Capacity: The Math That Manufacturers Don't Show
Battery capacity is measured in milliamp-hours (mAh) or watt-hours (Wh). mAh alone isn't directly comparable across products because it doesn't account for voltage. A battery rated at 10,000mAh at 3.7V (nominal lithium cell voltage) stores 37Wh of energy. Two power banks with identical mAh ratings can have meaningfully different actual capacity if their battery architectures differ.
Real-world efficiency: no lithium battery pack delivers 100% of rated capacity. Expect 80–90% efficiency in converting stored energy to device charge. A 10,000mAh power bank delivers approximately 7,500–8,500mAh to your devices after conversion losses and voltage transformation. This is normal and expected — it's a consequence of how lithium-ion batteries and USB voltage conversion work.
Charging efficiency also varies by output standard. USB-C Power Delivery is more efficient than standard USB-A because it negotiates voltage and current precisely between the charger and device. Charging a phone from 0% to 50% on a PD power bank takes less of the bank's capacity than the same charge on a standard 5V/1A USB-A output.
For airlines: most carriers limit lithium batteries to 100Wh per pack without prior approval. 100Wh is approximately 27,000mAh at 3.7V. A 20,000mAh pack is universally accepted; a 26,800mAh pack is at the borderline; anything above 27,000mAh requires airline approval. Check before you fly.
USB Standards: What Your Devices Actually Speak
USB-A — legacy port, 5V up to 2.4A (12W) using USB Battery Charging 1.2. Charges most phones at their maximum rate if the phone supports it. Does not support modern fast-charging protocols. Included on most power banks for cable compatibility, but not the primary output to use.
USB-C Power Delivery (PD) — the modern standard. Negotiates voltage and current between charger and device, supporting 5V/3A (15W), 9V/3A (27W), 15V/3A (45W), and 20V/5A (100W). USB-C PD is what you need for charging laptops, tablets at full speed, and modern phones that support fast charging. The actual wattage delivered depends on what the device asks for — a phone that only accepts 15W won't draw 45W even if the power bank can supply it.
Quick Charge and PPS — Qualcomm's Quick Charge (QC4+/QC5) is a proprietary protocol separate from USB PD, used in many Android phones alongside PD. PPS (Programmable Power Supply) is the newer standard allowing finer voltage/current negotiation — used in Samsung Super Fast Charging and some other implementations. A power bank supporting PD3.0 and PPS covers the majority of fast-charging protocols in current phones.
MagSafe and Qi2 — wireless charging standards for iPhone (MagSafe) and Qi2-compatible Android phones. Maximum wireless charging is 15W for both. Wireless charging is 10–20% less efficient than wired and generates more heat. Convenient but not the most efficient way to use a power bank's capacity.
Matching Capacity to Your Use Case
Pocket-sized: 5,000–10,000mAh
Approximately the size of a modern smartphone, weighing 180–280g. Good for daily commute, a full day away from power, or an emergency backup. Won't charge a laptop. Will fully charge most phones once. The sweet spot for most daily users is 10,000mAh — it fits in a jacket pocket and provides meaningful backup without being heavy enough to notice.
Travel-sized: 10,000–20,000mAh
The mainstream travel power bank range. 20,000mAh is the practical maximum for carry-on airline luggage. These are too heavy for pockets but fit in a day bag. Expect 2–3 full phone charges, one tablet charge, and for higher-output models (45W+), partial laptop charging while in use. This is the right range for anyone traveling internationally or spending long days away from power.
High-capacity: 20,000–27,000mAh
For extended travel, field work, or multiple devices. At 400g+ these are not pocketable. Most can fully charge a USB-C laptop (50–80Wh battery) at least once. For laptop charging to be practical rather than just emergency measure, look for 65W+ output — anything lower will charge very slowly, often not keeping up with laptop power consumption during active use.
For more options, see our best portable power stations 2026 guide for larger battery systems that can fully power a laptop and multiple devices for multi-day trips.
Input Rate and Pass-Through Charging
A power bank's recharge speed matters more than most buyers expect. A power bank that takes 12 hours to refill from empty is significantly less useful than one that refills in 3 hours — yet input speed is rarely highlighted in product marketing.
Look for 30W or higher USB-C PD input as the modern standard. Some models support 45W or 65W input, refilling in 1.5–2 hours. A power bank that takes more than 6 hours to fully recharge is poorly suited for travel use unless it's primarily charged overnight.
Pass-through charging (charging the power bank while it simultaneously charges connected devices) is a feature on some models. The practical limitation: total power is shared, so both charge more slowly. Useful as a bedside charger configuration or when you have limited outlets, but not a primary reason to choose any particular model.
What to Buy in 2026
Best for most people: Anker 533 (10,000mAh, 30W PD, built-in USB-C cable) or Belkin 10K (similar spec). Both are compact, reliable, $50–60, and charge phones and tablets quickly. The built-in cable on the Anker reduces what you need to carry.
Best for laptop charging: Anker 737 (24,000mAh, 140W PD) — can charge most USB-C laptops at full speed and recharge in under 2 hours at 140W input. For a more portable option, EcoFlow River 2 Max (20,000mAh, 60W PD) works for laptops with lower power draw (ultrabooks, MacBook Air).
Best budget option: Monoprice 10,000mAh (QC3.0, 18W output, USB-A + USB-C) is reliable and compact for under $30. Not PD, but charges most phones near their maximum rate. Capacity claims at this price point tend to be optimistic — plan for ~7,500mAh actual output.
What to Avoid
Power banks with misleading specs are endemic to the market. Red flags:
"50,000mAh" in a pocket-sized unit at a low price — physically impossible. Large capacity batteries are large and heavy. If the size and price seem too good to be true, the capacity is either fictional or the cells are low-quality with rapid degradation. A genuine 20,000mAh lithium-polymer pack weighs 350–450g.
"Maximum output 100W+" without per-port PD spec — often means combined output of all ports, not per-port. If you need 65W for a laptop, the bank must specify 65W PD output per port.
No safety certifications — lithium battery safety requires overcharge protection, thermal protection, and short-circuit protection circuits. Cheap packs with poor battery management are a fire risk. Stick to established brands: Anker, Belkin, Mophie, Samsung, or well-reviewed alternatives with UL, CE, or FCC certification.
The Bottom Line
Match capacity to your actual need: 10,000mAh covers a full day away from power for most phones. 20,000mAh covers a weekend trip. Anything beyond that is for specialized use cases. The most common mistake is buying too large a power bank and leaving it at home because it's too heavy to carry.
USB-C PD is the minimum standard for any new purchase. USB-A is legacy and disappearing. A power bank without USB-C PD is not future-proof. And verify that any power bank you're buying actually supports the output wattage your laptop or tablet needs — particularly for laptops that charge at 45W or 65W via USB-C, not all "USB-C" power banks deliver that much per port.
Marcus Chen is an electronics engineer and maker with 15 years of experience in PCB design and repair. He writes about electronics tools and safety practices for GearPrism.