Inverters
Inverters bring a new level of enjoyment and safety aboard your boat.
By supplying 120vAC power, inverters allow you to use TVs, VCRs, computers,
microwaves, hair dryers and power tools when away
from shorepower. They can also recharge dead cell
phone or hand-held VHF batteries. Choosing the right
size inverter depends mostly on whether you want to operate one appliance or
many at the same time.
120vAC power on board can come from three sources: shorepower,
a generator (genset), or an inverter. Nearly
unlimited AC appliances can be operated from shorepower,
of course. Assuming you don't want to stay tethered to the dock, however,
your choice between a genset or an inverter (or
both) also depends on your power requirements underway.
Gensets produce large quantities of continuous
power, ideal for running air conditioners, large freezers,
and other high-load, long-duration applications. They also charge the ship's
batteries. The downside is that gensets require
fuel, an exhaust system and engine maintenance. Although modern technology
has improved gensets, they do consume space,
vibrate, and aren't always very quiet.
W here AC power needs are smaller or more intermittent,
inverters have a number of advantages. They
are an economical, maintenance-free, and relatively compact source of AC
power, and the inverter’s solidstate circuitry is
quiet and nonintrusive. Although many inverters
produce a modified sine wave, not the
true sine wave of a genset or shorepower,
the power from these inverters is adequate to run most electronic and
motor-driven appliances. True sine wave inverters are on the market (for an
additional cost.)
True sine wave inverters do not produce lines on TVs, and allow clocks to
keep accurate time. Additionally,
some printers will not run on modified sine wave and reportedly bread makers
require true sine to operate.
Generators and inverters are not incompatible, and many genset
owners also have an inverter on board.
The generator provides the muscle to charge batteries and power an air
conditioner, while the inverter
can silently run the TV or brew a cup of coffee in a quiet harbor.
Whether an inverter, genset, or a combination of
both is the best solution for you requires an honest
accounting of your power needs and an understanding of how inverters work.
How Inverters Work
Inverters operate somewhat like battery chargers in reverse: they take 12vDC
power from a battery and run it through modern circuitry to change it into
120vAC current. The ship's batteries are the inverter's fuel tank and, by
nature, inverters are real gas guzzlers. Since they will only provide 120vAC
power as long as there is ample juice left on the 12v side, it is strongly
recommended that you have a separate, dedicated engine starting battery or
battery isolators to avoid accidentally draining the entire 12v system.
Because inverters and battery chargers share certain electrical components,
many models are available
as combination inverter/charger units. Most switch automatically to battery
charger mode when plugged into shore power or if the genset
is turned on, changing back into an inverter when the power input is
disconnected or turned off. Most models have multi-stage chargers that
prolong the life of your batteries
by monitoring them and altering the charge rate as the batteries approach
full capacity. Since overcharging is one of the leading causes of death among
batteries, this is an important feature.
Choosing an Inverter
Before you can select the appropriate inverter, you must first analyze your
power needs, including future
additions. The key is to determine the maximum amount of 120vAC power you and
your crew will use at any one time. Small inverters will operate one or two
smaller appliances simultaneously. If you want to run
the TV, VCR, and computer in the salon while a crew makes microwave popcorn,
however, you'll need a larger capacity unit.
Inverters are rated by maximum continuous watts. The most accurate way to
tote up the watts you might
use is to check the data plate on each 120vAC appliance you plan to have on
board. Be aware that items like hair dryers can vary in wattage from 500 to
1,500 watts depending on the setting chosen. Also note that most motor-driven
equipment such as blenders and power tools draw a power surge for a few
seconds at start up— modern inverters generally accommodate such momentary
surges as long as you don't switch everything on at the same time.
To determine the proper inverter size for your boat, make a list or
spreadsheet of all the 120vAC equipment you will run, adding its wattage in a
second column. If the appliances data plate states the power in amps, convert
to watts using this formula:
Volts x Amps = Watts
i.e. an item stated at 120 volts and 3 amps will
draw 360 watts.
Add the wattage of all the appliances that you want to operate
simultaneously, making sure you include any
items such as a refrigerator, that run continuously.
The total wattage run at one time is the minimum size inverter you should
consider. If in doubt, purchase one size larger, as you will probably
discover more uses for your inverter after you’ve installed it—and if you
turn too many loads on at the same time, the inverter will overheat and shut
down. Note that the total adds up to 3,200 watts in our sample chart, but
this boat could squeak by with a 1,000 watt inverter by running only one item
at a time.
Choosing Batteries
Inverters depend on adequate type, size and number of 12v batteries for
proper operation. To determine the size and number of batteries you'll need,
expand your list of appliance wattages by adding a third column for the
amount of time you’ll want to run each piece of gear in a 24-hour day. Since
battery capacity is measured in amp hours, you’ll need to build a forth
column for adding up the amp hours required. Convert the watts to amp hours
using this formula:
AC Watts ÷ 12v x 1.1 x Hours Use=Amp hours
Example: the 13" TV uses 50 watts and you want to use it 2 hours each
day, so 50/12 x 1.1 x 2 = 9 amp hours. If the data plate lists AC amps rather
than watts, use this formula:
AC
Amps x 10 x 1.1 x Hours of Use = Amp hours.
When your chart is complete, you will see immediately which appliances
are going to pull the most juice out of the batteries. Note that it isn’t
always the highdraw items that cause problems—the
blender draws six times the watts of the TV, but since its use is short, the
TV has a much larger total. Now is a good time to
reconsider luxury items, especially those with heating elements such as
toasters, hair dryers, and waffle
irons.
In our example, a total of 397 amp hours is required to meet the power needs
from the inverter in 24 hours.
Since you won’t want to draw the batteries flat, however, and since batteries
are only 20-50% efficient
depending on their age, the temperature and the charging source, more battery
power will be needed than the simple total. At a minimum you would want to
add 50% more battery capacity for a total of 596 amp
hours. Be aware that batteries age and die faster if “cycled,” (discharged
and recharged) frequently or
deeply. Many battery manufacturers recommend that you double the needed
capacity to avoid premature
destruction of batteries—in our example this would result in a bank with 794
amp hours.
When constructing your appliance list, don’t forget that the demands from the
inverter aren't the only items your batteries have to handle. They still have
to keep pace with all the 12vDC lights, electric head, water pumps, electronics and entertainment systems. The amp hour total
for the inverter is what you must add to your existing banks to keep the
inverter running.
Inverter Options
Just as you monitor your car's fuel gauge, you must be aware of the status of
your batteries. Most inverter
manufacturers offer excellent remote panels, some with sophisticated battery
monitoring systems. A few
of the features available include low battery alarm, overload warnings and
the number of amp hours consumed —all on a panel mounted in a conveniently
visible location.
Every amp hour drawn out of the batteries must be replaced. If your boat has
a genset or you plug into
shorepower at the end of each day, this is of
little concern. If you anchor for more than a few days without a
generator, however, methods of replacing the batteries’ expended juice is a
necessity. High-output alternators on the main engine, wind generators and
solar panels can all be arranged to keep the system supplied with fresh
energy.
All inverter installations require a large, dedicated 12v fuse in the main
power line. Be sure to purchase the correct fuse block and fuse with the
inverter—it’s always wise to carry a spare fuse if you plan a longterm
cruise.
|
