Xantrex DR 1524, Installation & Operator'S Manual, Page: 65 / 84

5.0 APPENDIX
57
©2000 Xantrex Technology Inc.
Battery Bank Sizing
The battery bank’s size determines the length of time the inverter can continue to supply AC
output power during a utility outage. The larger the bank, the longer the inverter can run. An
undersized battery bank results in reduced battery life and short inverter run times.
In general, the battery bank should be designed so the batteries do not discharge more than
50% of their capacity on a regular basis. Discharging up to 80% is acceptable on a limited basis
such as a prolonged utility outage. Totally discharging a battery results in permanent damage and
reduced battery life.
For stand-alone applications, design a battery bank that can power the loads for 3–5 days
without requiring recharging. To duplicate the conditions on sun-less days or windless periods, the
power supplied from other sources (i.e., solar, wind, hydro, etc.) is not included in this calculation.
This is often referred to as the “number of days of autonomy.” If the system is a hybrid, with daily
generator run periods, the battery bank size can be smaller.
Estimating Battery Requirements
To determine the proper battery bank size, it is necessary to compute the number of amp-hours
that are required between charging cycles. When the required amp-hours are known, size the
batteries at twice this amount to ensure the batteries are not regularly over-discharged.
To compute the amp-hour requirements, the amp-hour ratings of each appliance powered by the
inverter must be added together. Use the figures from the nameplate label on the appliances, then
use the formula WATTS = VOLTS x AMPS. Then divide the calculated wattage of the load by the
system battery voltage to determine the amperage the load will draw from the batteries.
(AC current) x (AC voltage)/(battery voltage) = DC amps.
Example:
Nameplate label specifies 6 amps at 120 VAC.
The system battery voltage is 24 volts DC.
• First determine the wattage by using the formula: WATTS= VOLTS x AMPS = 120 x 6 =720
watts.
• Then divide the wattage by the system battery voltage to determine the DC amperage.
720/24 = 30 amps DC amps.
If the AC wattage is specified on the nameplate label, the battery amperage will be:
(watts)/(battery voltage) = DC amps (720/24 = 30 DC amps).
Multiply the amperage by the number of hours the load will operate to roughly calculate amp-
hours. Double this figure to reach the 50% battery capacity level.
Refer to the example and work sheet on the following pages as a guide to determine the battery
bank’s amp-hour requirements.
NOTE: Motors typically require 3–6 times their running current when starting. Check the
manufacturer’s data sheets for their starting current requirements. If large motors will be started from
the inverter, increase the battery bank size to allow for the higher start-up current.