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post #1 of Old 02-11-2003 Thread Starter
Kevin Jeffrey
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Electrical Power on Board

This maze may seem intimidating at first, but with the right approach managing your boat's electrical supply can be easy.
Electrical power is rapidly becoming one of the most important systems on board a cruising sailboat, since so many other systems now depend on a steady supply of electricity. With the right approach, understanding electrical power can be easy. 

First, you need to accept that as the skipper of a boat, you essentially become the manager of a small private utility company. You have to know how to make and manage your own supply of electricity, especially after leaving the dock. There are rewards and responsibilities that go with this position, and your best chance of success is to realize that the job has to be taken seriously. 

Secondly, it is important to make your electrical power needs less mysterious. In part, this is done by having electrical concepts and system components presented to you in understandable terms. Additionally, the use of system monitoring gear that makes electricity on your boat become "visible" is a major aid in understanding your system's condition. We hope to cover both of those topics and more over the course of this series of articles. 

Marine electrical power systems consist of four major components: 

  1. Gear that produces electricity (whether renewable or engine-driven power sources)   
  2. Gear that stores electrical power for later use (batteries)   
  3. Gear that distributes electrical power (wiring, switches, etc.)   
  4. Gear that consumes electrical power (all electrical appliances on board)

Above all else, the manager of an electrical power system attempts to create a balance between the energy produced and the energy consumed. Most horror stories associated with electrical systems on boats come from something being radically out of balance in the system. Therefore, one of the first tasks of an electrical system manager is to determine the boat's daily electrical load. 

Before undertaking any electrical task on board, make sure that you have the appropriate tools for the job.
There are two types of power on a boat, DC (direct current) power using batteries as the source, and AC (alternating current) power that comes from either shorepower, a motor generator, or an inverter. Marine DC systems typically operate at 12 volts, although some boats are wired for 24 or 32 volts. AC power on a boat is similar to that in a home, and allows you to operate standard household appliances on board. 

The terms used to state power production and consumption are a matter that commonly confuses sailors. Electrical power sources and appliances are usually rated in watts, which describe the amount of power they can produce or consume at a given moment, usually under peak conditions. As you can see, power rating alone doesn't tell you much about the total energy a piece of gear can produce or consume until time is brought into the equation. The 110-volt electrical energy in a home or at a dock is measured in terms of watt-hours or kilowatt-hours (1 kW-hour =1,000 watt-hours). However, since 12-volt batteries are rated in terms of amp-hours instead of watt-hours, energy on a boat is typically measured in amp-hours. These amount to the same thing if you assume that the voltage is fairly constant and you have the formulas to convert between amps and watts. These are: 

Amps (current flow) x Volts (system voltage) = Watts (power, either production or consumption)

Bringing time back into the equation:
Watts (power) x Hours of operation (time) = Watt-hours (total energy produced or consumed).

Details of how to calculate your total electrical load will be covered in a future article, but some interesting things come to light when you begin to apply the above formulas to your electrical use on board. It is clear that the total electrical energy produced by a charging source, such as solar panels, wind and water generators, alternators, and AC battery chargers, is found by simply multiplying the charger's power rating for the conditions it's operating in (sunlight level, wind strength, boat speed, engine rpm, etc.) by the hours, or fractions of an hour, that the charger operates. It also becomes apparent that appliances with a high power rating operating only for a short time actually use a modest amount of energy, while the energy consumption of some low-draw appliances that operate for long periods of time can be quite substantial. Thus the draw from a 400 amp-hour starter (rated at 4,800 watts at 12 volts) used for only one minute (400 amp-hours used for 1/60 of an hour) totals only 6.66 amps consumed. On the other hand, a one amp-hour light bulb (rated at 12 watts at 12 volts) left on for 24 hours eats up 24 amps-hours. 

Whether outfitting a new boat or upgrading an existing boat, the steps required for proper electrical power system planning and equipment selection are the same: 

  1. Determine your daily electrical load   
  2. Size the house bank of batteries to give you the amount of total storage capacity you need   
  3. Choose what type of charging sources you'll need to replenish the batteries on a regular basis   
  4. Determine what other gear is needed in your system (monitors, controls, distribution components, etc.)   
  5. Select manufacturer's individual equipment models that fit your situation

    A simple circuit diagram of the boat will come in handy for trouble-shooting electrical woes.

    Once your system is planned and the equipment selected, make sure the gear is installed according to recognized standards such as the ABYC (American Boat and Yacht Council) guidelines. Things to keep in mind when installing electrical power gear include proper wire type, wire size, connectors, and installation techniques, as well as proper power switches, circuit protection devices, and distribution posts and bus bars.  

I recommend that sailors make a circuit diagram of their completed system for future modifications and trouble-shooting. This diagram doesn't have to be fancy, but it should include all major components and wire paths, clearly labeled for quick reference. 

In Part Two, Calculating Your Electrical Load , Kevin explains how to calculate expected electrical loads and build a load chart. 

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