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.
Walkthrough
Example 
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 intypically 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. 
Example 
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. 
Example 
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.) 
Example 
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. 
Example 
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. 
Example 
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. 
11 Responses
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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.
Tech
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
Mark
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.
Tech
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 selfballasted 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
Mark
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.
Tech
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
Mark
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.
Tech
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?
Mark
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.
Tech
February 5, 2013 a 10:21 am




my load is 15 Watts LED DC what size of battery for me to run 15watts LED DC for 8hrs?
JB
March 12, 2014 a 6:02 pm
Can i use 3 battries (1 Wet Cell 150Amp and 2 AGM Dry Cell 18Amp) same time on local Transformer base UPS ??