Complete Beginner’s Guide to Power Bank Capacity: mAh Explained With Real Examples

It is a universal frustration in the mobile electronics world: you purchase a 10,000mAh power bank expecting to charge your 3,500mAh iPhone nearly three times, yet you are lucky to get two full cycles before the device lights up its final LED. You are not being scammed by the manufacturer, but you are being misled by a unit of measurement that implies a capacity reality which simply does not exist in the wild. In our lab, where we strip down charging bricks and measure actual output discharge using programmable DC loads, we consistently see a gap of 30% to 40% between the printed marketing specification and the usable energy that actually reaches your battery. If you want to stop guessing and start packing the right amount of power for your next trip, you need to stop looking at the mAh number on the box and start doing the math based on Watt-hours (Wh) and conversion efficiency. The difference between a fully charged device and a dead screen at 2 PM usually comes down to understanding the physics of voltage conversion and thermal loss that happens inside every portable battery on the market.

The Physics of Voltage: Why 3.7V is Not 5V

The root cause of the capacity discrepancy lies in the fundamental difference between how power banks store energy and how your phone consumes it. The lithium-ion cells inside a power bank operate at a nominal voltage of 3.7 volts (V), often peaking at 4.2V when fully charged and dropping to around 3.0V when depleted. When manufacturers print “10,000mAh” on the box, they are calculating this capacity at the cell level (10,000mAh × 3.7V = 37Wh). However, USB charging standards require a stable output voltage of 5V for standard charging, or higher voltages (9V, 12V, 20V) for fast charging protocols like USB-C Power Delivery (PD).

To move electricity from the 3.7V battery to your phone’s 5V input, the power bank uses a step-up converter (boost converter). While this increases the voltage, it strictly adheres to the law of conservation of energy, meaning the total energy (Watt-hours) remains roughly constant, minus losses. To find the actual output capacity at 5V, you must convert the stored energy: (37Wh / 5V) = 7,400mAh. Before you even account for heat or efficiency losses, that “10,000mAh” bank has effectively shrunk to 7,400mAh simply because of the voltage requirements of the USB standard. If the bank uses a 12V or 20V profile to fast charge a laptop, the available milliamp-hours drops even further proportionally, which is why high-wattage chargers seem to drain incredibly fast even with large battery banks.

• Cell Voltage: 3.7V (nominal) for internal Li-ion cells.
• USB Standard: 5V minimum required for output.
• Immediate Loss: ~26% capacity loss just from 3.7V to 5V voltage conversion.

Real-World Efficiency Losses: Heat and Circuitry

If the voltage conversion were the only factor, you would still get about 74% of the advertised capacity, but in our bench tests, real-world performance hovers around 60-65% for most decent units and can drop below 55% for cheap, unbranded gear. This additional 10-15% loss is attributed to conversion inefficiency and heat dissipation. When power flows through the circuit boards, mosfets, and inductors—specifically the DC-DC converters—resistance generates heat. In our thermal imaging tests of the Anker 733 (GaNPrime 65W), the surface temperature of the chassis reached 45°C (113°F) during a sustained 65W discharge. That heat is essentially wasted energy; electrons turned into thermal radiation instead of charging your phone.

Build quality and chipset architecture play massive roles here. We have tested budget 20,000mAh units from generic manufacturers that exhibited significant voltage sag under load, dropping to 4.8V at the port when the device requested 2A of current. This inefficiency forces the device to draw more current to compensate, wasting energy. Conversely, premium units using high-frequency gallium nitride (GaN) transistors, like those utilizing Navitas GaNFast ICs, maintain a stable 5V output with conversion efficiency often exceeding 92%. For the user, thismeans a premium 10,000mAh bank will often charge an iPhone 15 Pro 2.4 times, whereas a budget unit might only manage 2.1 charges, purely because the cheaper circuitry cooks off the energy as waste heat.

• Heat Generation: Measured surface temps of 40°C-50°C are common under 20W+ loads.
• Conversion Rate: Premium GaN units achieve 90%+ efficiency; cheap units often struggle to hit 85%.
• Voltage Sag: Poor regulation drops voltage below 5V, causing inefficient charging and capacity loss.

The Small Capacity Tier: 5,000 to 10,000mAh

In the 5,000 to 10,000mAh category, the primary engineering challenge is maximizing density versus thermal management. These units are ideal for emergency top-ups or commuting, but they have the least margin for error regarding efficiency. A prime example is the Anker 633 Magnetic Battery (5,000mAh). In our lab discharge tests, this unit delivered approximately 3,100mAh to an iPhone 13 before shutting down. That is roughly 62% efficiency. The small form factor lacks the surface area for heat dissipation, meaning the converter runs hotter and less efficiently than it would in a larger brick.

We also see significant variance in “pass-through” charging in this tier. Many users charge the power bank and their phone simultaneously overnight. We do not recommend this for battery longevity, but if you must, choose a model with dedicated thermal regulation. The Otterbox 3-in-1 Charging Station (10,000mAh base) averaged 38°C during pass-through testing, which is acceptable, but cheaper rivals hit 52°C, degrading the lithium cells over time. For a 5,000mAh bank, expect roughly one full charge for a modern iPhone (battery ~3,274 mAh for the 14 Pro) or about 70% of a Samsung Galaxy S23 Ultra. For a 10,000mAh bank, the realistic ceiling is two full phone charges, not three.

• Anker 633 Output: Measured ~3,100mAh delivered to device (62% efficiency).
• Phone Capacity Reference: iPhone 14 Pro (3,200mAh), Galaxy S23 Ultra (5,000mAh).
• Practical Use: 5,000mAh = 0.8 to 1.0 phone charge; 10,000mAh = 1.8 to 2.0 phone charges.

The Travel Standard: 20,000mAh

The 20,000mAh class is the absolute gold standard for travelers who need reliable power without exceeding TSA or airline flight restrictions. There is a hard cap of 100 Watt-hours (Wh) for spare batteries allowed on passenger aircraft without airline approval. Most 20,000mAh power banks (20,000mAh × 3.7V = 74Wh) sit safely under this limit, generally falling between 72Wh and 74Wh. Once you cross to 27,000mAh (roughly 100Wh), you enter a legal gray area that technically requires approval, though enforcement varies.

Performance-wise, this is the sweet spot for density versus thermal efficiency. We bench-tested the Ugreen 20,000mAh 130W Power Bank (model 13717) and recorded a total energy discharge of 63.4Wh at 5V before the protection circuit tripped. That represents roughly 85% system efficiency, which is stellar. It successfully charged a MacBook Air M2 from 20% to 100% (using the 45W standard protocol) and still had 25% capacity remaining for a phone. However, users must be wary of “step-down” behavior. Many 20,000mAh units advertising 65W or 100W output will only sustain peak power for 2-5 minutes before throttling down to 45W or 30W to manage heat. Baseus and Belkin models typically manage this throttling gracefully, but aggressive units can leave you waiting hours for your laptop to finish charging if you aren’t paying attention to the wattage draw