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Remember the good old days, when a boat’s electrical system was little more than a single battery, some running lights and a bulb or three in the cabin?

It’s fun to think about, when you consider today’s vessels, where electronics suites, power winches, entertainment systems and even full-size electric stoves and refrigerators require banks of batteries and a hold full of gear to charge them.

The swollen electrical loads and enlarged consumption on modern boats has increased the burden on captain and crew, too. We need to understand these systems—and it’s not always easy.

While the thought of dissecting your power system may cause your brain to short circuit, a failure to understand what keeps the lights on can have consequences. You don’t want to awake in an anchorage to find that the coffee machine won’t brew—or that the engines won’t start.

The good news is that taking stock of your system—call it a power inventory or an energy audit—is something that almost every boat owner can do, with a weekend of work and a little bit of patience. It’s a low-cost way to get better acquainted with your boat. Veterans can do this when they add equipment or revamp systems. Beginners will find that it is quite an education.

The basic drill is simple: listing the power requirements of your equipment, figuring out how much you use at any given time it and then using that information to develop a profile of your energy needs. You can then compare that profile with the amount of power your boat can produce at any given time and adjust accordingly.

You may decide to restrict your power consumption. Or perhaps you will decide to invest in equipment—maybe a generator or a second alternator—to produce more energy. Either way, you will have more control over your fate—and less chance of waking up to a blackout.

When addressing the DC side of the equation, the first step in our energy audit is to list all the equipment on the boat that draws power from the batteries. Because electrical devices consume power measured in amperes, or amps, the goal is to list the draw in amps for each device. We’ll then use that to calculate a measure called amp-hours (often abbreviated Ah). An amp-hour is simply the amount of power consumed by a device during a period of one hour. We’ll use that measure to calculate the boat’s daily power consumption.

Most gear will have the amperage or wattage (sometimes both) listed on the manufacturer’s label, or in the manual. We are going to list all the gear, with its draw in amps and watts. Start by simply inspecting the equipment, and then turn to the Internet to fill in the blanks. Listing the numbers for each piece of gear on paper will work fine, but a spreadsheet on a computer is even easier. It can do the math for you—and there will be some math.

Evaluating a boat’s electrical system requires some basic math skills. Nothing elaborate, just a couple of easy formulae. In cases where you only have partial information, for example, a simple formula can help you calculate what you need, which in this case is amps. Let’s say that you come across a piece of gear in your 12-volt system that is rated only in watts, which is fairly common. You can use this simple formula to calculate the draw in amps: Watts ÷ Volts = Amps. So, for example, to calculate the amperage of a 12-volt, 10-watt light, we would plug the numbers into the formula: 10 ÷ 12 = .83. The light draws .83 amps.

When the numbers are complete, we use a second useful formula to calculate amp hours: Amps x Time = Amp-Hours. Time is simply the amount of time you use the device each day. To use our example from above, if the 10 watt, 12-volt light was used for four hours a day, it would consume .83 amps x 4 hours, or 3.32 amp-hours (you can see where putting this all into a spreadsheet makes the job easier).

Obviously, part of arriving at a list of equipment with amp-hours is estimating how much time, measured in hours, that you use your gear each day on the water. This will be different for every boater—and it pays to be honest. Record what you do, not what you should do. If you routinely leave the chartplotter and the radar on all night at anchor, then count those hours. If the kids watch television for five hours and then sleep with the cabin lights on, reflect that in your audit. Low-balling the numbers won’t put anymore amp-hours in your batteries, so be as realistic as possible.

When you are finished, you will have a list of DC-powered gear and how much it draws from the batteries during the amount of time that you use it. By adding it up, you have the estimated number of amp-hours of 12-volt DC power required each day.

ADDING IN AC

So far, things have been relatively simple. Here is where the complications start. Most modern boats have 120-volt AC power systems in addition to DC power.

In times past, these were only used when connected to shore side power, but today air conditioners, hair dryers, blenders and coffee makers are just as often run on the water, using a generator or inverter. (Yes, they do make 12-volt appliances, but the 120-volt household models are typically cheaper, more powerful and certainly easier to come by.)

So, it’s time to add another section to the energy audit worksheet, this one for AC devices. Use the same process, recording voltage, amps and, this time, the watts. If watts aren’t provided, use the formula to calculate them (Volts x Amps = Watts). Give it a watt-hour rating (watts x time) and estimate daily usage. As a last step, add in how each item on your list is powered: generator, inverted or shore power.

When you are done, you will have a complete energy audit, listing the consumption for both the AC and DC systems and calibrated to how you use your boat.

Note, however, that you are probably not going to use every single thing, all at once. Rather, you probably use certain gear in combination, while other items sit idle. For example, you might run lights and your radar overnight in an anchorage, but not the windlass. Play around with your numbers so that they reflect accurate usage—and add in real-world experience. Remember that the audit starts as an estimate. By paying attention to your actual usage and refining your calculations, you can make it more accurate over time. And be sure to update it when you add devices.

To help illustrate this process, we developed a sample energy audit for the hypothetical vessel M/V Slo-Poke (see table). The numbers and usage times are estimates—so don’t use them as your own. But it should give you an idea of what your audit should look like.