Chuck Husick: Techno-Talk,
May 2001 from BoatUS Magazine - Updated October 2008
(120v) On Board
While power may be coveted by politicians and academicians, many boat
owners are still somewhat confused about how to get and use it. Many
of the electrically powered devices and appliances we want on our boats
can operate directly from 12 volt, DC battery power, however others
require 120-volt, 60Hz shore power. When away from shore we can satisfy
our AC power requirements from a genset (an engine driven, 120-volt
AC output alternator) that converts the energy in gasoline or diesel
fuel into electrical energy. If our AC power needs are more modest and
the required operating time is not too long, an inverter that draws
on the chemical energy stored in a battery will do the trick.
genset will deliver as much AC power as you may need, for as long
as the fuel lasts. Gensets with integral sound shields can be very
quiet, especially when fitted with exhaust systems that separate
cooling water from the exhaust gas, avoiding the annoying burble/splash
of the conventional exhaust system. Even small gensets can supply
the high in-rush current required to start air conditioners. However,
gensets take up scarce space and can be costly to install. A typical
genset suitable for an air conditioned 32-foot boat will cost about
silently convert 12-volt 4DC power from a battery into 120-volt
AC power, providing as much as 3,000 watts to operate microwave
ovens, toasters, TV/VCR sets and other appliances. The AC current delivered from an inverter may be a true sine wave, almost identical to the power supplied from a genset or the local electric power system. Alternatively it may be a "modified sine wave", a form of AC power that is less costly to produce. True sine wave power is generally preferred for powering inductive loads such as air conditioning systems. Modified sine wave power is satisfactory for use with most microwave ovens and all heat generating appliances such as hair dryers, toasters, electric fry pans and the like.
compact, relatively light and generally easy to install. A modified sine wave, 3000-watt
inverter will cost about $1500 and an 8-D battery can run from $210-$500.
Many inverters also function as battery chargers when the boat is
plugged into shore power.
major limitation is that inverters draw energy from the boat's batteries
and even the largest batteries store only limited amounts of energy.
Four of the biggest standard size marine batteries (160 pound, size
8-D,) would be needed to provide as much electrical energy as you
can obtain from burning one gallon of gasoline or diesel fuel in
a genset. If you are thinking of powering an air conditioner drawing
1,200 watts, plan on having one fully charged size 8-D battery on
board for each hour you want to be cool.
an inverter provide the power you want? An inverter powering a microwave
will consume between 840 and 920 watts from the battery while powering
an 800-watt microwave. The typical size 27, 60-pound storage battery
used on many boats can be used to power the microwave for about
25 minutes, long enough for popcorn and dinner. There are creative
alternatives: Reduce the rate at which you draw energy from the
battery and you can take more out before having to recharge. Do
the popcorn in the microwave, cook dinner on the charcoal grill,
and the inverter can power a TV and VCR consuming 150 watts for
about three and a half hours. Want to cook longer or run other gadgets?
Add more batteries.
you install an inverter place it as close as possible to the battery,
and connect it to the battery using proper size (LARGE) cables.
Inverters that are routinely required to deliver more than about
400 watts are best powered from large size 4-D or 8-D batteries.
Connecting two or more batteries in parallel when the load on the
inverter exceeds about 400 watts for more than a short time will
reduce the current demand on each battery, improving battery efficiency
and reducing recharge time. Provide ventilation for the inverter,
as they get hot when powering large loads.
assume that the inverter is always on when working on the electrical
system. Unlike the 12 volts from the battery, the inverter's 120-volt
output can be as unpleasant or deadly as shore power.
the boat's engine is running the alternator, it will supply some
or perhaps all of the power consumed by the inverter, extending
the permissible operating time. However, running the engine with
the gear in neutral to power the alternator may ultimately damage
the engine, resulting in repair costs that can exceed the cost of
a genset. Also, running the engine for an extended period of time
when at anchor or on a mooring will not endear you to nearby boaters
who simply want to get away from it all for some peace and quiet.