How Are Power Banks Measured? | Clear Capacity Guide

Shopping for a new charger gets confusing fast. Ports promise speed and labels mix volts, amps, watts, and big mAh counts. This guide turns that noise into clear steps so you can compare models and pick the right size without paying for capacity you never use.

What Those Numbers Really Mean

Every pack lists three core ideas. Charge (mAh) is stored charge. Energy (Wh) is voltage × charge. Power (W) is how fast energy moves to your device. You will also see voltage (V) and current (A), tied by the relation W = V × A.

Label On Box	What It Measures	What It Tells You
mAh (milliamp-hours)	Stored charge in the internal cells at their nominal voltage	Rough size of the battery inside
Wh (watt-hours)	Total energy (voltage × charge)	Best number for apples-to-apples comparisons
V (volts)	Electrical potential	Nominal cell voltage is about 3.6–3.7 V; USB output is higher
A (amps)	Flow of current	Higher current means faster charging when the device agrees
W (watts)	Power delivered at an instant (V × A)	Speed of charging; 20 W beats 10 W
PD / PPS	Fast-charge protocols	Negotiates higher voltages and currents safely
Cycles	Charge-to-discharge life rating	How many full cycles before capacity fades

From mAh To Wh: The Quick Conversion

Many brands print only charge. Energy is what matters. Use this math to translate charge to energy: Wh = (mAh ÷ 1000) × V. When a pack lists 20,000 mAh and 3.7 V cells, energy is about 74 Wh. If only the USB output voltage is shown, stick with the cell voltage for a fair capacity comparison because the internal battery stores energy at that lower number.

You will also see travel rules written in those energy units. The FAA battery guidance explains the Wh calculation and uses it to set flying limits. That same math is the reason many packs print both mAh and Wh on the label.

Ways To Measure A Power Bank In Practice

Marketing numbers are one thing; measured output is another. Labs use controlled tests to rate cells and finished packs at set discharge rates, temperatures, and end-of-discharge voltages. Makers report that figure. At the USB port, results depend on conversion electronics, cable quality, and your device’s requests.

Bench Testing That Mirrors Real Use

To check performance at home, use a USB power meter and a steady load, or a phone app that logs energy. Run a few discharges at 5 V 1 A, then try a higher PD step such as 9 V 2 A. Good packs stay efficient across runs; weak ones sag or throttle.

Rated Capacity Versus Delivered Energy

Why the gap between a bold mAh figure and what reaches your phone? Cells store energy near 3.7 V. The pack boosts that to the higher USB voltage your device negotiates, and conversion wastes some energy as heat. Cables add small losses. Real-world efficiency lands near 70–90% depending on load and temperature.

Output Power And Fast-Charge Labels

Power banks also advertise how quickly they can charge laptops, tablets, and phones. Output power is the product of voltage and current after the protocol handshake. Modern USB-C models speak USB Power Delivery, which added new fixed steps at 28, 36, and 48 V to enable higher power tiers. The USB-IF PD page summarizes the expansion up to 240 W when paired with compliant 240 W cables and devices.

Reading Port Labels Without Guesswork

Look for clear markings like “USB-C: 5 V⎓3 A, 9 V⎓3 A, 15 V⎓3 A, 20 V⎓5 A (100 W)” or “EPR 48 V⎓5 A (240 W).” Multi-port packs divide total output across sockets, so a “100 W max” body may cap each port at lower levels when more than one device is plugged in. Good packaging includes a per-port table and a combined limit.

Capacity Versus Speed: Pick The Right Balance

Energy size and charge speed serve different needs. For all-day phone backup, a compact 10,000 mAh class pack is plenty. For tablets and handheld gaming, 15,000–20,000 mAh keeps screens lit. For 13-inch laptops, favor a strong 65–100 W port; energy above 60 Wh helps, but speed matters more.

How Cell Voltage Moves The Needle

Most packs use 3.6–3.7 V lithium-ion cells. Some larger models stack cells in series for 7.2 V class packs and use buck converters to deliver stable USB-C outputs. Energy stays the same either way; the layout just changes how the electronics move it.

Real-World Examples With Math

Turn the spec sheet into clear expectations. Use energy in Wh and a realistic efficiency figure to estimate charges, then check that the port speed matches your gadget’s needs.

Phone And Tablet Estimates

A typical modern phone holds around 11–14 Wh. A small tablet lands near 20–30 Wh. If your pack lists 74 Wh and runs near 85% efficiency, usable energy at the USB port is about 63 Wh. That yields four to five phone charges or two small-tablet charges, depending on screen-on use while charging.

Bank Energy (Wh)	Device Battery (Wh)	Estimated Full Charges
37	12	About 2
74	12	About 4–5
74	28	About 2
100	50	About 1 (with headroom)
140	99	About 1 (laptop top-off)

What Labels And Standards Add To The Story

Two markings help buyers compare across brands. The Wh figure, required on many batteries for transport, ties back to a clear energy number. Fast-charge logos and port print point to supported voltage steps and currents. Packs and cables that pass independent compliance programs reduce guesswork and often perform closer to their printed specs, especially at higher power tiers.

Travel Limits And Why Wh Matters

Air travel rules use Wh, not mAh. Under common policies, spare batteries and power packs up to 100 Wh usually travel in carry-on bags. Larger units up to 160 Wh often need airline approval first, and bigger packs are not accepted for passengers. Always check your route. The math and the limits are explained in the FAA page linked above, and international guidance follows the same Wh logic.

Standards That Govern Fast Charging

USB-C and Power Delivery set the language that devices use to request safe voltage and current. Newer versions add higher steps and tighter safety checks. When a device and a pack both speak the same version, the port negotiates the best match, from simple 5 V phones to high draw laptops. Certified cables rated for the needed current complete the chain. These steps include fixed voltages (5, 9, 15, 20, 28, 36, 48 V) and optional PPS modes that fine-tune voltage in small increments to reduce heat while keeping speed. The same documents also define basic safety checks, such as cable current ratings and fault handling, which keep negotiations stable when a device requests more power.

How Makers Rate Capacity Under Lab Conditions

Battery standards define the terms that appear in spec sheets. “Rated capacity” is the charge a cell can deliver over a set time window at a defined temperature when discharged to a specified end-of-discharge voltage. Vendors choose cells that meet the rating, then design the boost or buck converters to deliver steady USB-C output across common steps like 5 V, 9 V, 15 V, 20 V, and EPR voltages. They also validate thermal limits and shutdown behaviors so a hot summer bench test does not push the pack outside safe ranges.

Why Your Results Vary

Small gear rarely runs at lab conditions. Cold days trim capacity. Hot dashboards age cells faster. Gaming while charging raises demand and losses. Long, thin cables drop extra voltage. None of this means the pack is faulty; it just reflects how physics, safety limits, and device requests interact outside a test room.

Buying Smart: A Short Checklist

Match Energy To The Job

Phones: 10,000 mAh class for daily carry. Tablets and handheld consoles: 15,000–20,000 mAh class. Light laptops: energy near 60–74 Wh with a 65 W or 100 W port. Bigger laptops: look for higher power tiers and compliant 240 W cables when needed.

Check Port Speeds

Compare printed per-port limits to your device’s needs. A notebook that accepts 20 V at 5 A needs a pack and a cable that both claim that tier. If you see “shared 100 W,” expect lower per-port speed with two devices connected.

Look For Clear Energy Labels

Prefer boxes that list both mAh and Wh. That lets you do the quick math across brands and cell voltages and spot inflated marketing claims.

Mind Efficiency And Cables

Short, certified USB-C cables help maintain voltage and reduce waste. Expect a healthy pack to deliver about three-quarters to nine-tenths of its energy at the port in steady tests. Spike loads and heat move that number down.

The Bottom Line: Read Energy, Then Power

If you boil this topic down to two moves, here they are. First, compare energy in Wh using the simple formula near the top. Second, match the port wattage and PD steps to your device. Those two checks turn a noisy box into a clear choice—do those two checks once, and spec sheets stop feeling cryptic—the right pick becomes obvious and easy.