Power bank capacity is measured in milliampere-hours (mAh) — a unit that describes how much electrical charge a battery can store. A 10,000 mAh power bank does not deliver 10,000 mAh to your devices, though. Energy is lost during the conversion process, so real-world output is consistently around 60–70% of the rated number. Understanding this gap is the single most important thing to know when evaluating power bank capacity.
Quick Answer
- mAh measures stored charge, not delivered charge
- Real-world usable capacity is roughly 60–70% of the rated mAh due to voltage conversion losses
- A 10,000 mAh power bank delivers approximately 6,000–7,000 mAh to your devices
- Higher mAh always means more charges — but physical size and weight increase with capacity
- For airline travel, Wh (watt-hours) is the relevant unit, not mAh
What mAh Actually Measures
Milliampere-hours (mAh) is a unit of electric charge. One milliampere-hour equals a current of one milliampere flowing for one hour. In practical terms, it tells you how much energy a battery can store.
Power banks store energy at a cell voltage of around 3.6–3.7V. Your devices, however, charge at 5V (or higher with fast charging). A voltage conversion circuit — called a boost converter — steps the voltage up, and this conversion is where energy is lost. The conversion efficiency of most power banks sits between 85–92%, and when combined with other losses (heat, self-discharge, circuit overhead), usable output typically lands at 60–70% of rated capacity.
This is not a flaw or a marketing trick — it is a fundamental property of how lithium-ion batteries and boost converters work.
How to Calculate Real-World Charges
The Efficiency Formula
To estimate how many times a power bank will charge a specific device, use this formula:
Usable capacity = Rated mAh × 0.65
Then divide by the device's battery capacity:
Number of charges = Usable capacity ÷ Device battery mAh
Using 65% as the efficiency factor gives a conservative, real-world estimate. Manufacturers often use 80–85% in their marketing figures, which overstates performance.
Calculation Examples
| Power Bank | Rated mAh | Usable mAh (×0.65) | iPhone 15 (3,349 mAh) | Samsung S24 (4,000 mAh) | iPad mini (5,124 mAh) |
|---|---|---|---|---|---|
| Small | 5,000 | 3,250 | ~0.9× | ~0.8× | ~0.6× |
| Mid-size | 10,000 | 6,500 | ~1.9× | ~1.6× | ~1.3× |
| Large | 20,000 | 13,000 | ~3.9× | ~3.3× | ~2.5× |
| Max carry-on | 26,800 | 17,420 | ~5.2× | ~4.4× | ~3.4× |
Charge counts assume the device starts empty and the power bank starts full. Partial charges are proportional.
Why Manufacturer Numbers Are Higher
Marketing specs for power banks typically show charges like "charges an iPhone 3× from a 10,000 mAh bank." This uses a higher efficiency assumption (80–85%) and sometimes a lower-capacity iPhone model as the reference device. The table above uses 65% efficiency, which matches independent lab testing more closely.
Related: What Nobody Tells You About mAh Ratings on Power Banks
mAh vs Wh: When the Unit Matters
Related: Power Bank Rules for Air Travel in 2026
Watt-hours (Wh) is the unit airlines use to regulate power banks. To convert mAh to Wh:
Wh = (mAh × V) ÷ 1,000
Power banks use a nominal cell voltage of 3.7V:
| Rated Capacity | Wh (at 3.7V) | Airline Status |
|---|---|---|
| 10,000 mAh | 37 Wh | ✅ Permitted (all airlines) |
| 20,000 mAh | 74 Wh | ✅ Permitted (all airlines) |
| 26,800 mAh | ~99 Wh | ✅ Permitted (under 100 Wh limit) |
| 27,000 mAh | ~100 Wh | ⚠️ Borderline — verify with airline |
| 30,000 mAh | 111 Wh | ⚠️ Requires airline approval (100–160 Wh) |
| 43,000+ mAh | 160+ Wh | ❌ Not permitted on passenger aircraft |
The standard carry-on limit is 100 Wh without approval and 160 Wh with airline approval. Most consumer power banks fall well under 100 Wh. The common 26,800 mAh size — the largest that comfortably fits under the limit — is sold specifically because of this regulatory ceiling.
Common Capacity Tiers and Their Use Cases
Power banks are not sold in arbitrary sizes. The market has converged on a handful of capacity tiers, each suited to specific use cases.
| Capacity | Wh | Weight (approx.) | Best For |
|---|---|---|---|
| 5,000 mAh | 18.5 Wh | 100–130 g | Daily commute, top-up charges |
| 10,000 mAh | 37 Wh | 200–250 g | Weekend travel, 1–2 full phone charges |
| 20,000 mAh | 74 Wh | 400–500 g | Multi-day trips, tablets, multiple devices |
| 26,800 mAh | ~99 Wh | 500–650 g | Max airline-legal capacity |
| 30,000+ mAh | 111+ Wh | 600 g+ | Ground travel only; not airline-legal without approval |
Weight scales roughly linearly with capacity. A 20,000 mAh power bank is approximately twice the weight of a 10,000 mAh model — there is no free lunch in energy density at the consumer level.
Related: How to Choose a Power Bank: Key Specs That Actually Matter
How Capacity Degrades Over Time
Related: Do Power Banks Damage Your Phone Battery?
Power bank capacity is not static. Lithium-ion cells lose capacity with each charge cycle, and rated mAh refers to the capacity when new. A typical power bank retains around 80% of original capacity after 500 charge cycles. At 300–500 full cycles, a 10,000 mAh bank may effectively behave like a 8,000–8,500 mAh unit.
Factors that accelerate capacity loss include: - Storing the power bank fully charged or fully depleted for extended periods - Charging at high ambient temperatures - Frequent use of the highest output wattage
Storing at around 50–80% charge extends cell longevity meaningfully.
Common Misconceptions
"A higher mAh power bank charges faster." Capacity (mAh) and charging speed (W) are unrelated. A 5,000 mAh power bank can output 65W; a 20,000 mAh unit might only output 10W. Output wattage depends on the converter circuitry, not cell capacity.
Related: Is It Safe to Charge Your Phone with a High-Wattage Power Bank?
"The rated mAh is what your device receives." No. As covered above, voltage conversion losses mean delivered capacity is 60–70% of rated. A power bank claiming "10,000 mAh" delivers roughly 6,500 mAh to a phone.
"More capacity is always better." Beyond a certain point, extra capacity adds weight without proportional benefit. If a 10,000 mAh bank is enough for a three-day trip, a 20,000 mAh unit adds 200+ grams without practical advantage.
"All power banks with the same mAh are equivalent." Capacity is equal on paper, but output wattage, number of ports, build quality, and cell quality vary considerably. Two 10,000 mAh power banks from different manufacturers can deliver meaningfully different real-world performance.
Frequently Asked Questions
What does mAh mean on a power bank? mAh stands for milliampere-hours. It is a measure of how much electrical charge the battery can store. A higher number means more total energy, which translates to more charges for your devices — though real-world output is 60–70% of the rated figure due to conversion losses.
Is 10,000 mAh enough for a week of travel? For most smartphones, a 10,000 mAh power bank provides roughly 1.5–2 full charges. For a week of heavy use, a 20,000 mAh unit is more appropriate — or access to wall outlets each evening.
Why does my power bank run out faster than expected? The most common reason is the gap between rated and usable capacity. A 10,000 mAh bank delivers approximately 6,500 mAh due to voltage conversion losses. Additional factors include aged cells (reduced capacity over charge cycles) and heat-related efficiency drops.
Can I bring a 20,000 mAh power bank on a plane? Yes. A 20,000 mAh power bank at 3.7V equals approximately 74 Wh, which is under the standard 100 Wh airline limit. It must be carried in hand luggage, not checked baggage.
What is the difference between mAh and Wh? mAh measures charge; Wh measures energy (charge × voltage). Airlines use Wh because it accounts for voltage and gives a more accurate measure of total stored energy. To convert: Wh = (mAh × 3.7) ÷ 1,000.
Does a higher mAh power bank charge my phone faster? No. Charging speed is determined by output wattage (W), not capacity (mAh). These are independent specifications. Check the output wattage rating to understand charging speed.
Summary
Power bank capacity in mAh tells you how much energy is stored, but usable output is consistently 60–70% of that figure after voltage conversion losses. To calculate real-world performance, multiply rated mAh by 0.65 and divide by your device's battery size. For airline travel, convert mAh to Wh using the 3.7V cell voltage — the legal carry-on limit is 100 Wh without airline approval, which corresponds to approximately 27,000 mAh.