Battery Amp Hour Capacity Calculator

Use our battery capacity calculator to easily convert your battery’s capacity from watt hours to amp hours (Wh to Ah), or amp hours to watt hours (Ah to Wh).

Battery Capacity Calculator

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Optional: If you select a battery type, we’ll tell you how much usable capacity your battery bank has.
How many batteries do you have in your battery bank? If you have more than 1, we’ll ask how they’re wired together.
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Your battery capacity is:
Battery voltage:

How to Use This Calculator

1. Enter your battery’s capacity and select its unit from the list. The unit options are milliamp hours (mAh), amp hours (Ah), watt hours (Wh), and kilowatt hours (kWh). For instance, if you have a 1200Wh battery, you’d enter the number 1200 and then select “Wh” from the list of unit options.

2. Enter your battery’s voltage. If you have a 12V battery, you’d enter the number 12.

3. Optional: Select your battery type from the list. If you select a battery type, we’ll estimate your battery’s usable capacity. For some battery types, such as lead acid batteries, you can’t use their full capacity without damaging them and shortening their lifespan.

4. Enter the number of batteries you have in your battery bank. If you’re calculating the capacity of 1 battery, you’d just enter the number 1. If you enter 2 or more, a field will appear asking how your batteries are wired together.

5. If you have multiple batteries in your battery bank, select how they’re wired together. The wiring options are series, parallel, and series-parallel. The series-parallel option is only available if you’ve entered a number of 4 or greater in the Quantity field. If you’ve selected the series-parallel option, a field will appear asking for the length of your series strings.

6. If your batteries are wired in a series-parallel configuration, enter the number of batteries wired together in each series string. For instance, if each of your series strings has 4 batteries in it, you’d enter the number 4. We’ll use this number to calculate the length of your parallel strings.

7. Click “Calculate Battery Capacity” to get your results. If you’ve entered your battery capacity in watt hours, we’ll calculate your battery’s amp hours. And if you’ve entered your battery capacity in amp hours, we’ll calculate your battery’s watt hours. For battery banks with multiple batteries wired together, we’ll also calculate your battery bank’s voltage.

How to Calculate Battery Amp Hours

To calculate a battery’s amp hours, divide its watt hours by its voltage.

Formula: battery amp hours = battery watt hours ÷ battery voltage

Abbreviated: Ah = Wh ÷ V

Calculator: Watt Hours to Amp Hours Calculator

Small batteries — such as those found in phones, tablets, and battery packs — more commonly express their battery capacity in milliamp hours. To calculate a battery’s milliamp hours, divide its watt hours by its voltage and then multiply by 1,000.

Formula: battery milliamp hours = battery watt hours ÷ battery voltage × 1,000

Abbreviated: mAh = Wh ÷ V × 1,000

Calculator: Watt Hours to Milliamp Hours Calculator

Example

Let’s say you have the following LiFePO4 battery.

You look at the battery specs and see that it has a voltage of 12.8 volts and a capacity of 1280 watt hours. Here’s how to calculate its amp hours:

1280Wh ÷ 12.8V = 100Ah

In this example, your battery has a capacity of 100 amp hours. Put another way, it’s a 100Ah battery.

How to Calculate Battery Watt Hours

To calculate a battery’s watt hours, multiply its amp hours by its voltage.

Formula: battery watt hours = battery amp hours × battery voltage

Abbreviated formula: Wh = Ah × V

Calculator: Amp Hours to Watt Hours Calculator

If your battery’s capacity is given in milliamp hours, multiply its milliamp hours by its voltage and then divide by 1,000.

Formula: battery watt hours = battery milliamp hours × battery voltage ÷ 1,000

Abbreviated formula: Wh = mAh × V ÷ 1,000

Calculator: Milliamp Hours to Watt Hours Calculator

Example: 1 Battery

Let’s say you have a 12V 50Ah car battery.

To calculate its watt hours, you multiple its amp hours by its voltage:

50Ah × 12V = 600Wh

Your car battery has a capacity of 600 watt hours.

Example: 2 Batteries Wired in Series

In certain cases – such as building an off-grid solar power system – you may need to wire multiple batteries together to build a battery bank. Batteries can be wired in series, parallel, or series-parallel.

Wiring batteries in series sums their voltages but keeps their amp hours the same.

For example, let’s say you wire two 12V 100Ah LiFePO4 batteries in series. Doing so sums their voltage for a total of 24 volts (12V + 12V = 24V), but keeps their amp hours the same at 100Ah.

The result is a 24V 100Ah battery bank. To calculate its watt hours, you multiply amp hours by volts.

100Ah × 24V = 2400Wh

Turns out your battery bank was a capacity of 2400 watt hours.

There is an alternative way to arrive at this number. You could instead calculate the watt hours of one of your batteries, and then multiply that value by the number of batteries you’re wiring together.

You know that each battery is a 12V 100Ah battery. So you can first calculate its watt hours.

100Ah × 12V = 1200Wh

Then you just need to multiply the watt hours of the one battery by the number of batteries you’re connecting together — 2, in this example.

1200Wh × 2 = 2400Wh

Using this approach, we arrive at the same value: 2400 watt hours. Use whichever method you prefer.

Note: The second method assumes that your batteries are identical. I strongly recommend that you only wire identical batteries together. By identical I mean the batteries should have the same age, voltage, and amp hours. They should be the same battery type and from the same brand.

Example: 2 Batteries Wired in Parallel

Let’s instead say that you wired your two 12V 100Ah LiFePO4 batteries in parallel. Wiring batteries in parallel sums their amp hours and keeps their voltages the same.

So, for this example, summing the amp hours gets us 200 amp hours (100Ah + 100Ah = 200Ah) and the voltages remain the same at 12 volts.

The result is a 12V 200Ah battery bank. You calculate its watt hours using the same formula:

200Ah × 12V = 2400Wh

In this example, your battery bank once again has a capacity of 2400 watt hours.

Notice that the battery bank has the same number of watt hours regardless of whether you wire the batteries in series or parallel. The way you wire batteries together affects their voltage and amp hours, but the resulting battery banks will always have the same number of watt hours.

How Many Battery Amp Hours Do I Need?

To get a very rough estimate of how many amp hours your battery needs to have, you need to know:

  • Device current draw in amps (A): How many amps does the device you’re powering use? You can usually find this number listed on the device’s label or power cord. If power consumption is listed in watts, convert watts to amps.
  • Desired runtime in hours: How many hours do you want to run your device for?
  • What is your battery’s recommended depth of discharge? Some brands will list a recommended depth of discharge (DoD) in the product manual. If yours doesn’t have one, use the following rules of thumb: lead acid batteries can be safely discharged to 50%, while lithium and nickel-based rechargeable batteries can often be safely discharged to 100%.

Once you have the above info, multiply the device’s current draw in amps by your desired runtime in hours. Divide all that by the recommended depth of discharge.

Formula: battery amp hours = device current draw in amps × desired runtime in hours ÷ depth of discharge

Abbreviated: Ah = A × hrs ÷ DoD

However, batteries don’t discharge with 100% efficiency. The number of amp hours you’ll actually get from your battery depends on how fast the battery is being discharged, something called C-rate. Rechargeable batteries also lose capacity as they age.

So you should treat the results you get from this calculation as the minimum number of amp hours you could possibly get away with. For most situations, I’d recommend oversizing your battery by at least 25%.

Example

Let’s say you want to buy a 12V lithium battery to power some 12V LED lights. According to the product label on the LED lights, they use 2 amps. You want to run them for up to 5 hours at a time.

The brand of lithium battery you’re looking at has a recommended depth of discharge of 80-100%. You decide to be conservative and size your battery based on an 80% depth of discharge.

To estimate how many amp hours your battery needs to have, you plug everything in to the above formula.

2A × 5 hrs ÷ 80% = 10Ah ÷ 80% = 12.5Ah

The smallest battery size you could consider is a 12.5Ah battery. However, you know that it’s always a good idea to oversize your battery to account for inefficiencies during discharging and reduction in capacity over time due to battery age. So you decide to oversize your battery by 25%.

12.5Ah + (12.5Ah × 25%) = 12.5Ah + 3.125Ah = 15.625Ah

Based on your calculations, you decide to get 12V lithium battery with a 16 amp hour capacity.

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Alex Beale
Alex Beale is the founder and owner of Footprint Hero. As a self-taught DIY solar enthusiast, Alex has spent 4 years producing educational solar content across YouTube, TikTok, Instagram, and the Footprint Hero blog. During that time, he's built Footprint Hero to over 7 million blog visits and 18 million YouTube views. He lives in Tennessee.