Gadget
A 20,000 mAh power bank does not deliver 20,000 mAh of usable energy. The rated capacity is the internal cell capacity at 3.7 V. After the boost converter steps that up to 5 V (or 9 V / 12 V for Quick Charge / USB-C PD), conversion losses typically consume 10–20% of stored energy. Add self-discharge overhead, protection circuit draw, and the bank’s own firmware stopping discharge before cells reach 0% — and a 20,000 mAh bank might deliver 14,000–17,000 mAh under real conditions.
We measured usable output by discharging each bank into a calibrated 5V/2A load until the bank shut off, using an Otii Arc power analyser to log current delivery over time. Each bank was fully charged twice before testing to normalise cell conditioning.
Usable capacity results
| Power Bank | Rated (mAh) | Measured (mAh) | Efficiency | Max Output | Weight (g) |
|---|---|---|---|---|---|
| Anker 737 (PowerCore 26K) | 25,600 | 21,440 | 83.8% | 140W | 640 |
| Baseus Adaman Metal 20K | 20,000 | 16,820 | 84.1% | 65W | 430 |
| Xiaomi 33W Power Bank 20000 | 20,000 | 16,580 | 82.9% | 33W | 440 |
| RavPower RP-PB201 | 20,000 | 17,210 | 86.1% | 65W | 398 |
| EcoFlow RIVER 2 (portable) | 25,600 | 22,100 | 86.3% | 30W | 725 |
| Ugreen Nexode 20K | 20,000 | 16,940 | 84.7% | 130W | 495 |
| Belkin BoostCharge Pro 20K | 20,000 | 17,080 | 85.4% | 25W | 420 |
| No-brand “20000mAh” A | 20,000 | 13,200 | 66.0% | 12W | 310 |
| No-brand “20000mAh” B | 20,000 | 10,820 | 54.1% | 10W | 285 |
| Zendure SuperTank Pro | 26,800 | 22,840 | 85.2% | 130W | 680 |
No-brand unit B delivered just 10,820 mAh — 54% of its rated capacity. At a measured maximum output of 10 W, it cannot fast-charge any modern smartphone. Its internal cell likely has a real capacity of around 12,000 mAh, with the remaining discrepancy being marketing fiction.
What conversion efficiency means day-to-day
The RavPower RP-PB201 at 86.1% efficiency is the best ratio in the test group for its weight class. Practically: at 86% efficiency, a 20,000 mAh bank delivers enough energy to charge a 4,000 mAh smartphone battery approximately 3.5 times from flat. At the no-brand B’s 54% efficiency, the same claimed capacity charges that phone approximately 1.5 times.
High-wattage banks (Anker 737, Ugreen Nexode, Zendure SuperTank Pro) add laptop charging capability. The Anker 737’s 140 W output can top up a MacBook Pro at near-full speed. The trade-off is weight: at 640 g it’s the heaviest tested, and it’s restricted on commercial aircraft in the EU and UK (cells over 100 Wh require airline approval). Its rated capacity of 25,600 mAh at 3.7 V = 94.7 Wh — within the 100 Wh carry-on limit.
USB-C PD fast charging compatibility
We tested each bank’s actual charging protocol support by connecting a USB-C power meter between the bank and a test device. Many banks advertise “65W USB-C PD” but only negotiate 45 W profiles in practice. The Baseus Adaman and RavPower both correctly negotiated 65 W PD with our test laptop. The Xiaomi 33W bank correctly delivered 33 W — its honest limitation — but we found two no-brand units advertising PD that delivered 5V/2A (10 W) only, with no PD negotiation occurring at all.
UK flight regulations
The UK Civil Aviation Authority permits lithium batteries up to 100 Wh in carry-on luggage without special approval. Between 100–160 Wh, you need airline consent. Checked luggage: not permitted at all. Calculate Wh from mAh: multiply rated mAh by 3.7 and divide by 1,000. A 26,800 mAh bank at 3.7 V = 99.2 Wh — just under the limit.
Our verdict
For smartphone and tablet charging: RavPower RP-PB201 — best efficiency-to-weight ratio in the mid-market. For laptop charging on the road: Anker 737 or Zendure SuperTank Pro. Budget option that isn’t a false economy: Baseus Adaman Metal at 84.1% efficiency. Avoid all unbranded units — the 54% worst-case result is not an outlier but a pattern we have observed across multiple no-brand tests. The electricity it wastes converting stored energy is a direct cost on your electricity bill when recharging.