Sizing a 12 Volt Battery to a Load

Calculator | Sizing a 12 Volt Battery to a Load

Do you need a 12 volt battery for your application but don't know what size? This calculator is designed to help you find a deep cycle battery when a continuous load is applied, not for cranking or starting purposes. If you know how much power your application takes to run, and the time you would like to run it, we'll recommend a 12 volt battery with a safe amount of AH (Amp Hours) that will give you the runtime you need.

Load Size
Enter in your application's load in terms of Amperage * 
Amps (Watts/Volts)
Load Duration
Enter the time, in hours, that you want your load to be powered
Hours
 
Temperature Adjustment
Check if Battery Temp is Over/Under 0-85°F **
 
Age Adjustment
Check if Battery is more than 6 months old
 
Battery Types: Gel
AGM
Flooded
Battery Needed is rounded to nearest whole number, and is rated in AH at a 20 Hour rate. If you find a battery with at least this many rated Amp Hours Your Load will run for the desired amount of time at a safe 50% discharge level.
Battery Needed   AH @ 20Hrs

(Note* if you are running AC devices, you will need to figure out the DC amperage using our AC to DC calculator found here).

(Note** if you are using Gel batteries in temperatures below 0 deg F but above -60 Deg F, there is no need to check the box.)

 

Walkthrough

The first Field to enter information into is labeled "Load Size".  This is typically found on the device you are running; for light bulbs it will be in watts and you need divide by the voltage you are running in-typically 12 volts. Other DC devices should be rated in amperage. (Note* if you are running AC devices, you will need to figure out the DC amperage using our AC to DC calculator found here). For our example we are running a 12 volt 15 amp swamp cooler.

The second field is labeled "Load Duration", which is completely up to the user. If you want your load to run for 5 hours, put 5 as in our example shown here.

The third field, "Temperature Adjustment", is for adjusting the calculations for extreme temperatures.  For our example, it is above 85 deg. F, so check the box. (Note** if you are using Gel batteries in temperatures below 0 deg F, and above -60 Deg F, there is no need to check the box.)

The fourth field is to adjust for the age of the battery being considered.  Since the greatest usage of the calculator is to figure out what battery to buy, usually the box will be left unchecked, as in our example, but it is there in case the batteries available are older.

The next three fields are for selecting what battery type you are going to use. Choose from Gel, AGM, and Flooded. For our example we select the AGM Battery.

The final Field is where the calculator waves its magic wand and tells you what you need. This number is rounded to the nearest whole number, and will tell you what battery Amp Hour rating to look for in the selected battery type.

For our example, our 15 amp swamp cooler will run safely for 5 hours with a 180AH, rated at 20Hours, AGM battery. For a little more detail on the math check out our Math Behind the Magic article.