mAh vs. Wh: What’s the Difference? A Simple Guide to Laptop Battery Capacity

Shopping for a replacement laptop battery, you’ve probably noticed that some list 6791mAh, some list 57Wh, and some list both. What do these numbers mean? And does a bigger number always mean longer battery life?

The answer comes down to a few formulas from middle-school physics. You don’t need to be good at math to follow along.

1. The Key Specs on a Battery Label

Take the Microsoft Surface MQ50 laptop battery sold at Store Shoppe. Its product page lists:

• Voltage: 7.58V
• Capacity: 6791mAh
• Energy: 51.48Wh

Here is what each one means.

Voltage (V): the “push” behind the current. In physics class, we learned that voltage is what drives current through a circuit. Think of a battery as a water supply: voltage is a bit like water pressure. A 7.58V rating means the battery’s nominal voltage is 7.58 volts. (This is only an analogy, since voltage and water pressure aren’t the same thing.)

Current (A): how fast charge flows. Current is measured in amperes (A). In the water analogy, it’s roughly how much charge passes a given point each second.

Capacity (Ah or mAh): how much charge the battery can deliver. Ah means amp-hours and mAh means milliamp-hours. The conversion is simple: 1Ah = 1000mAh, so 6791mAh = 6.791Ah.

Here is the important part: mAh measures electric charge, not energy. To measure energy, we need another unit: Wh.

2. What Is Wh, and Why Is It Different from mAh?

Wh stands for watt-hour. It measures energy, meaning how much electrical work the battery can supply.

From physics class, energy equals power multiplied by time:

E = P × t

A device drawing 1 watt for 1 hour uses 1 watt-hour (1Wh) of energy. That’s why Wh tells you how much energy a battery actually stores.

So how are mAh and Wh connected? Recall the other formula:

P = U × I (power = voltage × current)

Current multiplied by time gives charge, and charge multiplied by voltage gives energy. For a battery rated with a nominal voltage and an amp-hour capacity, that gives us this approximation:

Wh ≈ V × Ah

Or, if capacity is in mAh:

Wh ≈ V × mAh ÷ 1000

This single formula is the key to understanding the difference.

3. Working Through Real Examples

Example 1: Checking a product listing.
The Surface MQ50 is rated 7.58V and 6791mAh.

• Convert to amp-hours: 6791 ÷ 1000 = 6.791Ah
• Multiply by voltage: 7.58 × 6.791 ≈ 51.48Wh

That matches the 51.48Wh on the product page.

Example 2: Finding Wh when only mAh and voltage are given.
The 13000162 (TDBW1) battery, made for the Tobii Dynavox TD Pilot, is listed as 11.1V and 6600mAh, with no Wh rating:

• 6600 ÷ 1000 = 6.6Ah
• 11.1 × 6.6 = 73.26Wh

So its nominal energy is about 73.26Wh. (This figure is calculated, not stated by the seller.)

Example 3: Working backward to find voltage.
The HIKMICRO thermal imager battery INR18650-35E-1S2P is rated 6.7Ah / 24.12Wh:

• V ≈ 24.12 ÷ 6.7 ≈ 3.6V

The formula works in any direction: if you know two values, you can find the third.

Keep in mind that this is a method for understanding and comparing labeled specs. Real-world usable energy also depends on battery design, temperature, age, and how the device is being used.

4. Why a Higher mAh Doesn’t Always Mean Longer Runtime

This is the trap that catches many buyers. Compare two real batteries from Store Shoppe:

• Battery A: HBC BA405031: 6400mAh, 23Wh (so roughly 3.6V)
• Battery B: SCHWING 98384265: 7.2V, 3300mAh

At a glance, Battery A has almost double the mAh. But let’s calculate Battery B’s energy:

7.2 × 3300 ÷ 1000 = 23.76Wh

A is about 23Wh and B is about 23.76Wh, so their nominal energy is nearly identical.

Now the reverse case:

The mAh ratings are almost the same (6791 vs. 6600), yet the TDBW1 stores about 42% more energy.

The lesson: very different mAh can mean similar energy, and similar mAh can mean very different energy. The difference is voltage.

Also note that equal energy does not make two batteries interchangeable. Voltage, connector, size, internal structure, and device compatibility all have to match. (The batteries above are made for remote controls and medical devices; they’re used here only because their listings show complete specs.)

So when comparing batteries with different voltages, don’t rely on mAh alone.

5. Can Wh Tell You How Long a Laptop Will Run?

Yes, as long as you know roughly how much power the laptop draws. Using the Surface MQ50’s 51.48Wh:

Runtime ≈ battery energy ÷ average power draw

• At 10W: 51.48 ÷ 10 ≈ 5.1 hours
• At 25W: 51.48 ÷ 25 ≈ 2.1 hours

The same battery gives very different runtimes depending on how hard the laptop works. In real use, screen brightness, CPU load, gaming, video playback, background apps, battery aging, and energy conversion losses all affect results. Treat these numbers as a simplified estimate, not a guarantee.

6. mAh or Wh: Which Should You Look at When Buying a Replacement Battery?

Start with Wh, then check the other specs. Wh reflects a battery’s nominal energy directly, so it’s usually more useful than mAh when comparing batteries with different voltages.

Here are some laptop batteries from Store Shoppe’s laptop battery collection (prices were as listed when I checked and may change):

Because each lists Wh, you can compare their energy at a glance. But they fit different laptops, so higher Wh alone doesn’t make one a substitute for another.

Before you buy, check:

1. Model and part number. Match the label on your old battery and confirm the replacement is compatible with your exact laptop.
2. Nominal voltage. It must match your device’s requirements. Never choose a different voltage just because the capacity is higher.
3. Energy (Wh). Once voltage and compatibility are confirmed, use Wh to compare capacity.
4. Connector and dimensions. The plug, connector, shape, and mounting must all fit.
5. Quality and condition. Rated capacity is only a reference. Real performance depends on cell quality, usage history, aging, and manufacturing.

If you’re unsure whether a battery fits your laptop, verify the exact model and specifications before ordering.

7. Quick Summary

• mAh: electric charge capacity.
• Wh: energy capacity, best for comparing batteries with different voltages.
• V: voltage, the key to converting between the two.

Wh ≈ V × mAh ÷ 1000

For example, the Surface MQ50 (7.58V, 6791mAh) has a nominal energy of about 51.48Wh, which matches its product listing.

Understanding these basics will help you read battery labels accurately, compare specs fairly, and find the right replacement for your laptop. Capacity matters, but the correct model and compatibility matter just as much.