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Knowledge Base : Tools : Calculator | 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
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
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.)



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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.

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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.

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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.)

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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.

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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.

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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.

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  • Can i use 3 battries (1 Wet Cell 150Amp and 2 AGM Dry Cell 18Amp) same time on local Transformer base UPS ??

    Ghulam Nabi
    April 11, 2012 a 2:34 am
    • If you use all three batteries to create a single battery bank, then I would advise against it. The Wet Cell is a much larger battery then the two AGMs. When you bank them together, they behave as one battery. The contrast of battery capacities will cause an uneven charge and discharge cycle.

      April 11, 2012 a 8:00 am
    • I want to run ten (10) 23w compact fluorescent bulbs rated at 0.380 amps on a 120 v AC system powered by a deep cycle 12 v dc system using a 400 w inverter

      run time per day = 10 hours. Maximum run hours between charges = 24 hours… Wasn‘t sure I was using the calulators correctly. How should I size the battery and do you have any comments or suggestions on the inverter I would use

      January 29, 2013 a 2:46 pm
      • 0.380 amps x 120 volts = 45.6 Watts, not 23w. Therefore 10 lights will be 456 Watts total per hour. A 400 Watt inverter will struggle with this power.

        456 Watts will pull ~41 amps from a 12v battery source per hour. For ten hours, that‘s a total of 410 amps pulled. For a battery recommendation I would at least double this number…so any AGM battery bank rated with 820 Amp Hours or more is my recommendation. More is needed for flooded batteries.

        January 29, 2013 a 3:11 pm
        • OK, I‘m just reading the information off the lamp. The lamp is rated 23 watts and then also rated 0.380 amps, followed by a color temperature. Can you explain the discrepancy.. Since they are self-ballasted is the difference the ballast? Are we looking at a starting current demand vs a run wattage rating? If so, how does this affect the calculations. My suspicion is that the .38 amp rating is based on the ballast… If this is the case, how does this affect my battery capacity planning?

          Thank you for your assistance

          January 29, 2013 a 3:38 pm
          • Unfortunately, I can‘t confirm to you the true power rating of the lamp. Given the uncertainty, I would use the highest power rating to make my calculations from (which I did in my previous comment). It‘s better to size a system for a worst case scenario than not.

            January 30, 2013 a 9:07 am
      • Hello,

        I did some checking here‘s what I got from a tech person:

        “The 0.38A is the operating amperage, the inrush current is typically much higher than that value. The part of the equation that is missing in your calculation is the Power Factor of the lamp, which for almost all CFL lamps is specified as >0.5. Taking into consideration the power factor value, the 23W CFL will consume 23W of power still, but would draw 0.38A at 120V”.

        Does this change the calculations? Assume 9 lamps, 8 hr run time between charges for simplification

        January 30, 2013 a 10:58 am
        • The draw amount is what we need to calculate with. There is a significant loss of power when energy going into the lamp is actually being used to run it. Therefore, each lamp will pull ~4.1 amps each per hour. For 9 lamps and 8 hours, that‘s a total of 295.2 amps pulled from a 12 volt battery source. My recommendation: 12 volts with at least 600 available Amp Hours. That‘s three 4D size batteries in parallel.

          January 30, 2013 a 12:46 pm
          • OK How about (9) 17w LED lamps… I do not yet have amps, but it should be significantly lower – trying to get this down to one manageable battery under 100lbs and running 8 hrs, with less than a 10 hour recharge time.

            I plan on using a 400 watt inverter to power this project..any comments?

            February 4, 2013 a 12:58 pm
            • 17 watts / 12.5 volts = 1.36 amps. 9 of these lights will pull 12.24 amps per hour. running 8 hours is a total of 97.92 amps. The calculator recommend a battery with ~ 200 AH, which is still over 100 lbs of weight. Inverters can add on average 15% inefficiency to the power draw.

              February 5, 2013 a 10:21 am
        • 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. 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.

          April 11, 2016 a 6:27 pm
          • I do agree figuring out a usage for everything is quite an undertaking, and be difficult if your the one designing the system, versus buying a RV with a manufacturer recommendation. With that said your best bet would be to average each device into the calculator, and then add it all up at the end. However I can tell you that most of our customer that dry camp a lot generally start with a pair of 6 volt deep cycles seen here: RV 6 Volt Batteries. Most people start in the 200 AH range and go and higher if your space permits. If you have any further questions I would suggest emailing us directly via our email support link.

            April 12, 2016 a 7:03 am
          • my load is 15 Watts LED DC what size of battery for me to run 15watts LED DC for 8hrs?

            March 12, 2014 a 6:02 pm
            • JB, if you put in the information into the calculator it will tell you. The one factor you still need to know is the system voltage, so that you can get the amperage of the load.

              May 12, 2014 a 12:11 pm
            • I have 4 cfl’s /12 volt each…they are rated at 15 watts each….wondered what size AH battery I should use to get maybe 5-7 hrs of use out of lights? Would an AGM type battery be okay?

              April 27, 2014 a 3:42 am
              • AGM would work fine. If you plug in those numbers, you will find that a 58AH battery will run your load for 5 hrs.

                May 12, 2014 a 12:13 pm
              • This is actualy for a question. On a 12vdc converted to 120v a/c., I want to run a water pump, 120v 8 Amp. But the problem is the starting amps of the pump, which is over 30 amps. How many amps of batteries do I need, and what size and type of inverter? Thanks

                Jerry McLean
                April 28, 2014 a 3:41 pm
                • please email your question to

                  May 12, 2014 a 12:13 pm
                  • Im looking at something similar for sump pump and believe you will need an inverter that can handle a 3600 watts start (120vac x 30amps inrush) so maybe a 2000 watt inverter with a 4000 watt peak may work. Thinking pure sine wave for motor..not sure. For Batteries something in the range of 400ah with 30% duty cyle should give you ~4 hours … you need AGM type battery not your typical car/start or marine style…any others our there agree/disagree? Big cables between batteries and inverter!

                    July 26, 2014 a 7:41 pm
                  • Hello, I have 50W DC fan i need to run 24 hours with the 12V battery. what capacity battery i need to run this fan 24 hours continuously.


                    May 13, 2014 a 1:28 am
                    • If you plug in the numbers in the calculator it will tell you. If the load is DC at 12 volts, then you are looking at about a 200AH battery.

                      May 14, 2014 a 10:35 am
                    • Hello,

                      Kindly shed light on my query. If i have a 12V inverter system, using a 1* 200AH battery and a 48V inverter system which requires 4*200AH battery to get the system to perform. For the 48V system, is the amp hrs calculated as 800AH (4*200ah) or is it still only 200AH since it is connected as a 48V?

                      May 15, 2014 a 6:23 am
                      • 200AH. Please refer to this article for more details.

                        May 16, 2014 a 2:17 pm
                        • /kb/articles/battery-articles/battery-bank-tutorial.html

                          May 16, 2014 a 2:17 pm
                      • For confirmaton: I have a houseboat that uses shore power and does not leave the docks. (No working engines). But I need battery power for occasional use for some overhead 12v lights (about 8 although all would not neccessarily be on, which I understand to be about an amp apiece, a toilet macerator which looks to draw about 13 amps when it is engaged, and 2 automatic bilge pumps that come on if there is water in the bilge (I am variously seeing 4-8 amp estimates on water pumps). In the unlikely event everything were on at once I guess I would be drawing up to 37 amps. I also have a 4 stage converter to keep my batter topped off. So it seems to me that a 50 AH deep cycle battery like your model UB12500-45977 would be sufficient for my needs. Am I missing anything?

                        Lee Callister
                        December 1, 2014 a 4:04 pm
                        • You don’t want a situation where you are discharging the battery more than 50%. If you lost shore power that would happen in less than an hour. If you feel comfortable with that, then the 50 AH would work.

                          December 30, 2014 a 8:22 am
                        • Thanks

                          Zafir Ali
                          October 12, 2014 a 6:54 am
                          • I have a question. I want to run a 4000#winch from a portable battery. The winch sucks 240 amp and would run for 20 min max. Should I use a deep cycle RV battery?? and at what AM- HR rating??

                            David Deane
                            October 15, 2014 a 3:53 pm
                            • Based on the large load your are pulling I would recommend at minimum a 232 AH AGM Battery for that 20min run time.

                              November 18, 2014 a 11:25 am
                            • Dear All,
                              Can anybody Suggest me number of Batteries (Acid) of 200 AH if i have to enpower Load of 1300 Watt for 13 Hours daily.
                              pls Guide Me Number of Batteries Which should i use???


                              November 5, 2014 a 1:45 am
                              • Watts/Volts = Amps. Once you have you amp figure you can use the calculator. If you need additional help please contact our TECH Department.

                                November 19, 2014 a 10:58 am
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