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 Energy Independent
When designing our new DC system, we broke the process into the five major steps described below to make it more manageable.
Step 1. Analyze Your Needs
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 will conduct a power survey and recommend a complete system to meet the customer’s needs.
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.
Step 3: Decide on AC Power Sources
Most equipment on boats is designed to run off 12-volt DC power. We do have a few things on Sea Spell, such as a sewing machine and a computer, that require 110-volt AC power. To operate these, we installed a 300-watt inverter to convert DC power into AC power (up to 15 percent of the energy passing through the inverter is lost). An inverter is simple to install.
An alternative is to install a generator. An advantage is that some generators produce AC power directly, eliminating the need to convert DC power to AC when the generator is running. A good-sized generator can produce enough power to run an air conditioner, something most DC systems cannot accommodate, and can also charge DC batteries. An inverter/charger combines both functions; it can change AC power to DC to charge the batteries and then convert the DC power back to AC when the generator is not running.
A good generator can serve as a boat’s primary source of power. In recent years several companies have developed suitcase-size, reliable, efficient, and quiet units. The downside is that you have to buy more fuel and maintain the generator.
Step 4: Select Batteries
The heart of every electrical system is the battery bank. We upgraded our batteries from two mid-size 12-volt batteries to four large 6-volt Trojan batteries connected together to form one extra-large 12-volt bank. The advantage of 6-volt batteries is that they are heavy-duty and mass-produced, so they are less costly than other options. We also have a separate battery for starting our engine. It is kept topped up with a trickle charger attached to the main bank.
There are several varieties of batteries on the market today. Traditional flooded, or wet-cell, batteries require regular checks to top up water levels. Newer sealed AGM (absorbed glass matt) and gel-cell batteries require no maintenance. When being charged, flooded batteries can release explosive hydrogen gasses that must be vented. Sealed batteries have a relief valve.
The three types have different charging requirements, so it is important to match your charging regulators to the type of battery you use. Selecting the best batteries for your needs involves weighing the cost, how often you use them, and the conditions they will be subjected to. Some battery types work better in cold (or hot) weather than others.
Whichever battery type you choose, select deep-cycle batteries that can be heavily discharged and recharged many times. Deep-cycle batteries last longer if you don’t discharge them below half of their rated capacity. This means your battery capacity should be at least double your anticipated demand. If you plan to use wind and solar power to help keep your batteries charged, you’ll want to have enough capacity for times when the wind isn’t blowing and the sun isn’t shining—as much as three times the projected demand.
Step 5: Put It All Together
Once you’ve selected the major components, it’s time to put them together. You’ll need a monitor to keep track of the power going in and out of your battery bank and regulators to keep your power sources from overcharging your batteries. You’ll also need properly sized cables to connect everything, along with safety fuses and disconnect switches.
You may feel intimidated once this pile of equipment is gathered in front of you. Your first instinct may be to throw up your hands and ask a professional to install it. But with a little perseverance and patience, you can do it yourself. Just break the job down into several smaller projects.
We wired up our new batteries first. Then we installed a battery monitor, which told us if each new component was doing its job. When we hooked up the solar panel, for instance, the monitor showed a negative 4 amps rather than 4 amps coming in, so we knew something was amiss.
After mounting the panel, wiring it, and connecting the monitor/regulator, we moved on to the wind generator, which we treated as a separate project. We raised the windmill mast, attached the blades, ran the cables, and connected them to the battery bank.
Replacing the alternator involved installing larger cables to handle the increased amperage and installing an external regulator. Finally, we wired up the inverter.
Most projects took under a day. Before beginning each one, we read the instructions until we understood them fully, and we referred to other reference books if we had questions. Additionally, I had taken a basic U.S. Power Squadron marine-electronics class to gain more confidence. The same information can be gained from other resources.
Knowledge gained from previous electric projects involving homes or cars is helpful, but bear in mind that marine installations generally require a higher grade of wire to prevent corrosion. Tin-coated multi-strand copper wire is the standard. While soldering connections is common practice ashore, on a boat we use crimp connectors and put heat-shrink tubes over the connections to keep moisture out.
Remember, too, that there are safety issues. “Someone who clearly is not handy probably shouldn’t try working with electricity,” Hamilton Ferris warns. “Safe wiring practices must be followed so people don’t get hurt or killed. System reliability comes down to quality of installation. People can do their own work if they read the right books. But you have to understand things like proper wiring gauges and connections. When you’re going through an inverter and stepping up to 110-volt AC power, there is a serious shock risk.”
Final Considerations
Designing and installing a DC power system is only part of achieving energy independence. Energy efficiency and conservation greatly affect the amount of power required on a boat. A 17-watt fluorescent bulb provides as much light as a 60-watt incandescent bulb and draws a fourth as much power. Better still, new LED lights use just a fraction of that amount. Similarly, it is important to turn off lights, instruments, and other appliances when not in use.
Also, certain high-draw appliances should not be run off DC batteries. Electric cooking appliances (like our toaster) require far more energy than most inverters and battery banks can reasonably produce. The same goes for hot-water heaters, home heaters, and air conditioners.
Aboard Sea Spell, we cook with propane and rely on the breeze, awnings, good ventilation, and fans to keep us cool. We added extra insulation to our refrigerator to reduce its power demand. We also added insulation above the headliner to help keep the cabin cool in the summer and warm in the winter.
The reward for our efforts is the ability to enjoy modern electric appliances wherever our boat is located. Two years after completing our upgrade, we know the effort and expense were worth it when we anchor in a remote cove, sip icy-cold cocktails as the sun goes down, and then step down into the cabin and turn on the lights.
Further Reading
It is helpful to understand a few basic concepts when working with electricity. Many excellent books have been written on the subject. A few of them are:
The 12-Volt Doctor’s Practical Handbook by Edgar J. Beyn, Weems and Plath
Independent Energy Guide by Kevin Jeffrey, Avalon House
12-Volt Bible for Boats by Miner Brotherton, Seven Seas Press. Free PDF!
Boatowner’s Mechanical and Electrical Manual by Nigel Calder, International Marine
Sailboat Electrics Simplified by Don Casey, International Marine
