Estimating Battery Requirements - Xantrex RS2000 Installation Manual

Estimating Battery Requirements

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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-hour 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 X 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 X 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 = 80A
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 running current. 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 RS2000 can surge to 5000VA for 5 seconds; this
means that the RS2000 should be able to start a motor with an LRA of less than
40A.
If large motors will be started, you may need to increase the battery size to allow
for the high start-up demand.
Installation
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