How Long Will my Batteries Last?

Unfortunately, this question can not be answered without knowing the size of the battery bank and the load to be supported by the inverter. Usually, this questions better phrased as “How long do you want your load to run?”, then specific calculations can be made to determine the power 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 form the battery.

4. For a 24-Volt inverter system, each 200 Watts of the inverter load requires approximately 10 DC Amps form 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 24-Volt system resulting in the number of  Amps requied form the battery bank.

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 V DC microwave. Taking 1000 Watts form a 12 Volt battery requires the battery to deliver approximately  84 Amps.

(1000 Watts ÷ 12 Volts = 84 Amps)

A full-size 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. Therefor, the A.H.drain will 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.

Example of Amp-Hour Calculations

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 power 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.

If the load is to operate for 3 hours:

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 Discharge 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.

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1. Vito

What happens if we connect some kind of motor, the load in each starting points would be increased much more, therefore you have to bear in mind this extra Amp’s that your battery would absorbs

• Vicky

Yes, you are right. This is very important for using a power inverter.

Have you ever used a power inverter before?