need. Go through each part of your boat, list all electric appliances, and estimate how many hours they are used each day.

Some appliances are tricky. A constant-cycling DC refrigerator, for example, is “used” 24 hours a day, but runs only periodically. You need to know roughly how many hours the compressor runs on average each day. This can be determined by checking periodically—say, every 15 minutes for a few hours—to see if it is running or not. If it is running half of the times you check, you can estimate that it runs 12 hours a day. If you check it 12 times and it’s running 7 times, then you can calculate (7/12) that it runs 58.3 percent of the time, or 14 hours out of every 24.

Next, determine how many amps the rest of the appliances draw. Most appliances have labels or manuals specifying their electrical draw in amps or watts. (To convert watts to amps at a fixed voltage, use the equation amps = watts/volts.) Multiply the amps by the hours used to find how many amp-hours a given appliance uses per day; then total all figures to get your total amp-hours per day.

A typical galley survey might look like the one in the table below. Once you get the total for your entire boat, you’ll have an idea of how much energy you need to generate each day.

Several companies specialize in designing and creating DC systems to meet the needs of individual boats. We worked with Hamilton Ferris. Others include Jack Rabbit Energy, Hotwire Enterprises, and Fourwinds Enterprises Inc. Hamilton Ferris charges \$200 to conduct a power survey and recommend a complete system to meet the customer’s needs. The \$200 is deducted from the cost of the recommended system if the customer orders it.

Alternatively, you can do your own research, design your own system, and shop around for the necessary components online or from most chandleries.

Step 2. Select DC Charging Devices

When we bought Sea Spell, she was equipped with a small 42-amp alternator, similar to those installed on most cars, and a battery charger for connecting to shore power. Our first choice was to upgrade our alternator to 125 amps so we could generate more power when running the engine. This tripled the engine’s electrical output for little additional cost. Now we can always crank up the diesel and top up our batteries more quickly.

Since we prefer not to run our engine every day, we wanted to utilize solar and wind energy. When the boat is at anchor on a sunny day, our twin 80-watt solar panels feed up to 9.5 amps to the batteries. When the wind gets up, our wind generator starts cranking out the amps. We’ve seen it hit nearly 30 amps in heavy gusts.

Wind and sunshine are never constant, so it is important to have more generating capacity than you need. An expert can help you decide what combination of solar and wind power is best suited to your needs. For cruising in the tropics, solar power often gets the nod if you have to pick one or the other.

All power, however generated—by AC shore power, solar, wind, or an alternator—should pass through voltage regulators to ensure your batteries are not overcharged. Many devices have built-in regulators; others require external regulators.