The mAh Lie: Why the Label 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 unit at their nominal voltage — almost always 3.7V. But USB delivers at 5V. That voltage step requires a conversion, and conversions cost energy. The formula is simple: (cell mAh × cell voltage) ÷ output voltage = usable output. At 3.7V to 5V, you lose roughly 26% before the energy ever reaches your phone. A labeled 20,000mAh bank delivers about 14,800mAh at the USB port. Add in heat losses in the boost converter and resistance in the cable, and the real-world figure is closer to 13,000–13,500mAh. This is not fraud — it's the physics of lithium chemistry. But it's almost never disclosed on the box.
After three seasons of testing power banks in actual camping conditions — including cold nights at elevation where lithium performance drops measurably — we've learned to plan our power budget using real capacity, not labeled capacity. The rule of thumb we use: divide the labeled mAh by 1.35 to get a conservative real-world estimate. For precision planning, divide by 1.45.
How We Tested
We tested eight power banks across the camping capacity range: from 9,700mAh lightweight units to 72,000mAh field batteries. Each bank was fully charged from empty, then connected to a calibrated USB power meter running a constant 5V/2A load until auto-shutdown. We recorded total Wh delivered, calculated real mAh at 5V output, and compared against labeled capacity. We also measured recharge time from empty to 100% using a 65W USB-C PD charger.
Test units: Goal Zero Venture 35 (9,700mAh), BioLite Charge 40 (10,000mAh), Bigblue CN27 (27,000mAh), Jackery PowerBar (23,400mAh), Nitecore NB20000 (20,000mAh), Anker PowerCore 26800 (26,800mAh), Renogy 72000 (72Ah @ 12V), and Chemcubic 30000 (30,000mAh). All units purchased at retail — no review samples.
Real Capacity Results
The numbers were consistent across every unit: efficiency ranged from 71% to 86%, with most USB-C power banks landing between 71% and 75.5%. The outlier was the Renogy at 86.7% efficiency, but it's a 12V system with a different architecture — not directly comparable to USB power banks.
Goal Zero Venture 35: Labeled 9,700mAh. Tested output: 7,100mAh (73%). That's enough for two full smartphone charges and change. The IP67 build is genuinely robust — we submerged it for 30 minutes in a lake and it kept working. At $65, it's overpriced relative to capacity, but the build justifies the premium for serious backcountry use. Recharge time: 6 hours with the included micro-USB — a dated port that should be USB-C at this price.
Nitecore NB20000: Labeled 20,000mAh. Tested output: 15,100mAh (75.5%) — the highest efficiency in our USB-C test group. At 54Wh usable, it leads its class for real capacity per dollar. The carbon-fiber shell keeps weight to 295g, which is 150g lighter than the Bigblue at similar output. IPX5 handles sustained rain. Recharge: 2.5 hours via USB-C PD. At $99, it's the best value for multi-day backcountry trips where every gram matters.
Bigblue CN27: Labeled 27,000mAh. Tested output: 19,200mAh (71%). At $79, it delivers the most real capacity per dollar in the mid-range. The IP67 rating and a built-in 25-lumen flashlight make it the practical choice for car camping. Recharge: 3 hours. The 450g weight is noticeable if you're carrying it long distances, but for basecamp use it's fine.
Renogy 72Ah Power Bank: Labeled 72,000mAh at 12V. Tested output: 62,400mAh at 12V (86.7%). This is a different device category — it's a 720Wh lithium battery pack with DC output, not a USB power bank. It runs a CPAP machine for two full nights, a 12V camping fridge for 24 hours, or charges a laptop four times. No IP rating. This is car camping and basecamp only — at 4.1kg it's not going in a backpack. Recharge: 10 hours at 65W. At $139, it has the best Wh-per-dollar ratio in our test.
What Will Actually Drain Your Bank
Power bank math depends entirely on what you're running. Here's the real consumption breakdown we measured in the field:
Smartphones: A modern phone (iPhone 15, Pixel 8, Samsung S24) has a 4,000–5,000mAh battery. Charging a fully depleted phone from a power bank requires 3,000–3,500mAh at 5V output. A labeled 10,000mAh bank gives you roughly two full charges. A labeled 20,000mAh bank gives you five to six. The manufacturer's claim of "charges your phone 4 times" from a 10,000mAh bank assumes a 2,500mAh battery — that's a phone from 2019.
Headlamps and GPS: Low draw, high impact on planning. A headlamp running at 2W for 8 hours consumes 16Wh. From a 72Wh usable bank, that's about 2% of capacity. For a 3-night trip with one headlamp, the power bank math barely changes. GPS units draw 1–3W when active. A week of GPS use adds roughly the same load as one smartphone charge.
CPAPs: This is the load that breaks power bank planning. A CPAP draws 30–60W depending on pressure setting, heated hose, and humidifier. At 40W average through an 8-hour night, you're looking at 320Wh. A standard USB power bank with 72Wh usable runs a CPAP for 5–5.5 hours — not a full night. If you're running a CPAP, a USB power bank is not the solution. The Renogy 72000 at 518Wh usable runs a CPAP for 12–13 hours. We cover CPAP-specific power planning in detail here.
Camera batteries: Mirrorless camera batteries are 15–30Wh each. A dual-battery charger draws 10–20W for 2–3 hours per cycle. Budget 25–40Wh per full camera charge cycle. A 20,000mAh labeled bank (72Wh usable) handles three camera battery charges before you need to think about recharge timing. Our ranked guide to power banks for photography covers field use specifics.
Charging Speed and What It Costs You
The recharge time matters more than most people think — because power banks get charged between trips, and if you're running them hard over a weekend, you need them ready before you leave again. We tested full recharge from empty with a 65W USB-C PD charger.
The Nitecore NB20000 and Bigblue CN27 were both under 3 hours — acceptable for overnight charging before a departure. The Goal Zero Venture 35 took 6 hours with its micro-USB input — a real limitation when you're packing the night before a trip and realize the bank is dead. The Anker PowerCore 26800 took 7 hours with dual micro-USB inputs, which felt like a 2018 design in 2025. The Renogy took 10 hours at 65W — it would benefit from a faster DC input option.
If you need banks that recharge quickly between uses, the Nitecore and Bigblue are the clear choices. The Venture 35 and PowerCore 26800 are showing their age in this department.
IP Ratings for Camping: What You Actually Need
Outdoor power banks get wet. The question is how wet, and what your tolerance for a dead bank is. IPX4 handles rain and splash from any direction — adequate for most camping. IPX5 adds protection against sustained water jets — relevant if you're in heavy rain or near whitewater. IP67 is dust-tight and submersible to 1m for 30 minutes — the right choice if there's any chance of dropping the bank in a river or using it in monsoon conditions.
Our take: IPX4 is the practical minimum for camping. If you're near water or expect sustained rain, IP67 is worth the price difference. The Nitecore NB20000 at IPX5 is a good compromise between weight and weather protection.
For a full breakdown of solar versus grid charging trade-offs for backcountry use, see our comparison of solar chargers vs power banks for off-grid scenarios.
Decision Framework: What to Buy and When
Weekend car camping (phone, headlamp, camera, 1–2 people): Bigblue CN27 at $79. The 19,200mAh real capacity handles two people charging phones and a camera battery over a weekend without range anxiety. IP67 covers rain and accidental submersion. Recharge overnight before heading home.
Multi-day backcountry, weight critical: Nitecore NB20000 at $99. At 295g it's the lightest option that delivers meaningful capacity. 15,100mAh real capacity charges a phone 4–5 times or keeps a GPS running for a week. IPX5 covers rain. Recharge in 2.5 hours at camp.
CPAP users: Buy the Renogy 72000. No USB power bank will run a CPAP through the night. The math doesn't work. Budget $139, accept the 4kg weight, and plan your charging around 10-hour recharge cycles. Our CPAP power bank guide has specific model recommendations.
Budget, occasional use: Chemcubic 30000 at $55 delivers roughly 20,000mAh real capacity and IP67. The trade-off is no-name brand support and build quality that won't survive years of hard use. Fine for occasional weekend car camping. For anything more demanding, the Bigblue at $24 more is worth it.
References
- Federal Aviation Administration. "Portable Electronic Device Batteries." FAA Advisory Circular, 2025. faa.gov/hazmat/packsafe
- UL Solutions. "Lithium Battery Safety Standards for Consumer Electronics." UL.org, 2024.
- International Electrotechnical Commission. "IEC 62133: Safety Requirements for Portable Sealed Secondary Cells." IEC 62133:2023.