Installation Guide

Planning
8 900-0144-01-01 Rev B
G. System DC voltage: The Radian inverter
requires nominal 48 Vdc to operate.
H. Battery voltage: Most individual battery
voltages are less than the system DC voltage.
The batteries need to be placed in series to
deliver the correct voltage.
I. Capacity: Battery capacity, which is measured
in amp-hours, is not usually a fixed number.
It is specified based on the rate of discharge.
For example, the OutBack EnergyCell 200RE is
rated at 154.7 Ahr when discharged at the
5-hour rate (to terminal voltage 1.85 Vpc). This is a high rate of discharge that would hypothetically
drain the battery in 5 hours. The same battery is rated at 215.8 Ahr when used at the 100-hour rate.
Use the appropriate discharge rate (correlated to the expected loads) to measure the capacity of a
battery. Use battery specifications for terminal voltage 1.85 Vpc whenever possible.
J. Maximum depth of discharge (DoD): Most batteries cannot be discharged below a certain level
without damage. The bank requires enough total capacity to keep this from happening.
To Calculate Minimum Battery Bank Size (refer to Table 2 for letter designations):
1. The load size, item A, is measured in watts. Compensate this figure for efficiency loss. Multiply the
conductor efficiency by the inverter efficiency (E x F). (These items are represented as percentages,
but may be displayed as decimals for calculation.) Divide item A by the result.
2. Convert the compensated load into amperes (Adc). Divide the step 1 result by the system voltage
(item G).
3. Determine the daily load consumption in ampere-hours (amp-hours, or Ahr). Multiply the step 2
result by the daily usage hours (item B).
4. Adjust the total for required days of autonomy (the days the system must operate without
recharging) and the maximum DoD. Multiply the step 3 result by C and divide by J.
The result is the total amp-hour capacity required for the battery bank.
5. Determine the number of parallel battery strings required. Divide the Ahr figure from step 4 by the
individual battery capacity (I). Round the result to the next highest whole number.
6. Determine the total number of batteries required. Divide the system voltage by the battery voltage
(G ÷ H). Multiply the result by the step 5 result.
The result is the total required quantity of the chosen battery model.
EXAMPLE #1
A. Backup loads: 1.0 kW (1000 W)
B. Hours of use: 8
C. Days of autonomy: 1
D. Grid-interactive system (GS3548E inverter)
E. Conductor efficiency: 98% (0.98)
F. Inverter efficiency: 92% (0.92)
G. System voltage: 48 Vdc
H. Batteries: OutBack EnergyCell 220GH (12 Vdc)
I. Capacity at 8-hour rate: 199.8 Ahr
J. Maximum DoD: 80% (0.8)
1) A ÷ [E x F] 1000 ÷ (0.98 x 0.92) = 1109 W
2) 1 ÷ G 1109 ÷ 48 = 23.1 Adc
3) 2 x B 23.1 x 8 = 184.9 Ahr
4) [3 x C] ÷ J [184.9 x 1] ÷ 0.8 = 231.1 Ahr
5) 4 ÷ I 231.1 ÷ 199.8 = 1.156
(rounded to 2)
6) [G ÷ H] x 5 [48 ÷ 12] x 2 strings = 8
batteries
Table 2 Battery Bank Elements
Item Source of information
A. Load Size Site-specific
B. Daily Hours Site-specific
C. Days of Autonomy Site-specific
D. Application Site-specific
E. Conductor Efficiency Site-specific
F. Inverter Efficiency Inverter manufacturer
G. System Vdc Inverter manufacturer
H. Battery Vdc Battery manufacturer
I. Capacity Battery manufacturer
J. Maximum DoD Battery manufacturer