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Estimating Battery Requirements
To determine the proper battery bank size, you need to compute the number of
amp-hours that will be used between charging cycles. When the required amp-
hours are known, size the batteries at approximately twice this amount. Doubling
the expected amp-hour usage ensures that the batteries will not be overly
discharged and extends battery life. To compute total amp-hour usage, determine
the amp-hour requirements of each appliance that is to be used and then add
together, or the watt-hours can be totaled and converted to amp-hours.
Start with the nameplate rating of your appliances. If the wattage is marked on the
appliance, you can use that number directly. Otherwise, multiply the marked
voltage and amperage: WATTS = VOLTS × AMPS.
Once you know the AC wattage drawn from the inverter, multiply that amount by
the length of time the appliance will be used to determine the energy the load will
require: WATT-HOURS = WATTS × HOURS. You can then convert this to an
estimate of the battery amp-hours that the appliance requires.
BATTERY AMP-HOURS USED = AC WATT-HOURS/10 (for a 12-volt battery)
—or—
BATTERY AMP-HOURS USED = AC WATT-HOURS/20 (for a 24-volt battery)
For example, a 100 W light bulb that is used for 4 hours will use 400 watt-hours
(Wh) and the inverter will consume approximately 40 Ah from a 12 volt battery,
or 20 Ah from a 24 volt battery.
Another useful rule of thumb is that the current drawn from the battery can be
estimated from the AC output watts by using these same factors (10 for 12-volt
systems, 20 for 24-volt systems). For example, when running an 800 W
microwave oven, the inverter will draw approximately 800 divided by 10 = 80 A
from a 12-volt battery.
Motors are normally marked with their running current rather than their starting
current. Starting current may be three to six times the running current. The
specification that is important in determining whether an inverter will start the
motor or not is the locked rotor amps. This specification may be abbreviated to
LRA or LRI. In general, if the surge capability of the inverter is in excess of the
listed LRA, the inverter will start the motor. The MS2000 can surge to 5000 VA
for 5 seconds; this means that the MS2000 should be able to start a motor with an
LRA of less than 40 A.
If large motors will be started, you may need to increase the battery size to allow
for the high start-up demand.