Wh to Ah Calculator for Battery Capacity & Sizing

How to Use Our Wh to Ah Converter

To find the amp-hour capacity of your battery, you only need two pieces of information that are fundamental to any battery system.

• Energy (in Wh): Enter the total energy capacity of your battery in watt-hours. This value is the universal standard for battery energy and is often printed on the battery label (e.g., 99Wh on a laptop battery or power bank).

• Voltage (in Volts): Enter the nominal voltage (V) of your battery or electrical system. This is a critical value, as amp-hours are directly dependent on the system’s voltage (e.g., 3.7V, 12V, 24V, 48V).

Once you enter both values, the calculator will immediately show you the equivalent charge capacity in Amp-hours (Ah).

Understanding Your Results

The number provided by the calculator is your battery’s charge capacity in Amp-hours (Ah). While Wh tells you the total energy stored, Ah tells you how much charge the battery can hold and deliver at its specific voltage. This is essential for calculating runtime and sizing components like wires and fuses.

Here is a breakdown of the components in this calculation:

• Watt-hour (Wh): Think of this as the total amount of energy in the fuel tank. It is the most accurate and universal way to compare the capacity of any two batteries, regardless of their voltage. This is the unit airlines use for their safety limits.

• Voltage (V): Think of this as the pressure of the electrical system. It’s the force at which the energy is delivered.

• Amp-hour (Ah): Think of this as the size of the fuel tank in terms of charge. It tells you how many amps (A) a battery can provide for a certain number of hours (h). A 100 Ah battery can theoretically deliver 10 amps of current for 10 hours.

The formula used by the calculator is simple and derived from the definition of a watt-hour:

Why This Conversion is Crucial

The same amount of energy can result in very different amp-hour ratings depending on the system’s voltage. This is why you can’t compare batteries using Ah alone unless their voltages are identical.

As you can see, a 1,200 Wh battery is a 100 Ah battery in a 12V system, but it’s only a 25 Ah battery in a more efficient 48V system. They all hold the same total energy, but their charge capacity rating is different.

Frequently Asked Questions

What is the difference between Ah and Wh again?

It’s the difference between charge and energy.

• Wh (Watt-hours): This is a measure of total energy. It’s the most important number for knowing how much work a battery can do. It’s the “what.”

• Ah (Amp-hours): This is a measure of charge capacity. It tells you how long a battery can provide a certain amount of current at its specific voltage. It’s the “how.”

You need both to get a complete picture of a battery’s capabilities. Wh is for comparing total capacity, while Ah is for practical calculations within a system of a known voltage.

Why do I need to know the voltage for this conversion?

Voltage is the essential link between energy (Wh) and charge (Ah). The formula Watts = Volts × Amps is the foundation of electronics. Because of this, you cannot determine a battery’s amp-hour rating from its watt-hour rating without knowing the voltage at which it operates. A high-voltage system needs fewer amp-hours to store the same amount of energy as a low-voltage system.

An airline says my battery must be under 100Wh. What is that in Ah?

This is a perfect practical question our calculator can answer, but the result depends on your battery’s voltage. Most consumer electronics use low-voltage lithium-ion cells.

• For a USB Power Bank (3.7V): 100 Wh / 3.7V = 27.02 Ah, or 27,027 mAh. Most power banks advertised as 20,000 mAh or 25,000 mAh are safely under this limit.

• For a 12V Battery System: 100 Wh / 12V = 8.33 Ah. Any 12V battery over 8.33 Ah is over the standard limit. This is why larger batteries for RVs or boats are not allowed on planes.

How can I use this for my solar power system?

This conversion is vital for sizing your battery bank. Let’s say an energy audit shows you need 4 kWh of stored energy per day. Your solar installer recommends a 48V system for efficiency.

1. First, convert kWh to Wh: 4 kWh × 1000 = 4000 Wh.

2. Now, use the calculator to find the required Ah: 4000 Wh / 48V = 83.3 Ah.

You now know you need a 48V battery bank with a capacity of at least 84 Ah to meet your energy needs.

My battery is rated in kWh. How do I use the calculator?

Easy! The “k” in kWh stands for “kilo,” which means 1,000. To convert kilowatt-hours to watt-hours, simply multiply the kWh value by 1,000.

Example: A 5.2 kWh battery has 5.2 × 1000 = 5200 Wh. You would then enter “5200” into the calculator.

How does the Ah rating relate to how long my battery will last?

You can get a good estimate of runtime if you know how much current your appliance draws in amps (A).

The formula is: Runtime (hours) = Battery Amp-hours (Ah) / Appliance Amp Draw (A)

Example: You have a 50 Ah battery (at 12V) and you want to run a pump that draws 5 amps. Runtime = 50 Ah / 5 A = 10 hours. Keep in mind this is a theoretical maximum. The usable capacity can be less due to temperature and the battery’s recommended depth of discharge.

How do I find my battery’s voltage if it’s not listed?

The voltage is a critical specification and is almost always printed clearly on the battery’s casing or label. Look for a number followed by the letter “V”. If you can’t find it, check the technical specifications in the product manual or on the manufacturer’s website for your specific model number.

Does a higher Ah rating always mean a better battery?

Not necessarily. As we’ve shown, a 50 Ah, 24V battery contains the exact same amount of energy (1,200 Wh) as a 100 Ah, 12V battery. The 24V system might be considered “better” because it can operate more efficiently with smaller wires, but it doesn’t have more energy. The best way to compare the raw capacity of two batteries is to convert them both to watt-hours (Wh).

What is “C-Rate” and how does it affect the usable Ah of my battery?

The C-Rate measures how fast you discharge a battery relative to its capacity. A 1C rate on a 100 Ah battery means you are drawing 100 amps. A C/20 rate means you are drawing 5 amps (100 Ah / 20 hours).

This matters because a battery’s stated Ah capacity is usually rated at a very low C-rate (like C/20). If you discharge the battery faster (at a higher C-rate), its effective, usable Ah capacity will be lower. This phenomenon is known as Peukert’s Law.

How many amp-hours is a 1200Wh battery?

Using the calculator:

• At 12V: 1200 Wh / 12V = 100 Ah

• At 24V: 1200 Wh / 24V = 50 Ah

• At 48V: 1200 Wh / 48V = 25 Ah

This perfectly illustrates why you must know the voltage to find the amp-hour rating.

After converting your battery’s energy rating to amp-hours, you might want to perform the reverse calculation. You can use our Ah to kWh Converter for that. To estimate how long your battery will run specific appliances, check out our detailed Battery Life Calculator.