One of the best ways to make sure there’s always enough battery power on board is to reduce your electrical demand where possible. This doesn’t mean you have to give up conveniences or safety gear you’ve come to enjoy and rely on. Most multihull sailors can reduce their electrical load significantly simply by choosing efficient appliances and operating them wisely.

Reducing electricity use before selecting charging gear is similar to insulating a house before selecting a heating system. Reducing demand can get you most of the way toward self-sufficiency and the ability to use renewable sources of power for a large portion of your electrical load.

The first step is to have a good estimate of your current electrical load. This approach allows you to view your total energy budget and target problem areas effectively. The results may surprise you. To estimate your electrical load, set up an appliance load chart as shown in the sidebar.

Estimating Your Electrical Load

List all electrical appliances you would like to have on board. If you plan on upgrading your boat soon, this is a good time to think about future electrical demand. List your DC loads first, then your AC loads, as this allows you to assess your DC charging sources as well as various methods of providing AC power. You may wish to set up separate charts for electrical use in port and on a passage, since electrical use and charging patterns vary significantly between the two.

Appliance avg. current draw (amps) avg. use (hours/day) avg. consumption (amp-hours/day)


Cabin Lights

Running Lights

Standard anchor Light

VHF Receiver/Transmit



Stereo/Tape Deck

Bilge Pump

Marine Refrigeration




Laptop Computer

Microwave Oven

Coffee Mill

Coffee-Maker (Brewing)


total average daily load 133. amp-hours per day

Next to each appliance note the average current draw. Average current draw takes into account the fact that for some appliances, such as autopilots, the current draw varies with how hard the appliance is working when operating. Average draw also accounts for appliances with multiple speed or other operation settings.

A digital system monitor provides a convenient method for finding the current draw of all appliances already on board. Begin by making sure all charging sources and appliance loads are off, then turn of appliances one at a time. The Link 10 (available from Heart Interface) and the Masterlink/BTM1 (available from Mastervolt) are good single-bank system monitors that displays charging or load amps, battery voltage, amp-hours used, and much more. The Link 20 and Masterlink/BTM2 monitor dual house banks. The SALT Systems Monitor (available from Sea, Air & Land Technologies) is an accurate, simple-to-use device that can monitor up to four battery banks or charging sources, has programmable alarm functions, and can even connect to your PC.

In the next column bring time into the equation by noting how many hours on average each appliance is used. Try to take into account that some appliances are used every day while others are used occasionally, and that some appliances are used mostly in port and some are used exclusively at sea. To find the average daily hours of use for occasional loads, calculate average hours of use per week and divide by 7 days.

Now multiply the average current draw in column 2 by the average daily hours of use in column 3 to get your average daily electrical energy consumption in amp-hours. You’ll find that some appliances with high power draws consume a modest amount of energy if operated for short periods of time, whereas a relatively low-draw appliance like an anchor light consumes a surprising amount of electricity when operated nightly for 10 or more hours.

Go through your appliance list and choose loads that can be reduced. Start with the most universal and easiest to alter, lighting.

Cabin Lights

Kerosene lanterns are hard to beat for atmosphere and low battery draw, but electric lighting is truly one of the simple luxuries of our times. Since cabin lighting typically accounts for a large portion of a boater’s electric load, it’s best to reduce consumption by up to 75 percent with energy-efficient fluorescents or up to 20 percent with halogen lamps. Lighting efficiency is measured in lumens per watt, or the light output in relation to the energy consumed. Incandescent bulbs are notorious energy wasters, operating in similar fashion to heating elements—only about 10 percent of the electricity consumed is turned into light, while 90 percent is dissipated as heat. A typical 8-watt fluorescent lamp delivering 60 lumens-per-watt gives the same light output (lumen level) as a 40-watt incandescent bulb yielding 12 lumens-per-watt. On one boat we owned there were eleven 8-watt fluorescent cabin fixtures, and if all were used at once the total draw was still less than a single 100-watt incandescent bulb. On a typical evening we would operate the equivalent of four cabin lights for about 4 hours, so our average cabin lighting load was 0.67 amps per light x 4 lights x 4 hours per night, or just over 10 amp-hours per night, about the same as using one 30- to 35-watt incandescent bulb.

Anchor Lights

Most powerboat and sailboat anchor lights, including those incorporated into a sailboat’s masthead tricolor light, have a sizeable electrical load. They consume between 10 amp-hours (if a 12-watt bulb is used) and 20 amp-hours (if a 25-watt bulb is used) in one night. A good alternative is to get a low-drain anchor light. Battery-powered versions that hang in the rigging, some with photoelectric switches that shut the unit off at dawn, are readily available. We made our own out of a fractured glass preserve jar, two 100-milliampere high-intensity DC lamps from Radio Shack, and some exterior-grade wire that doubled as the hanging lanyard. It produced as much or more light than a kerosene lantern, consumed only 2 amp-hours per night, and cost around $10. A plastic jar or brighter bulbs can be substituted if desired.

Running Lights

Sailors can greatly reduce the total electrical draw of their boat running lights by switching to a masthead tricolor fixture. It uses only one 25-watt incandescent bulb instead of three separate deck-mounted lights for port, starboard, and stern. Your deck-mounted running lights can be left in place for use when under power or in areas of heavy traffic. Make sure that your energy system is sized so that you always have enough energy for running lights. I’ve met a number of sailors who conserve electricity at the expense of running lights, since one night of operation can amount to 60 amp-hours of electricity if three separate 25-watt fixtures are on for 10 hours. The risk of “going without”, however, is irresponsible, and the load can be reduced to 20 amp-hours if a tricolor is used. Even a modest energy system can handle this load. Powerboaters don’t need to worry so much about the load from running lights, since they are creating electricity with their alternator anytime they are underway.

Part II

Last issue we reviewed the importance of reducing your electrical demand where possible in order to comfortably ensure a steady supply of electricity away from the dock. We discussed how reductions in electrical load can often come through making smart appliance choices and using those appliances wisely, precluding the need for major changes in your lifestyle. We started with ways to reduce your electrical load from lighting. In this issue we’ll continue on with ways to reduce other appliance loads.

Refrigeration & Water Making

On-board refrigeration places you in a fairly high energy category, especially when cruising in warm climates. Powerboaters can reduce their electrical load by by choosing modest-size, efficient standard or marine refrigerators (check the energy use rating on the label), or cut their electrical load significantly by converting to a marine built-in system with a super-insulated box similar to those used on sailboats.

Sailors have several types of refrigeration systems to choose from. Engine-driven refrigeration using holding plates (plates that “store” cold and keep box temperatures low for 24 hours or more) is perhaps the most complete answer to reducing electrical consumption, since it eliminates it altogether. In fact, the boat’s main engine, or one of the many small diesel chargers on the market, can be powering a refrigeration compressor as well as a water-maker pump and a high-output alternator for charging batteries. The downside of this type of system is that you are tied to running an engine every day, and you’ll get no contribution to your refrigeration or water-making loads from renewables such as solar or wind power. An alternative is to have an efficient 12-volt holding plate refrigeration system such as the Technautics Coastal 12, which comes completely pre-charged with 134a HFC refrigerant, ready to be owner installed (cost: $1,600 US, $2,160 CN), and an efficient 12-volt water-maker such as the Village Marine Little Wonder, which produces 6 gallons of water in an hour using only 15 amp-hours of electricity (cost: $2,899 US, $3,900 CN). With a 12-volt holding plate system, if renewable charging sources can’t keep up with the load you simply run an engine-driven high-output alternator to supply electricity for refrigeration, water-making, and storing surplus electricity.

Here’s some helpful hints regarding other appliances:


Eliminate appliances with heating elements where possible by cooking with propane. If you like to use a coffee-maker despite its 100-amp draw when brewing, drip the coffee into a thermal carafe and turn the machine off. If your model has a built-in clock or timer, disconnect it when it’s not being used to eliminate the “phantom” load (see below). Microwave ovens are fairly efficient, especially when used for warming food quickly, but they can drain batteries in no time if used indiscriminately.

Rechargeable Appliances

Rechargeable communication devices and cordless appliances use an AC plug-in transformer or “power cube”—a small device that transforms standard AC power into low-voltage AC or DC power. These appliances draw very modest amounts of power when charging or running, but the problem is they never completely shut off. If the power is supplied by a large inverter, this phantom load means the inverter may never go into its standby mode, wasting a great deal of power. Since most gear can be recharged quickly, it’s best to recharge the battery pack, then disconnect the power cube from the circuit. You can do this by placing all your rechargeable gear on one power strip that can be manually turned on for an hour or two when needed (caution: don’t use an electric timer for the charging cycles—it also has a constant power draw).


Today’s inverters are very efficient at converting battery power to household AC power, but they still consume about 10{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} of the available electricity in the process. Use 12-volt appliances where possible, and disconnect rechargeable AC gear when not in use so the inverter can revert to its standby mode (see above).


Laptop computers use only a fraction of the power of desktop models, especially once the laptop’s internal battery is charged. Use a direct DC charger where possible, or if an AC power cube is used, disconnect it from the power source whenever possible.


TVs, VCRs, stereos, and other entertainment appliances vary widely in the amount of power they draw, and many have phantom loads as described above. In general, DC equipment for the automotive and marine markets is typically much more energy efficient. Electricity use is proportional to a stereo’s wattage ratings, so be selective when buying. Phantom loads are those that draw power even when the appliance is turned off. In the entertainment arena, they come in the form of timers, lights, and the instant-on feature found on most TVs. To conserve energy, simply disconnect these appliances when not in use.


Here’s a time when it’s not better to give than receive. Communication devices such as VHF and SSB radios draw 10 times more power when transmitting as when receiving. Keep your communication transmissions to the minimum.


Keeping your boat clean is important, but don’t sacrifice your batteries for the sake of convention. Non-electric sweepers and small hand brooms have lost favour over the years, but they can be almost as convenient, less noisy, and certainly more energy efficient than high-draw electric vacuums.

Water Pumps

Most water pumps on board draw between 3 and 5 amps when running, but they are only on for short periods of time. Using hand- or foot-operated pumps helps conserve electricity while regulating fresh water use. For seawater electric pumps, keep intake strainers and waterlines clear of debris and marine growth. A seawater pump can draw many times more electrical power than normal if restrictions are present.

As you can see, reducing your electrical demand has a lot to do with making intelligent choices about appliances and how you use them. If after making those choices your batteries are still crying for help, you may have to alter your lifestyle a bit or upgrade your charging ability. Only you can decide when conservation measures become draconian and rob you of simple pleasures on board.


About the Author

Independent Energy Guide Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher. He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide. Sailors Multihull Guide