25-08-2017, 09:32 PM
Sizing an Inverter Battery Bank
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How LongWill 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 questionis better phrased as "Howlong do youwant 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.