Battery Capacity Calculator

What is Battery Capacity?

Generally, battery capacity is the amount of electricity a battery can generate. It may also be defined as the amount of energy stored in a battery.

Typically, battery capacity is expressed in amp-hours (Ah). However, other units like milliamp-hours (mAh), watt-hours (Wh), and kilowatt-hours (kWh) may also be used to represent battery capacity.

What is a Battery Amp-Hours (Ah)?

Battery amp-hours is a unit that defines how much current or charge a battery can discharge for one hour. Amp-hours is a combination of how many amps a battery has and how long those amps can last.

What is a Battery Milliamp-Hours (mAh)?

Like a battery’s amp-hours, a battery’s milliamp-hours define how much current (in milliamperes) a battery can discharge over one hour.

What is a Battery Watt-Hours (Wh)?

A battery’s watt-hours defines the amount of energy stored in the battery. It is a combination of how much power a battery can discharge and how many hours it can discharge the power for.

Battery watt-hours can be a good way to estimate battery life/run time when a certain load is placed on a battery.

For instance, if a 30W load is placed on a 270Wh battery, the battery would last approximately 9 hours (270/30). Of course, this is only an estimation; the run time might be shorter.

What is a Battery Kilowatt-Hours (kWh)?

Like a battery’s watt-hours, a battery’s kWh defines the amount of energy stored in a battery. It combines the total power a battery bank can store (in kilowatts) and how long the stored power can last.

How to Use Our Battery Capacity Calculator

• Enter the battery capacity in the corresponding field.
• Choose the correct battery capacity unit from the options: There are four options to choose from: mAh, Ah, Wh, and kWh. You can choose any of them, depending on the unit used for your battery’s capacity rating. So, if your battery is rated 10Ah, you’ll enter the capacity in amp-hours.
• Enter the battery voltage in its corresponding input field.
• You may or may not select a battery type: Selecting a battery type helps the calculator estimate your battery’s capacity even better. Some batteries – e.g. lead-acid batteries – do not use their full battery capacity under regular conditions. So, when you select your battery type, the calculator will consider the available battery capacity for that battery type.
• Enter the number of batteries in the “Quantity” input field.
• In the “How Are Your Batteries Wired Together?” input field, choose the batteries’ connection type – series, parallel, or series-parallel.
• If the connection type is series-parallel, specify how many batteries you wired in series in the “Length of Series Strings” input field.
• Then click on Calculate.
• If you enter the battery capacity in amp-hours or milliamp-hours, the battery capacity calculator will return your answer in watt-hours. But if you enter the capacity in watt-hours or kilowatt-hour, the battery capacity calculator will display the capacity in amp-hours.

How to Calculate Battery Capacity in Amp-Hours

To calculate the amp-hours of a battery, we need two values: the battery’s watt-hours and battery voltage.

When we divide watt-hours by voltage, we’ll get amp-hours:

Ah = Wh/V

So, when you enter watt-hours and voltage into a watt-hours to amp-hours calculator or our battery capacity calculator, this is what happens.

Example

How much battery capacity does a 12V, 12 Wh lithium battery have in amp-hours?

= 12/12 = 1 Ah

How to Calculate Battery Capacity in Milliamp-Hours

While larger batteries typically use amp-hours (Ah), watt-hours (Wh), and kilowatt-hour (kWh), smaller batteries – like those found in phones and game consoles – express battery capacity in milliamp-hours (mAh).

So, how do estimate the capacity of a battery in mAh? Well, we can do this in two ways, depending on the values we have.

If we know the battery’s capacity in amp-hours (Ah), we can calculate its capacity in mAh by multiplying Ah by 1000:

mAh = Ah x 1000

If we do not know the battery’s Ah but know the battery voltage and watt-hours, we can calculate its mAh by dividing Wh by voltage and multiplying by 1000:

mAh = Wh/V x 1000

How to Calculate Battery Capacity in Watt-Hours

To calculate a battery capacity in watt-hours, multiply the battery voltage and amp-hours:

Wh = Ah x V

But what if the capacity is in mAh? In that case, we’ll multiply mAh by voltage then divide by 1000:

Wh = mAh x V ÷ 1000

This is pretty much what happens when you enter amp-hours or milliamp-hours and voltage in our battery capacity calculator.

Example 1:

Say we have one battery rated 20V, 10Ah, how many watt-hours can the battery discharge?

To calculate how many watt-hours the battery can produce, we’ll multiply 20V by 10Ah:

= 20 x 10 = 200 Wh

Example 2:

If we have a battery system with three 20V, 10 Ah batteries connected in series. How many watt-hours can the battery bank give off?

The batteries are in series and have the same capacity in Ah. So, we can calculate the battery’s watt-hours by summing the individual voltages of the batteries then multiplying their cumulative voltage by their capacity in amp-hours.

The average voltage of the batteries = 20 + 20 + 20 = 60V

We added the voltages of the batteries because they are in series. If they were in parallel, their cumulative voltage would simply be 20V.

We didn’t add the Ah of each battery because in series, all components of the circuit carry the same current.

The watt-hours of the battery bank = 60 x 10 = 600 Wh

Example 3:

Say we have 2 sets of battery banks connected in parallel, and each set of battery bank contained three 12V, 50Ah batteries in series. How many watt-hours will this connection of battery banks discharge?

In this example, we have a series-parallel connection. So, the calculation wouldn’t be straightforward.

First, we’ll calculate the volts and amp-hour capacity of each battery bank:

Since each battery bank has three 12V batteries in series, their voltage would be 12 + 12 + 12 = 36 volts.

Of course, since the three batteries in each battery bank are in series, they’ll experience the same Ah all through. So, their amp-hours would be 50 Ah.

So, battery bank A is 36V, 50Ah and battery bank B is also 36V, 50Ah.

Next, we’ll calculate the average voltage and amp-hour of the two battery banks (in parallel):

In parallel connections, the components experience the same electrical pressure throughout. So, the parallel voltage would be 36V.

However, the amp-hour capacity in parallel circuits is summative. So, the amp-hours of the two battery banks in parallel would be 50 + 50 = 100Ah.

Now that we know the Ah and voltage of the battery banks in parallel, we can calculate the watt-hour of the connection:

= 36 x 100 = 3600 Wh

How to Calculate Battery Capacity in Kilowatt-Hour (kWh)

When we know the battery’s watt-hours (Wh), we can calculate its kWh by dividing Wh by 1000:

kWh = Wh/1000

However, if we do not know the battery’s Wh but know the voltage and amp-hours, we can estimate kWh by multiplying Ah and V and dividing by 1000:

kWh = Ah x V ÷ 1000

Example 1:

Say we have a battery rated 24V, 30Ah, how much energy can the battery store in kWh?

The battery capacity in kWh:

= 30 x 24 ÷ 1000 = 720/1000 = 0.72 kWh

Example 2:

How much energy does a 650 Wh battery store in kWh?

The battery capacity in kWh:

= 650/1000 = 0.65 kWh

What Factors Affect Battery Capacity?

Battery Type

As mentioned earlier, when using our battery capacity calculator, you can indicate battery type for a more accurate estimation.

Different battery types undergo different electrochemical reactions to generate energy. They also have different cycles. Of course, these two factors affect battery capacity.

A lead-acid battery for instance has a limited discharge cycle. Discharging below 50% of the rated capacity might just damage the battery or reduce battery life. In essence, you can only use half of the energy stored in a lead-acid battery.

Temperature

Generally, at higher temperatures, batteries have higher capacity, However, its best to keep batteries at their operating temperature. Raising battery temperature on purpose will reduce battery life.

Battery Size

As may be obvious, larger batteries will have a higher capacity than smaller batteries of the same type.

Discharge Rate

While discharge rate would not necessarily change how much battery capacity a battery has to offer, it will affect how fast the battery gets depleted. With higher discharge rates, batteries get depleted faster. This may create a perception that such batteries have low capacities. But in actuality, they only discharge faster.

On the flip side, batteries with slower discharge rates would be perceived to have higher capacities because they get depleted slowly.

Battery Age

The effect of battery age on capacity is pretty straightforward. Generally, as a battery gets older, its electrochemistry becomes less efficient: electrodes degrade, ions are lost, amongst other things. Consequently, they see a drop in capacity.

Factors like charge/discharge cycles also contribute to the drop in capacity seen in aging batteries.

Circuit Type

In our battery capacity calculator, we made provision for users to specify how their batteries are wired together. We did so because when estimating the battery capacity of multiple batteries connected together, the circuit matters.

In series connections, the voltage of the individual batteries in the connection sum up but the Ah and mAh don’t. Of course, this will affect the Wh and kWh of the ensuing battery bank.

In parallel connections, the Ah and mAh of the individual batteries sum up while the voltage doesn’t. Understandably, the Wh and kWh of the ensuing battery bank will be different from a series connection.

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