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# Calculating Battery Size

Updated: May 21, 2020

How Many Batteries Do I Need?

Usually, this question is better phrased as "How long do you want your air conditioner to RUN?", then specific calculations can be made to determine the proper battery bank size.

Remember "RUN" is the important word. Calculate battery size for constant "RUN" time and for normal use, since the air conditioner will turn on/off depending on the desired temperature and the external ambient temperature.

Time for some math!

## Step 1

Amps Calculations

Watts = Volts x Amps

Battery capacity is expressed by how many Amps for how many hours a battery will last. Amp-Hour (A.H.) capacity

Estimate the total Watts (or Amps) the air conditioner uses while operating, and how long the air conditioner needs to operate. The electrical loads are not constant, so estimations must be made.

Suppose you were to run an air conditioner for one hour, which draw 1000 Watts (for simple calculations).

1000 Watts from a 12-Volt battery requires the battery to deliver approximately 84 Amps.

(1000 Watts ÷ 12 Volts = 84 Amps)

1000 Watts from a 24-Volt battery requires the battery to deliver approximately 41 Amps.

(1000 Watts ÷ 24 Volts = 41 Amps)

1000 Watts from a 48-Volt battery requires the battery to deliver approximately 21 Amps.

(1000 Watts ÷ 48 Volts = 21 Amps)

## Step 2

Amp-Hour Calculations

If the hypothetical air conditioner is to operate for 3 hours:

Taking 1000 Watts from a 12-Volt battery requires the battery to deliver approximately 84 Amps.

(1000 Watts ÷ 12 Volts = 84 Amps) x 3 = 252 Amps

Taking 1000 Watts from a 24-Volt battery requires the battery to deliver approximately 41 Amps.

(1000 Watts ÷ 24 Volts = 41 Amps) x 3= 123 Amps

Taking 1000 Watts from a 48-Volt battery requires the battery to deliver approximately 21 Amps.

(1000 Watts ÷ 48 Volts = 21 Amps) x 3 = 63 Amps

For a 12-Volt battery: 84 Amps DC x 3 hours = 252 A.H.

For a 24-Volt battery: 41 Amps DC x 3 hours = 123 A.H.

For a 48-Volt battery: 21 Amps DC x 3 hours = 63 A.H.

​Example 1

An air conditioner running at 1000-Watt, running 1/3 of the time would be estimated at 333.3 Watts-per-hour. (1000/3 = 333.3 watts )

Example 2

A hypothetical air conditioner draws 25 Amps at 12 Volts DC. By multiplying 25 Amps x 12 Volts, you find out the air conditioner uses 300 Watts. The batteries will need to deliver 25 Amps to run the air conditioner (300 Watts/12 Volts = 25 Amps).

Example 3

Let's say you have a 100A.H battery for your system that is 12 volts (push) and 100 amp hours (storage capacity).

You could find out approximately how much energy this battery will store / provide by calculating the watt hours. To do this, just multiply the volts (V) x the amp hours (AH) and divide by 1000 (the hypothetical air conditioner).

Volts x Amp Hours / 1000 = Watt Hours

12V x 100AH = 1200 / 1000 = 1.2 Watt Hours

What this means is that you can power a 1000 watt air conditioner for 1.2 hours on a fully charged battery.

## Step 3

Battery Usage

You do not want to use 100% of the batteries capacity in fear of ruining the battery. When calculating battery size remember to also calculate a safe usable percentage of the batteries.

(battery capacity)mAh ÷ (air conditioner consumption)mA * 0.7 = estmated hours

(battery capacity)mAh ÷ (air conditioner consumption)mA * 0.6 = estmated hours

(battery capacity)mAh ÷ (air conditioner consumption)mA * 0.5 = estmated hours

(battery capacity)mAh ÷ (air conditioner consumption)mA * 0.4 = estmated hours

Battery Usage Example

Suppose you operate an air conditioner for one hour, which draw 1000 Watts (for simple calculations).

Taking 1000 Watts from a 12-Volt battery requires the battery to deliver approximately 84 Amps.

(1000 Watts ÷ 12 Volts = 84 Amps)

(100)mAh ÷ (84)mA * 0.7 = .83 estimated hours

(100)mAh ÷ (84)mA * 0.6 = .7 estimated hours

(100)mAh ÷ (84)mA * 0.5 = .59 estimated hours

(100)mAh ÷ (84)mA * 0.4 = .47 estimated hours

Let's put this all together.

Step 1:

​Determine Air Conditioner Watts.

Watts = Volts x Amps

Step 2:

Determine Air Conditioner Amp Hours.

Amps = Volts x Watts

Step 3:

Determine Battery Amp Hours.

Batteries x Amp Hours

Step 4

Determine total battery system capability based on percentages.

mah÷mA*(percentage of charge)= total estimated run time

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