Why the Labeled mAh Number Is Not What You Get
Every power bank on the market lists a capacity in milliamp-hours (mAh). That number refers to the cells inside the battery at their nominal voltage of 3.7V. USB, however, operates at 5V (or 9V/15V/20V with USB-C PD). When energy converts from 3.7V to 5V, physics requires a corresponding drop in current — you cannot create or destroy energy during conversion. The result is a 15–25% loss as heat during voltage conversion alone. A bank rated at 20,000mAh at the cell level delivers approximately 14,000–16,000mAh at a USB port. Manufacturers are not lying about this number — it is just not the number that matters to you.
Beyond voltage conversion, real-world factors compound the gap. Cable resistance saps small amounts on every charge cycle. Temperature matters more than most people realize: lithium batteries lose significant capacity below 10°C and can struggle in very cold field conditions. A bank stored in an external pocket on a winter flight will deliver noticeably less than the same bank at room temperature. Cell age also plays a role. After 300–500 full charge cycles, lithium cells typically hold 70–80% of their original capacity. A two-year-old travel power bank that has been used weekly is measurably diminished compared to new — even if it looks fine on the outside.
The practical implication: treat rated mAh as the maximum possible, not the expected output. Our camping power bank real-capacity study covers this discrepancy in more depth, with cycle-age data to show how capacity degrades over time.
Testing Protocol: How We Measured Real Performance
We purchased all 10 units at retail — no manufacturer samples, no pre-release units, no influence on results. Each bank was tested across three metrics that correspond directly to field use:
- Actual delivered capacity: USB power meter (FNIRSI FNB58) between the bank and the load. Load: an iPhone 15 Pro running a standardized discharge cycle and a Sony A7IV with dummy battery. We measured from full bank to empty at 5V USB output.
- Recharge time: From fully discharged to 100% indicated charge, using a 65W GaN wall charger and the included or recommended cable. This matters practically — a bank that takes 8 hours to recharge overnight is not useful if you need to leave at 6am after a long travel day.
- Pass-through charging: Whether the bank can charge connected devices while being recharged itself, and at what combined output rate. This is relevant for overnight charging: one wall plug charging both the bank and your phone simultaneously.
Banks tested: Anker 533 (10,000mAh), Zendure SuperMini (10,000mAh), Anker 20K 30W, Baseus 65W 20,000mAh, Xiaomi 50W Power Bank 20,000mAh, Goal Zero Sherpa 100PD (25,600mAh), Nitecore NPS200 (19,200mAh), BioLite Charge 80 (25,600mAh), Litionite P700 (25,600mAh), and EcoFlow River 2 Max (512Wh, included as a power station reference point).
Pocket Class: 5,000–10,000mAh
These are the banks that live in a jacket pocket or a carry-on accessory pouch. For day trips, urban travel, or as a secondary unit alongside a larger bank, they are the category most people actually needs — but the performance variation within this class is wider than the specs suggest.
Anker 533 (10,000mAh, $45) — Delivered 7,420mAh at 5V. This is a solid, honest performer in the pocket class. The 12W USB-A-only output is dated — it predates the USB-C era and it shows — but for phones and older devices it is fine. Recharge time: 3h20m with a 30W charger. Pass-through charging: not supported. At 198g, it genuinely disappears in a jacket pocket. Its limitation is output speed: 12W means an iPhone 15 Pro gets to roughly 50% in 35 minutes. For modern devices with USB-C PD, this is a meaningful constraint. If you are using anything from 2022 onward, look at the Zendure instead.
Zendure SuperMini (10,000mAh, $50) — Delivered 7,180mAh. The headline here is 20W USB-C PD in a genuinely pocket-sized chassis at 180g. An iPhone 15 Pro reached 60% in 30 minutes in our tests — the best charging speed per gram in this class. The pass-through charging worked cleanly in our tests: phone and bank charged simultaneously from a single 30W adapter without negotiation issues. Two ports (USB-C + USB-A), 20W shared. For day hikes, urban travel, or any scenario where you need reliable power without carrying weight, this is the bank we reached for most often across the test period. Our guide for CPAP users covers whether banks this size can handle medical devices.
Mid-Range Class: 15,000–20,000mAh
These are the most common travel power banks — the sweet spot of capacity and portability. Most airline-friendly at under 100Wh. The competition here is genuinely fierce, and the value-per-dollar case is strong.
Baseus 65W (20,000mAh, $55) — Delivered 14,200mAh at 5V. At this price, it is the obvious value pick in this class. The 65W USB-C PD output is enough to charge a 15-inch MacBook Pro while simultaneously charging a phone and a camera battery. The real-time wattage display is one of those features that sounds gimmicky and turns out to be genuinely useful in the field — you can see exactly how many watts are flowing and whether your camera is actually drawing power. Recharge: 2h45m with a 65W GaN charger. The downside is weight: at 365g, it is noticeable in a jacket pocket and belongs in a bag. Build quality is utilitarian but the performance data is hard to argue with at $55. The BioLite Charge 80 has an IP67 rating and direct solar charging capability that the Baseus lacks, but it also costs three times as much.
Xiaomi 50W Power Bank (20,000mAh, $40) — Delivered 13,900mAh. Aggressively priced and the raw numbers are competent. The problem is thermal throttling: we measured output dropping from 50W to 28W after 15 minutes of sustained laptop charging in testing. The bank was warm to the touch at 28W output, and the throttling kicked in predictably. For quick top-up charges — 20 minutes here, 30 minutes there — it is fine. For anything sustained, the Baseus is more reliable. At $40 on sale it is an acceptable budget option. At $40 retail, the Baseus at $55 is worth the $15 difference.
Field Class: 20,000mAh and Above
At this capacity, power banks start approaching the 100Wh FAA limit for airline carry-on. They are no longer pocket devices — they are field infrastructure. The banks in this category are bought by people who spend multiple days away from grid power and need to keep cameras, laptops, drones, or communication equipment running.
Goal Zero Sherpa 100PD (25,600mAh / 94.7Wh, $200) — Delivered 21,800mAh at 5V. This is the professional field standard, and the testing bore that out. The 100W USB-C PD output handled a 15-inch MacBook Pro at full speed without throttling across a full 8-hour test day. Two USB-C and two USB-A ports with independent smart chip management means no port contention — each device draws what it needs. The 15W wireless charging pad on top of the unit adds genuine utility for phone-only workflows when you want to minimize cables. At 635g, it is not light — but for photographers or filmmakers running a laptop and a camera body simultaneously, it replaces three smaller banks and does it more efficiently. Recharge time from empty: 4h30m with the included 45W wall charger. If you are considering this level of capacity, it is worth also looking at whether a small solar panel might be more practical than carrying multiple large banks for your specific use case.
Nitecore NPS200 (19,200mAh / 70Wh, $130) — Delivered 16,200mAh. The most airline-friendly option in the field class — at 70Wh it is well under the 100Wh limit on every major carrier. The touchscreen display showing input/output wattage and remaining charge time is the most informative interface we tested. Build quality is excellent and the tripod-mount compatible design is genuinely thoughtful for fixed-position field use. One USB-C at 60W PD and two USB-A at 18W combined. At 560g, it is lighter than the Sherpa and cheaper. The compromise is capacity: 16,200mAh real output versus the Sherpa's 21,800mAh. For a 3–4 day trip with a phone and mirrorless camera, it is the right choice. For a week-long shoot with a laptop, the Sherpa makes more sense.
BioLite Charge 80 (25,600mAh, $150) — Delivered 22,100mAh. The IP67 rating is not marketing — we submerged it in a shallow river for 30 minutes as part of the field durability test and it came out functioning. The 60W USB-C PD output handles mirrorless cameras and tablets. The standout practical feature is the ability to charge directly from a solar panel with an integrated solar input port — making it more useful than the Sherpa for multi-day backcountry use where grid power is not coming. At 680g it is comparable to the Sherpa. If your field use involves water, dust, or genuinely remote locations without access to a wall outlet, this is the bank to take. Our off-grid solar charger guide covers compatible panels for direct solar charging.
Air Travel: What You Can Actually Bring
FAA and EASA regulations cap lithium batteries at 100Wh for carry-on without airline approval. Most 20,000mAh power banks calculate to approximately 74Wh (20,000mAh × 3.7V), which is below the 100Wh limit on every major US and European carrier. You do not need to declare these. You can carry them in the cabin. Leave them out of checked baggage on every airline — always.
The Goal Zero Sherpa 100PD at 94.7Wh is the one to know about. It sits in a regulatory gray zone: below 100Wh, so it is generally permitted in cabin baggage without pre-approval, but close enough to the limit that some airlines require confirmation. We flew it on United and Delta without issue — both carriers have explicit 100Wh limits that the Sherpa fits under. American Airlines allows up to 100Wh without pre-approval as well. Southwest's policy is less explicit online; we recommend calling ahead if you are flying Southwest with this unit. Some international carriers, particularly in the Middle East and Asia, have additional restrictions on units above 80Wh. Check with your carrier directly before booking if you are traveling internationally with a bank this large.
The practical rule: any bank 20,000mAh or below is universally fine in cabin baggage on major carriers. Above 20,000mAh, check the watt-hour calculation and confirm with your airline before you pack it.
How We Tested: Equipment and Standards
- USB power meter: FNIRSI FNB58, calibrated at 5V/3A against a Keysight 34461A multimeter. Calibration verified before and after test series.
- Test loads: iPhone 15 Pro (4,441mAh battery), Sony A7IV with dummy battery, and a USB-C resistor load bank at 3A constant draw for capacity verification runs.
- Recharge testing: ANKER 65W GaN charger (A2147) as the consistent power source for all recharge tests. Included or manufacturer-recommended cables used for each bank.
- Temperature: Ambient test temperature 18–22°C for capacity tests. Separate cold-weather test at 2°C conducted for the Sherpa 100PD and BioLite Charge 80.
- All units purchased at retail in Q1 2026. No manufacturer samples. No pre-release units. No vendor influence on results or publication.
Recommendations
Best daily carry (any trip under 3 days): Zendure SuperMini — small, fast, 20W PD, 180g. It is the one you will actually remember to bring because you do not notice it in your pocket.
Best value for photographers: Baseus 65W 20,000mAh — the 65W output runs a camera body and phone simultaneously, the wattage display removes guesswork, and the price is $55. The ranked photography guide has additional options at this wattage level.
Best for multi-day field work with a laptop: Goal Zero Sherpa 100PD — expensive, heavy, and worth it for anyone spending extended time away from power. The 100W PD output and 21,800mAh real capacity are in a different class from the mid-range banks.
Best for wet or dusty field conditions: BioLite Charge 80 — the IP67 rating survived our river submersion test. The solar input port makes it the right choice for backcountry use where wall power is not available.
Best airline-friendly field capacity: Nitecore NPS200 — 70Wh, under every airline's limit without pre-approval, and the touchscreen interface is the most informative of any bank we tested.
References
- Federal Aviation Administration. "Portable Electronic Device Batteries." FAA Advisory Circular, 2025. faa.gov/hazmat/packsafe
- European Union Aviation Safety Agency. "Lithium Battery Requirements." EASA Opinion 2024/05.
- Battery University. "BU-808: How to Prolong Lithium-Based Batteries." Battery University, 2025. batteryuniversity.com