Sizing a 12 Volt Battery to a Load – Calculator _ BatteryStuff multinational debit card owned by mastercard

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


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:




Battery needed is rounded to nearest whole number, and is rated in AH at a 20 hour rate.Multinational debit card owned by mastercard

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.)

Choose your battery 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).Multinational debit card owned by mastercard 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.Multinational debit card owned by mastercard


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.

Mark hi tech, I have 2× 105ah batteries in the tray of the ute, running parallel, using a red arc dual battery system which seems so far keep these batteries topped up, using HD welding cable.Multinational debit card owned by mastercard from what I have learnt by reading all of the post that means I have 210ah and a 12 volt battery bank. I purchased a10ah smart battery charger, it was half the price and I got a bit excited, my question is will this fully charge these batteries eventually? I realise that 10% amp size for the charger is the recommendation. Also I’m looking at 2× 80w solar panels (160w) foldable with a regulator for each can I plug these two panels together (dual anderson plug) or will the two regulators confuse each other. Also not keen on burning the car down.

Lastly, trying to fit a square peg in a round hole, what if I was to get a manual battery isolator to split the batteries back to 105ah then just switch the alligator clips between batteries, a bit of stuffing around but wil his charge the batteries fully?Multinational debit card owned by mastercard

I would like to go camping every month but the reality is 2-3 times a year for only weekends away and 1 week somewhere special. The 4×4 drives around enough to keep the batteries topped up most of the time. But I would still like it set up to work as it should. I run a small fridge led lights all around and basic charging necessities ipad etc.


All the gear and no idea!!! Is my favourite saying….

DS I have totaled my setup that I am building in my cargo trailer conversion to be 16.3 amps. 9 LED 12v lights, A 12volt water pump, DC plug/USB charger/monitor combo, and another USB/DC charger. My lights I will use only when it is dark and when I am awake, the pump is only when I turn the water on. The lights are the lowest amps at .341. The pump is at the highest at 7amps.Multinational debit card owned by mastercard with such variables in frequency of usage how do i figure the battery needed? The calculator, which has helped me a lot, figures I am using all DC 12volt items the entire time which is not the case. Any suggestion on getting a better feel for how many AH I need in my deep cycle battery? I am looking at one that says @25amps 389 capacity minutes. What is that compared to 200 AH at 20-hr rate? Would this be sufficient for my setup? The calculator shows I would need a 216 AH battery running all items for 6 hours. I could enter each item separately to get amp hours then total the amp hours at the end. Would this be accurate?

Thank you again for the calculator resource.