The Capacity Math: What mAh Actually Means
Battery capacity is measured in milliamp-hours (mAh) or watt-hours (Wh). mAh alone is not 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 the same mAh rating but different battery chemistries or internal architectures can have meaningfully different actual capacity.
Real capacity vs. rated capacity: no lithium battery pack delivers 100% of its rated capacity. Expect 80–90% efficiency in the conversion from stored energy to usable device charge. A 10,000mAh power bank delivers approximately 7,500–8,500mAh to your devices after conversion losses and voltage transformation.
Charging efficiency also varies by output standard. USB-C Power Delivery (PD) is more efficient than standard USB-A because it negotiates voltage and current precisely. Charging a phone from 0% to 50% on a PD power bank takes less of the power bank's capacity than charging the same phone on a standard 5V/1A USB-A output.
The practical numbers: a 5,000mAh power bank will fully charge most modern phones once (with some loss). A 10,000mAh bank will fully charge most phones 1.5–2 times. A 20,000mAh bank will charge a phone 3–4 times and can charge a tablet at least once. For a laptop (50–80Wh battery), you need at least a 20,000mAh power bank to deliver one full charge, and many laptops with USB-C charging accept 60–100W input, which requires a power bank that supports 60W+ output.
USB Standards: What Your Devices Actually Need
USB-A — The legacy port. 5V, up to 2.4A (12W max) using the USB Battery Charging 1.2 standard. Charges most phones at their maximum rate if the phone supports it. Does not support modern fast-charging protocols. Most battery packs include USB-A for compatibility with older cables and devices.
USB-C Power Delivery (PD) — The modern standard. Negotiates voltage and current between the 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 (QC) and PPS — Qualcomm's Quick Charge is a proprietary protocol separate from USB PD. Many Android phones support QC4+ or QC5 alongside PD. PPS (Programmable Power Supply) is the newer standard that allows finer voltage/current negotiation, used in Samsung Super Fast Charging and some other implementations. A power bank that supports PD3.0 and PPS covers the majority of fast-charging protocols.
MagSafe and Qi2 — Wireless charging standards for iPhone (MagSafe) and compatible Android phones (Qi2). Maximum wireless charging power is 15W for both standards. Wireless charging is less efficient than wired (10–20% loss) and generates more heat. Useful for convenience but not for efficient power bank use.
Input Rate: How Fast You Can Recharge the Bank
Power banks are charged through their own input port, and the speed at which they recharge affects how useful they are in practice. A power bank with a 5W input recharge that takes 10 hours to refill is a much less useful product than one that refills in 3 hours at 30W input.
Look for input specs: 30W or higher USB-C PD input is the standard for modern power banks. Some support dual-input (USB-C + micro-USB) but USB-C is the standard now. A power bank that takes more than 6 hours to fully recharge from empty is not well-suited for travel use unless it's left charging overnight.
Pass-through charging (charging the power bank while it simultaneously charges connected devices) is a feature some power banks offer. The practical limitation: total output is shared between the pass-through device and the bank itself, so both charge more slowly. Not a major use case, but useful as a bedside charger configuration.
Form Factor and Practical Use Cases
Pocket-sized (5,000–10,000mAh) — These are the most portable. A 10,000mAh pack is approximately the size of a phone and weighs 200–300g. Appropriate for daily commute, a full day away from power, or an emergency backup. Won't charge a laptop. Good for phones and tablets at partial capacity.
Mid-size (10,000–20,000mAh) — The mainstream travel power bank size. 20,000mAh is the maximum capacity you can typically carry on a plane (check airline rules — most limit to 100Wh per battery, which is approximately 27,000mAh at 3.7V). These are too heavy for pockets but fit in a bag. Can charge most phones 2–3 times, charge a tablet once, and some support laptop charging with 45–65W output.
Large capacity (20,000–30,000mAh) — For extended travel, international trips without reliable power access, or powering multiple devices. At 20,000mAh+ and 400g+, these are not pocketable. Most can charge a laptop once to fully. Look for 65W+ output for laptop charging to be practical — anything lower will charge very slowly.
What to Buy
Best overall for most people
For a 10,000–15,000mAh PD power bank with 30W output and USB-C PD input: the Anker 533 (10,000mAh, 30W PD) or the Belkin 10K (similar spec). Both are $50–60, compact, reliable, and charge most phones and tablets quickly. The Anker has a built-in USB-C cable which reduces cable carrying requirements.
Best for laptop charging
For a laptop-capable power bank: the Anker 737 (24,000mAh, 140W PD) or the EcoFlow River 2 Max (20,000mAh, 60W PD). The 140W unit can charge most USB-C laptops at full speed; the 60W unit will charge laptops slowly while in use but works. The EcoFlow unit also has a built-in AC inverter option for non-USB-C devices.
Best budget option
For under $30: the Monopser 10,000mAh (QC3.0, 18W output, USB-A + USB-C) is reliable and compact. Not PD, but charges most phones at close to their maximum rate. At this price point, accept that the capacity claim is optimistic and plan accordingly.
What to Avoid
Power banks with misleading specs are common. Red flags:
Claims of "50,000mAh" in a pocket-sized unit at a low price: impossible. Large capacity batteries are physically large and heavy. If it's small and cheap and claims huge capacity, the capacity is either fictional or the cells are low-quality with rapid degradation.
"Maximum output" listed as 100W+ without specifying PD standards: probably means the combined output of all ports, not per-port. If you need 65W for a laptop, the power bank needs to specify 65W PD output per port.
No brand name or unknown brand with no reviews: lithium battery safety matters. A cheap power bank with poor battery management (overcharge protection, thermal protection, short-circuit protection) is a fire risk. Stick to Anker, Belkin, Mophie, Samsung, or well-reviewed alternatives with explicit safety certifications (UL, CE, FCC).
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 or a long international flight. Anything beyond that is for specialized use cases. The most common buying mistake is buying too large a power bank and then leaving it at home because it's too heavy to carry.
For output standard: USB-C PD is the minimum you should accept on any new purchase. USB-A is legacy and disappearing. A power bank without USB-C PD is not future-proof. And always verify that the power bank you're buying actually supports the output wattage your laptop or tablet needs before purchasing.