SunXtender Technical Manual

Document No. 6-0100 Rev. G
Page 24 of 43
CHAPTER 6 – APPLICATION GUIDE
The following section contains guidelines for sizing a battery system that should provide a
reliable energy storage system for stand-alone Renewable Energy systems. The primary
emphasis is for photovoltaic (PV) systems but other renewable energy source systems would
have similar requirements.
6.1 Load Calculations
DC Loads
To calculate the DC Ampere Hours per Day required to power the system:
DC Load Amps = 1000 x kW ÷ DC System Voltage
Total Daily Load [AH] = DC Load Amps x No. of Operating Hours per Day
Example:
For a 0.12 kW DC load at 48 VDC,
DC Load Amps = 1000 x 0.12kW ÷ 48VDC = 2.5A.
Total Daily Load = 2.5A x 24 Hours/Day = 60 AH/Day.
For variable DC Loads, establish the duty cycle based on percentages of the daily operations.
(P1% of day at xx Amps) + (P2% of day at yy Amps) + Etc = Total AH Consumed/Day
Example:
A system operates at 5A for 70% of the day and 10A for 30% of the day:
Total Daily Load = (70% x 5A x 24 Hrs) + (30% x 10A x 24 Hrs)
Total Daily Load = 84 AH + 72 AH = 156 AH/Day.
AC Loads
When an inverter is used to power 120 or 240 VAC appliances, such as pumps, refrigerators,
lighting, etc., the AC voltage must be converted to the Battery’s DC voltage and the efficiency of
the inverter must be considered.
If the inverter AC voltage is 120 VAC and the battery DC voltage is 24 VDC, then the conversion
factor is 5.0. For every AC amp drawn there will be 5 times as many DC amps required. Also,
the inverter’s conversion efficiency from DC to AC is not 100%. There is an internal loss in the
inverter which is normally about 10% to 15%. See inverter/charger manufacturer’s data for
efficiency specifications.
Example:
For a 2.4 kW AC Load at 120VAC with a 48VDC battery and Inverter operating at 90%
efficiency,
AC Load = 1000 x 2.4 kW ÷ 120 VAC = 20 Amps @ 120 VAC
DC Load = 20 Amps AC x 120/48 ÷ 0.90 = 55.6 Amps DC
Total Daily Load = 55.6 A x 24 Hours/Day = 1,334 AH/Day
Note: When sizing the battery for a load profile that has high peaks compared to the average
load, the available capacity will be less than the rated capacity at the average load. More
detailed calculations may be required in these cases.