How Long Will my Batteries Last?

Sizing an Inverter Battery Bank
How Long Will my Batteries Last?
Unfortunately, this question cannot be answered without knowing the size of the battery bank and the load to be
supported by the inverter. Usually, this question is better phrased as "How long do you want your load to run?", then specific
calculations can be made to determine the proper battery bank size.
Formulas and Estimation Rules
1. Watts = Volts x Amps
2. Battery capacity is expressed by how many Amps for how many hours a battery will last - Amp-Hour (A.H.) capacity
3. For a 12-Volt inverter system, each 100 Watts of the inverter load requires approximately 10 DC Amps from the battery
4. For a 24-Volt inverter system, each 200 Watts of the inverter load requires approximately 10 DC Amps from the battery
The first step is to estimate the total Watts (or Amps) of load, and how long the load needs to operate. This can be determined
by looking at the input electrical nameplate for each appliance or piece of equipment and adding up the total requirement. Some
loads are not constant, so estimations must be made. For example, a full-sized refrigerator (750-Watt compressor), running 1/3
of the time would be estimated at 250 Watts-per-hour.
After the load and running time is established, the battery bank size can be calculated. The first calculation is to divide the load (in
Watts) by 10 for a 12-Volt system or by 20 for a 24-Volt system resulting in the number of Amps required from the battery
bank.
Exam ple of Load Calculations
Suppose you were to run a microwave oven for 10 minutes a day, which draw about 1000 Watts, despite their size. To keep it
simple, think of the inverter as electrically transparent. In other words, the 1000 Watts required to run the oven come directly
from the batteries as if it were a 12 VDC microwave. Taking 1000 Watts from a 12-Volt battery requires the battery to deliver
approximately 84 Amps.
(1000 Watts ÷ 12 Volts = 84 Amps)
A full-sized refrigerator draws about 2 Amps at 120 Volts AC. By multiplying 2 Amps x 120 Volts, you find out the refrigerator
uses 240 Watts. The batteries will need to deliver 20 Amps to run the refrigerator (240 Watts/12 Volts = 20 Amps). Typically,
refrigerators operate about 1/3 of the time (1/3 "duty cycle"), or 8 hours a day. Therefore, the A.H. drain will be 160 A.H.
(8 hours x 20 Amps = 160 A.H.).
After the load and running time is established, the battery bank size can be calculated. The first calculation is to divide the load (in
Watts) by 10 for a 12-Volt system or by 20 for a 24-Volt system resulting in the number of Amps required from the battery
bank.
Example of Input Calculations
1. Total Watts = 1000 W
2. Amps from 12-Volt battery = 1000 ÷ 10= 100 Amps DC
3. Amps from 24-Volt battery = 1000 ÷ 20= 50 Amps DC
Next, the number of DC Amps must be multiplied by the time in hours that the load is to operate.If the load is to operate for 3 hours:
For a 12-Volt battery: 100 Amps DC x 3 hours = 300 A.H. For
a 24-Volt battery: 50 Amps DC x 3 hours = 150 A.H.
Now, the proper type and amount of batteries must be selected. Traction batteries, (also called deep cycle or golf cart type),
should be used in order to be able to handle the repeated discharge/charge cycles that are required.
Choosing the Correct Number of Batteries
This is a little more difficult due to the rating method used by the battery manufacturers. Also, because of the nature of the
battery, the higher the discharge rate, the lower the capacity of the battery.
Battery Capacity Hours of Di scharge
100 20
90 10
87 8
83 6
80 5
70 3
60 2
50
1
Most batteries' A.H. capacity is stated for the 20-hour rate of discharge. This means that a battery has a 100 A.H. capacity if it is
discharged over 20 hours, or at about 5 Amps-per-hour (100 A.H. / 20 hours = 5 Amps DC). However, this same battery would
last only one hour if the discharge rate was 50 Amps-per-hour (50 Amps DC x 1 hour = 50 A.H.) because of the high rate of
discharge.
The chart above indicates that for 3 hours of discharge rate, the battery has only 70% capacity. Therefore, we must have 428
A.H. of battery capacity. (Figured by dividing the A.H. capacity by the percentage of loss, or 300 A.H. ÷ 0.7 (70%)). Therefore
we would require 428 A.H. of batteries at a stated 20-hour rate. If the standard 12-Volt battery is 105 A.H., four batteries are
needed.
Finally, two more items must be considered. The more deeply the battery is discharged on each cycle, the shorter the battery
life will remain. Therefore, using more batteries than the minimum will result in longer life for the battery bank. Keep in mind
that batteries lose capacity as the ambient temperature lowers. If the air temperature near the battery bank is lower than 77°F
(25°C), more batteries will be needed to maintain the required capacity.