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Knowledge Base : Tools : Calculator | Determine Run Time for Specific Load

Calculator | Determine Run Time for Specific Load

This calculator is intended to help you figure out how long your lead-acid (Wet, AGM, Gel) battery will last under a specified load. In order to use this calculator you will need two separate AH ratings, given by the manufacturer, as well as the amperage, in direct current of your load. For an explanation of why a calculator is necessary to figure out the true run time of your battery see Puekert’s Law.

Capacity AH rating 1 C1 AH
Hour Rate AH rating is at R1 Hour Rate
Capacity AH rating 2 C2 AH
Hour Rate AH rating is at R2 Hour Rate
Battery Temperature  Check if Battery Temp is Over/Under 0-85 °F

Age of Battery

 Check if Battery is more than 6 months old.
Peukerts Constant    
Amperage of Load Applied Amps
Capacity At Given Load   AH @   Hr Rate under a   Amp Load



  This calculator is designed to tell you exactly how long your battery will last given a set amperage load that is put on it.  It requires 2 separate AH ratings at different hour ratings.  If you do not know, you will need to contact the manufacturer to find out.  Typically you will have a 100hr rate, a 20 hr rate and a 10 hr rate readily available from the manufacturer.
  If you cannot find out this information, then you can use our Modified Battery Run Time Calculator.
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C1 and R1 - The first field of this calculator is for the first AH rating for the battery.  In our example, it is 200 AH.  This leads to the second field, which is the hour rate that the AH is given at.  In our example it is the 20 Hr rate.
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C2 and R2 - The third and fourth fields are for the second set of ratings, in this case 216AH at a 100 Hr rate.
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Battery Temperature and Age - The fifth field is to adjust for extreme environmental temperatures, in our example it is over 85 deg F. The sixth field accounts for age of the battery.
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Peukert's Constant - The seventh field is Peukert’s constant, and is figured using the previously given information. If you want to see the formula yourself, see our Math behind the Magic article.
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Amperage of Load Applied - The next field is where the amperage of the load you are placing on the battery is entered. This is figured in DC amperage, so if your load is being ran through an inverter; use our AC to DC amperage conversion calculator. For our example, we have calculated a total load of 15 amps.
Example Capacity At Given Load - The nineth field tells you exactly what the adjusted capacity of this battery is at your specific load.
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Runtime with 50% Safe Discharge Level - The last field tells you approximately how long your battery will last under the given load and circumstances. Under a 15 amp load, our 100 AH Battery should be discharged no more than 6 hours and 9 minutes.

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13 Responses,   4.2 Rating

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  • this has been really helpful and not to hard to understand .
    Thanks very much.
    andrew Greaves

    October 1, 2012 a 4:54 pm
    • Glad you are finding it useful.

      BatteryStuff Tech
      October 2, 2012 a 8:05 am
    • Can you plug in AH and AH ratings for NiMh and NiCad batteries into these formulas and depend on the answers being correct.
      Also, you indicate in the text that you use a 15% ineficiency for inverter loss, but I only see about 10.4% used in the calculations in the example given. Am I missing something? This information is very informative. Thanks for the effort.

      November 2, 2012 a 11:51 am
      • The 15% loss is for the AC to DC Amperage Conversion Calculator. This Run Time Calculator assumes your DC amps have already been calculated accordingly. NiMH and NiCD batteries will not work with this calculators because they have different discharge variables. Peukert‘s Constant only refers to Lead Acid batteries.

        BatteryStuff Tech
        November 2, 2012 a 1:02 pm
      • if we add tow batteries ( same AH ) in parallel we should double the C for each one and keep the same R ?

        December 31, 2012 a 6:13 pm
        • Capacity doubles, voltage stays the same. Therefore, AH, CCA, RC, and other capacity ratings will all double.

          BatteryStuff Tech
          January 2, 2013 a 9:56 am
        • I have a 1500 watt load and a 12volt DC to AC 220 volt inverter which is 2000 watt. also, with a 250 watt 36 volt solar panel. how many ah of 12 volt batteries do i need to operate this my 2500 watt load for 15 hours

          Lawrence McCratty
          April 11, 2014 a 9:30 pm
          • Your first problem is your system is mismatched. A 36 volt source is designed to charge a 3 battery SEREIS bank. 3 batteries x 12V = 36V, max amp output is unchanged. Your inverter is designed to be run by a multiple battery PARALLEL system. 3 batteries x 15A = 45A or 3 batteries x 25A = 75A, 12V output is unchanged.

            May 22, 2014 a 7:48 am
            • PCbored nailed it. Good answer.

              BatteryStuff Tech
              May 23, 2014 a 2:00 pm
            • i have a 250 watt 36 volt solar panel to charge my batteries. how many ah of 12 volt batteries do i need to operate my 1500 watt load for 15 hours if i am using a 12 to 220 volt DC to AC inverter.

              Lawrence McCratty
              April 11, 2014 a 9:37 pm
              • A helpful hint: RC is the minutes to discharge at 25A draw, so if RC = 120 then 120 = 2 hours and 25A X 2 hours = 50 Ah. Therefore MOST batteries already have 2 different Ah ratings right there on the label. Example: I have a class 24 Marine RV battery, the label shows 20 hour rate = 65 and RC = 120, so in this case C1 = 65, R1 = 20, C2 = 50, R2 = 2. This calculator is extremely easy to use if you know how to interpret the given specifications.

                May 22, 2014 a 8:07 am
                • I found this web site while trying to understand charge time and time in use (load) for eletric carts using 6 6v deep cycle batteries in series, 36v. the run time calcutator looks as though it would be for 1 battery. With regards to the manufactures ratings used in the calcultor and series batrteries how do we handle that?

                  June 25, 2014 a 6:38 pm
                  • Please email your full question with system specs to

                    BatteryStuff Tech
                    June 27, 2014 a 8:54 am
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