Milliamp-Hours (mAh) to Watt-Hours (Wh): The Clear, Correct Way to Convert
ZacharyWilliamIf you want the quick answer, here it is: Wh = (mAh × V) ÷ 1000. That is the correct way to convert milliamp-hours (mAh) into watt-hours (Wh). The part many people miss is the voltage. Without voltage, you cannot convert mAh to Wh accurately, because mAh measures charge while Wh measures energy.
Quick Answer
What Most People Want to Know
- You need mAh + voltage to get Wh.
- A 20,000mAh power bank is usually about 74Wh, not 100Wh.
- Wh is the better number for comparing battery energy.
Why People Convert mAh to Wh in the First Place
Most small batteries and power banks are advertised in mAh, but Wh is the better unit when you want to compare actual stored energy. That is why airlines, battery labels, and many product spec sheets use watt-hours instead of milliamp-hours.
In plain English:
- mAh tells you how much electrical charge a battery can hold.
- Wh tells you how much total energy the battery stores.
- Voltage is the missing piece that connects the two.
The Formula You Need
Use this formula when converting mAh to Wh:
Use this one if you want to go the other direction:
That is the whole conversion. The only trick is using the correct nominal voltage for the battery or battery pack.
How to Convert mAh to Wh Step by Step
- Find the battery capacity in mAh.
- Find the battery’s nominal voltage.
- Multiply the two numbers.
- Divide by 1000.
Example 1: 5,000mAh at 3.7V
Example 2: 10,000mAh at 3.7V
Example 3: 20,000mAh at 3.7V
Example 4: 30,000mAh at 3.7V
These examples use 3.7V, which is a common nominal voltage for lithium-ion cells used inside many power banks. LiFePO4 cells are commonly around 3.2V, while multi-cell battery packs may use values like 7.4V, 11.1V, or 14.8V depending on how the cells are arranged.
Why a 20,000mAh Power Bank Is Usually About 74Wh, Not 100Wh
This is one of the biggest points of confusion. Many people look at 20,000mAh and assume it must equal 100Wh because they are thinking in terms of 5V USB output. But most power banks rate their mAh using the internal cell voltage, which is typically about 3.7V, not the 5V output voltage on the USB port.
So a 20,000mAh power bank is usually about 74Wh, not 100Wh.
Which Voltage Should You Use?
Always use the nominal battery voltage, not whichever voltage happens to be printed on the charger, cable, or USB output.
- For a single lithium-ion cell, use about 3.6V to 3.7V.
- For a single LiFePO4 cell, use about 3.2V.
- For a 2S lithium-ion pack, use about 7.4V.
- For a 3S lithium-ion pack, use about 11.1V.
- For a 4S lithium-ion pack, use about 14.8V.
- For a 12V battery system, check the spec sheet because “12V” systems may use a nominal value around 12.0V to 12.8V depending on chemistry.
Quick mAh to Wh Chart
The table below assumes a 3.7V lithium-ion battery:
| mAh | Wh |
|---|---|
| 1,000mAh | 3.7Wh |
| 2,000mAh | 7.4Wh |
| 3,000mAh | 11.1Wh |
| 5,000mAh | 18.5Wh |
| 10,000mAh | 37Wh |
| 20,000mAh | 74Wh |
| 27,000mAh | 99.9Wh |
| 30,000mAh | 111Wh |
This chart is accurate only when the battery uses a 3.7V nominal voltage.
How to Estimate Runtime After Converting to Wh
Once you know the Wh, you can make a simple runtime estimate:
If your device uses 10W and your battery stores 74Wh, the ideal runtime is:
But real life is never ideal. USB conversion losses, inverter losses, temperature, and battery reserve all reduce usable energy. That is why real-world runtime is usually lower than the simple math suggests.
For a more realistic estimate, use:
Example:
Common Mistakes People Make
1. Forgetting Voltage
You cannot convert mAh to Wh with mAh alone. Voltage is required.
2. Using 5V Output Instead of Battery Voltage
For power banks, the advertised mAh is usually based on internal cell voltage, not USB output voltage.
3. Confusing Rated Energy With Delivered Energy
A battery may store a certain number of Wh, but the energy delivered to your phone, laptop, or AC device will be lower after conversion losses.
4. Comparing Batteries by mAh Alone
mAh only works as a fair comparison when the voltage is the same. Otherwise, use Wh.
FAQ
Do I always need voltage to convert mAh to Wh?
Yes. Voltage is essential because mAh measures charge and Wh measures energy. The correct formula is Wh = (mAh × V) ÷ 1000.
Is 10,000mAh always 37Wh?
No. It is 37Wh only if the battery is 3.7V. Change the voltage and the Wh changes too.
Can I convert Wh back to mAh?
Yes. Use mAh = (Wh × 1000) ÷ V. For example, 100Wh at 3.7V is about 27,027mAh.
What is the easiest way to compare battery capacity?
Compare in Wh, not just mAh. It gives you a clearer apples-to-apples number, especially when voltages differ.
Why does real runtime look lower than the Wh math suggests?
Because conversion losses, heat, battery reserve, and the device’s actual power draw all affect the usable energy you get in real use.
Bottom Line
If you remember only one thing, remember this: mAh by itself does not tell the whole story.
To convert it correctly, you need the battery’s nominal voltage.
That one step helps you compare batteries more accurately, understand power bank labels more clearly, estimate runtime more realistically, and avoid the common mistake of assuming that a bigger mAh number always means more usable energy.
