Why the Label Is Not What You Get
The capacity printed on a power bank — 10,000mAh, 20,000mAh, 30,000mAh — is measured at the cell level, at the cell's nominal voltage of 3.7V. That is the rating of the lithium-ion cells inside the enclosure. USB, however, operates at 5V (or 9V, 12V for PD). When a power bank steps up 3.7V to 5V to charge your phone, it uses a boost converter. That conversion is not 100% efficient. Energy is lost in the process, and the amount lost depends on the quality of the converter, the current being drawn, and the cable resistance. The result: you receive fewer mAh at the USB port than the cells originally held.
Beyond the voltage conversion, there are losses in the cable, in the charging circuitry of your phone, and in the power bank's own output controller. Each link in the chain costs a small percentage. Adding them together, a well-designed power bank delivers 70–85% of its labeled capacity to your device. A poorly designed one can deliver under 60%. This is not fraud — it is physics that most manufacturers do not bother to explain.
The Formula: How to Calculate Real Output
The calculation is straightforward. Start with the labeled capacity, apply the conversion, and account for efficiency losses:
Step 1 — Convert labeled mAh to watt-hours:
Labeled mAh × 3.7V ÷ 1,000 = Labeled Wh
Example: 20,000mAh × 3.7V ÷ 1,000 = 74Wh
Step 2 — Apply output efficiency:
Labeled Wh × efficiency (0.70–0.85) = Usable Wh
Example: 74Wh × 0.75 = 55.5Wh
Step 3 — Convert back to usable mAh at 5V:
Usable Wh × 1,000 ÷ 5V = Usable mAh at USB port
Example: 55.5Wh × 1,000 ÷ 5V = 11,100mAh
That labeled 20,000mAh power bank delivers roughly 11,000mAh to your phone. The remaining 9,000mAh is lost in voltage conversion and heat. For a quick mental shorthand: divide the labeled mAh by 1.4 to get a conservative real-world estimate. For 20,000mAh, that is about 14,300mAh at the port — close to what independent testing consistently finds for quality units.
If you want a precise figure for planning a trip, our camping power bank real-capacity testing covers eight units with measured efficiency data.
What Efficiency Actually Depends On
Power bank efficiency is not a fixed number. It varies based on several conditions:
Load Current
Efficiency drops at very low and very high draw rates. At 0.5A output, a typical power bank may only achieve 65–70% efficiency because the boost converter's quiescent current — the power it draws just to run itself — represents a larger proportion of total throughput. At 2A, efficiency typically peaks at 75–85%. At 3A and above with USB-C PD, efficiency can dip again due to heat in the connector and cable. The practical sweet spot for most devices is 1A–2A, where efficiency is highest.
Temperature
Cold temperatures significantly reduce effective capacity. Lithium-ion cells lose capacity at low temperatures because the electrochemical reactions slow down. At 0°C, a lithium cell may deliver only 60–70% of its rated capacity. At -10°C, that drops to 40–50%. This is why power banks stored in an outside jacket pocket in winter behave disappointingly — the cells are too cold to deliver their rated output. Conversely, high temperatures above 40°C accelerate aging and can trigger the BMS to throttle output or shut down entirely. The optimal operating temperature range is 15–35°C.
Battery Age and Cycle Count
All lithium-ion batteries degrade over charge-discharge cycles. After 300–500 cycles, a typical consumer lithium cell retains roughly 80% of its original capacity. After 1,000 cycles, it may be at 60%. A power bank that delivered 15,000mAh new will deliver 12,000mAh after two years of regular use. This degradation is normal and unavoidable, though quality cells (Samsung INR, LG MJ1, Panasonic NCR) degrade more slowly than unbranded commodity cells. If you are buying a used power bank, the age and number of cycles matters as much as the labeled capacity.
Standby Drain
A power bank that sits fully charged for weeks slowly discharges itself. Lithium-ion self-discharge rate is roughly 2–3% per month at room temperature. A power bank left on a shelf for three months at 100% charge has lost 6–9% of its capacity before you ever use it. Some power banks also have a "standby mode" that draws measurable current to keep the USB-C controller active, adding a small daily drain. For emergency kits, store power banks at 40–60% charge, not 100%.
USB-C PD vs. Standard USB-A: Does the Protocol Matter?
Standard USB-A output at 5V uses a basic boost converter that typically achieves 70–78% end-to-end efficiency. USB-C Power Delivery (PD) is more complex but generally more efficient, because the negotiated voltage can match the device's needs more precisely. A laptop requesting 20V from a 100W PD power bank avoids the large step-up conversion that a 5V-only bank must perform.
In practice, USB-C PD power banks typically achieve 75–85% efficiency under optimal conditions, versus 70–78% for USB-A-only units. The gap is real but not dramatic. What matters more for real-world capacity is whether the power bank and device negotiate the optimal voltage. Using a USB-A cable to charge a device that supports USB-C PD means the charger and device cannot negotiate above 5V/3A — and that lower voltage path can be less efficient for devices that expect 9V or 15V input.
For getting the most from your power bank, use the cable and port type your device supports. Our GaN charger explainer covers why cable and charger compatibility matters for efficiency.
The 100Wh Airline Limit and Why It Matters for Capacity
Airline regulations define power bank capacity limits in watt-hours, not milliamp-hours. The FAA limits lithium battery power banks to 100Wh per unit for carry-on without airline approval. This limit exists because of the fire risk posed by large lithium battery fires in aircraft cargo holds, where they cannot be managed. All consumer power banks up to 27,000mAh at 3.7V (99.9Wh) fall within the standard 100Wh limit. Anything above that requires airline approval at check-in.
The conversion matters because watt-hours are what the airline measures: Labeled mAh × 3.7V ÷ 1,000 = Wh. A 26,800mAh bank is 99.16Wh — just under the limit. A 30,000mAh bank is 111Wh — over it. Knowing this calculation before you travel prevents the unpleasant discovery at security that your new 30,000mAh purchase is not legal to bring on board without pre-approval.
For the full safety and regulatory context around carrying power banks on aircraft, see our power bank safety guide, which covers airline rules, BMS requirements, and the specific warning signs of a problematic unit.
How to Verify Real Capacity Before You Buy
Marketing numbers are unreliable. Here is how to assess real capacity before purchasing:
Check the Weight
Lithium-ion cells are dense. A 20,000mAh (74Wh) battery pack using quality 18650 or 21700 cells weighs approximately 350–400g before the PCB, casing, and cables. If a 20,000mAh power bank weighs under 280g, it is almost certainly using lower-capacity cells than labeled — or fewer of them. Weight is one of the most reliable proxies for actual capacity, because the energy density of lithium cells is not easily faked at the cell level.
Look for Cell Specifications
Quality manufacturers specify the cells inside. Anker typically uses Samsung or LG cells and lists them. If a power bank lists "high-quality lithium polymer cells" with no model number, it is using unbranded commodity cells. This is not automatically dangerous, but it means the capacity claim is unverified. For ranked power bank recommendations for photography and field use, we only include units where cell specifications are verifiable.
Read Third-Party Tests
The gold standard for independent power bank testing is USB-meter-based discharge testing — a calibrated load connected to the USB output, measuring total energy delivered until the power bank shuts off. This is different from the manufacturer's claimed output at cell level. Look for tests that report Wh or mAh at the USB port, not just cell-level specifications. Our camping real-capacity guide uses this method: controlled USB discharge testing with calibrated equipment.
Calculate the Airline-Watt-Hour Figure
Before any purchase, do the math: multiply the labeled mAh by 3.7 and divide by 1,000. If the result is close to or over 100Wh, confirm the airline rules. If it is well under 100Wh (most 10,000–20,000mAh banks are 37–74Wh), you are within the standard carry-on limit. This quick calculation also tells you whether the capacity claim is plausible — a 50,000mAh power bank at 3.7V is 185Wh and would require special airline approval, weigh over 700g in cells alone, and cost significantly more than the typical online listing price. If the price seems too low for the labeled capacity, it almost certainly is.
Real-World Charging Examples
The best way to internalize real capacity is through examples:
iPhone 15 Pro (4,422mAh battery): From a labeled 10,000mAh (37Wh) power bank at 75% efficiency, you receive approximately 7,500mAh at 3.7V, or 27.75Wh at 5V. Your iPhone accepts roughly 4,500mAh at 3.85V (17.3Wh) per full charge from near-empty. Accounting for charging losses in the phone itself (typically 10–15%), a 27.75Wh usable delivery gets you 1.4 to 1.6 full charges. A labeled "4 charges" claim would require a 20,000mAh bank.
Sony A7 IV (two NP-FZ100 batteries, 16.4Wh each): A full camera battery cycle — from empty to 100% — requires approximately 20–22Wh accounting for charger inefficiency. From a labeled 20,000mAh bank (74Wh, 55.5Wh usable at 5V), you can complete roughly 2.5 full camera battery cycles. For a day of field photography with two spare batteries, you are looking at 40–45Wh of capacity used. A labeled 20,000mAh bank handles this, but it is not leaving much margin.
MacBook Pro 14" (70Wh battery): USB-C PD charging of a laptop is the most demanding use case. A 20,000mAh labeled power bank (55.5Wh usable) charges a 70Wh laptop from empty to roughly 70–75% — not a full charge. A 27,000mAh bank (73.7Wh labeled, ~55Wh usable) adds marginally more. For laptop users who need a full charge in the field, you need a 100Wh-class power bank, which approaches the airline carry-on limit. A 72,000mAh at 12V bank (266Wh) is well over the airline limit and must be shipped as cargo.
The Summary Numbers to Remember
- Divide labeled mAh by 1.4 for a quick conservative estimate of what reaches your phone at 5V.
- Divide by 1.5 if the power bank is old, stored in heat, or used in cold weather.
- Multiply labeled mAh by 3.7 and divide by 1,000 to get the Wh figure for airline limits.
- Quality USB-C PD power banks achieve 75–85% efficiency. USB-A-only units typically 70–78%.
- A 20,000mAh bank delivers roughly 11,000–13,500mAh at the USB port, depending on efficiency and conditions.
- Weight is a reliable capacity proxy. Under 280g for a claimed 20,000mAh is a red flag.
- Store at 40–60% charge for long periods to minimize self-discharge and calendar aging.
The labeled number is the starting point, not the planning figure. Understanding the gap between rated and real capacity changes how you plan trips, evaluate purchases, and interpret manufacturer claims. For detailed real-capacity testing data across a range of camping and field use power banks, see our camping power bank real-capacity results.