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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.
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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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15 people commented, TECH, Guy Bradley, Tech, Stacey, and 11 others
This article is rated 4.4 out of 5
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  • Guy Bradley
    Great calculator and and the nerds explanation page is great as well. Question. How do I use this calculator with two battery banks connected in parallel? I have (4) 445Ah 6V batteries in series for 24 volts. Another set of (4) 445Ah 6V batteries in series. These are Rolls Flooded Deep Cycle Battery S6 L16-HC. In theory I should have 890Ahrs till dead combining both battery banks in parallel. 50%, I should have 445Ahrs. I only see around 300Ahrs at a 17 amp discharge rate over 14 hours. Kinda hurts that I don't get the 445 being that the batteries are rated for 20 hrs at 22.25 amps. The batteries are a year and 7 months old and since I had them brand new, this is all I have ever been able to see. Thanks for your time.

    Reply  •  Rated article 5  •  February 19, 2023 at 7:11 am
    • TECH
      You just need to input the total AH rating of your battery pack for each capacity rating. I typically recommend using the AH rating closest to the amp draw you are applying to the battery in order to be the most accurate.

      Reply  •  Rated article 5  •  March 15, 2023 at 11:39 am
  • Stacey
    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?

    Reply  •  June 25, 2014 at 6:38 pm
    • BatteryStuff Tech
      Please email your full question with system specs to

      Reply  •  June 27, 2014 at 8:54 am
  • PCbored
    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.

    Reply  •  Rated article 5  •  May 22, 2014 at 8:07 am
  • Lawrence McCratty
    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.

    Reply  •  April 11, 2014 at 9:37 pm
  • Lawrence McCratty
    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

    Reply  •  April 11, 2014 at 9:30 pm
    • BatteryStuff Tech
      PCbored nailed it. Good answer.

      Reply  •  May 23, 2014 at 2:00 pm
    • PCbored
      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.

      Reply  •  Rated article 5  •  May 22, 2014 at 7:48 am
  • Mar
    if we add tow batteries ( same AH ) in parallel we should double the C for each one and keep the same R ?

    Reply  •  Rated article 5  •  December 31, 2012 at 6:13 pm
    • BatteryStuff Tech
      Capacity doubles, voltage stays the same. Therefore, AH, CCA, RC, and other capacity ratings will all double.

      Reply  •  January 2, 2013 at 9:56 am
  • Forrest
    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.

    Reply  •  Rated article 5  •  November 2, 2012 at 11:51 am
    • BatteryStuff Tech
      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.

      Reply  •  November 2, 2012 at 1:02 pm
  • Andrew
    this has been really helpful and not to hard to understand .
    Thanks very much.
    andrew Greaves

    Reply  •  Rated article 1  •  October 1, 2012 at 4:54 pm
    • BatteryStuff Tech
      Glad you are finding it useful.

      Reply  •  October 2, 2012 at 8:05 am