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mAh vs Wh Explained: Why You Can't Charge the Full Rated Capacity
When picking a power bank you see numbers like “10,000mAh” and “37Wh”—but what’s the difference? And why can’t a 10,000mAh unit charge your phone by the full rated amount?
This guide explains the difference between mAh and Wh, and why you never get the full rated capacity, from how it works. Once you can read the numbers, choosing a battery gets much easier.
The short answer
- mAh (milliamp-hours) is “current × time.” But it doesn’t include voltage, so it’s slightly incomplete for comparing devices.
- Wh (watt-hours) is “power × time”—the true energy including voltage. Convert with
Wh = mAh × voltage(V) ÷ 1000. - You can’t use the full rated capacity mainly because of ① voltage-conversion loss and ② the internal voltage difference. Expect about 60–70% of the rating in practice.
What is mAh? (current × time)
mAh expresses “how much current, for how long.” 1,000mA for one hour is 1,000mAh. A bigger number does mean “stores more electricity,” which is roughly right—but it leaves out voltage (V).
Two “10,000mAh” units with different internal voltages actually hold different amounts of energy. So for a strict comparison, Wh below is better.
What is Wh? (power × time = true energy)
Wh is “power (W) × time.” Since W = voltage(V) × current(A), Wh is the true energy including voltage.
The conversion is just:
Wh = mAh × voltage(V) ÷ 1000
Most power banks have a cell voltage of 3.6–3.7V, so 10,000mAh ≈ about 37Wh. Airline rules are set in Wh precisely because Wh is a common energy figure independent of the device (see airplane rules).
Why can’t you charge the full rated capacity?
You’d think “10,000mAh charges a ~3,000mAh phone three times,” but it’s really about two. Two main reasons:
- Voltage-conversion loss: the battery inside is ~3.7V, while phone charging steps up to 5V or more. That conversion loses energy as heat (which is why it gets a little warm).
- Shrinkage from the voltage difference: raising 3.7V to 5V makes the “mAh number” look smaller for the same energy.
As a result, usable capacity is typically 60–70% of the rating. The “10,000mAh ≈ two phone charges” feeling comes from this. It’s not a defect—it’s an expected loss by design.
Rough sizing
- About two phone charges → 10,000mAh class (everyday/travel standard)
- One quick top-up → 5,000mAh class (light)
- Several days / multiple devices → 20,000mAh+ (high-capacity power banks / for lightness, lightweight models)
Anker Power Bank (10,000mAh, 22.5W)
FAQ
Q. Should I look at mAh or Wh? A. For a strict comparison between devices, Wh (includes voltage). For everyday rough sizing, mAh is fine. Airline rules use Wh.
Q. How many times can 10,000mAh charge a phone? A. With a ~3,000mAh phone battery and 60–70% usable, about two times.
Q. Why does the battery get warm while charging? A. Because the voltage conversion loses some energy as heat. Some warmth is normal—just avoid hot environments (summer heat tips).
Q. Is bigger Wh always better? A. It lasts longer, but it’s heavier and bulkier. Match the capacity to your use.
Summary
mAh is current × time; Wh is the true energy including voltage (Wh = mAh × V ÷ 1000). You can’t use the full rating because of conversion loss and the voltage difference—expect 60–70%. Once the numbers make sense, sizing and airline rules stop being confusing. For the basics, see also volts, amps, and watts.
This article compares products based on public specifications and trends in customer reviews (not hands-on testing by our team). Prices, availability, and review counts change, so please check each product page for the latest. About us: About
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