The short answer
A 20,000 mAh power bank holds about 74 Wh, and Anker says you get roughly 65% of that out in practice, which is 48.1 Wh. That is just under one full charge of a 13-inch MacBook Air (52.6 Wh), about two thirds of a 14-inch MacBook Pro, and under half of a 16-inch Pro.
For most laptops a 20,000 mAh bank gives less than one full charge. A 10,000 mAh bank gives about a third to a half of one charge, and a 50,000 mAh bank about two charges of a MacBook Air.
From mAh to Wh
Power banks are sold in milliamp-hours, laptops are rated in watt-hours, and you cannot compare the two without the battery voltage. Anker gives the formula: Wh = (mAh x V) / 1000. It says common voltages for lithium-ion batteries are 3.6V to 3.7V. At 3.7 V it works out a 20,000 mAh bank at 74 Wh. At 3.6 V the same bank is 72 Wh, so the choice of voltage moves the answer by about 3%.
The label is the better source when it has one. Anker's own 26,250 mAh Prime power bank is listed at 99.75 Wh, which means its cells are rated at 3.8 V, not 3.7. If the pack prints a Wh figure, use that and skip the conversion. If it only prints mAh, 3.7 V is a fair assumption.
Why you do not get all of it
Charging a laptop from a power bank loses energy in the cells and the electronics. Anker's support text says this results in a 30% to 45% capacity reduction. For a fully charged 25,000 mAh bank, Anker says it typically delivers about 13,750 to 17,500 mAh. Its formula for the number of charges multiplies the bank's capacity by 0.65, then divides by the device battery.
We use 0.65 in the tables below because it is the only usable-share figure we found stated by a manufacturer. It is Anker's number for Anker banks, so treat it as a typical value and not a promise for every brand. Anker's own range of 13,750 to 17,500 mAh from a 25,000 mAh bank means a usable share of 55% to 70%. On a 20,000 mAh bank that is anywhere from 40.7 Wh to 51.8 Wh.
Worked table: three bank sizes
All numbers below use 3.7 V and Anker's 0.65. The last four columns are full charges of a laptop from empty to full.
| Power bank | Label energy | Usable energy | MacBook Air 13" (52.6 Wh) | MacBook Air 15" (66.5 Wh) | MacBook Pro 14" (72.4 Wh) | MacBook Pro 16" (100 Wh) |
|---|---|---|---|---|---|---|
| 10,000 mAh | 37 Wh | 24.05 Wh | 0.46 | 0.36 | 0.33 | 0.24 |
| 20,000 mAh | 74 Wh | 48.1 Wh | 0.91 | 0.72 | 0.66 | 0.48 |
| 50,000 mAh | 185 Wh | 120.25 Wh | 2.29 | 1.81 | 1.66 | 1.20 |
The sums are usable energy divided by the laptop battery. For the 20,000 mAh bank and a 13-inch Air that is 48.1 / 52.6, or 0.91. At 3.6 V it would be 72 Wh x 0.65 = 46.8 Wh, and 0.89 charges.
Laptop batteries by class, from the makers
The laptop figures come from Apple's own tech specs. The 13-inch MacBook Air (M2, 2022) has a 52.6-watt-hour battery, the 15-inch Air (M2, 2023) has 66.5-watt-hour, the 14-inch MacBook Pro (M4, 2024) has 72.4-watt-hour, and the 16-inch Pro (2024) is listed at 100-watt-hour. Apple adds that the 16-inch's actual rating is 99.6 watt-hours.
We did not find a Windows laptop maker whose spec page we could read as plain text, so we quote none. For a Windows laptop, the number to use is the Wh figure on your laptop's battery label or specification, not our table.
What changes the answer
The table assumes you charge from empty to full with the laptop idle. In real use you rarely do. Charging from a half-full battery needs only half the energy, so the bank goes further. Working while it charges takes energy that never reaches the battery, and the charge count drops.
Output power matters too. Apple says to use a power adapter or display that provides at least the minimum wattage of the adapter recommended for your Mac. A bank that cannot output that much may charge slowly or only slow the drain. Check the bank's USB-C output in watts before you buy it for a laptop.
Flying with a big power bank
The FAA says lithium ion batteries are limited to a rating of 100 watt hours per battery. With airline approval, passengers may carry up to two spare larger batteries in the range of 101 to 160 Wh. By those numbers, a 20,000 mAh bank at 74 Wh is under the limit. A 30,000 mAh bank is not: 30,000 mAh x 3.7 V / 1,000 = 111 Wh, so it needs airline approval. A 50,000 mAh bank at 185 Wh is above 160 Wh. We have only quoted the US rule here, so check your airline and the UK or EU rules before you travel.
What this means in a power cut
If your plan is to keep a laptop alive through an outage, a 20,000 mAh bank is a top-up, not a day's power. Our guide to charging a laptop without mains power covers power stations, car charging and solar, and the laptop runtime tool turns watt-hours and your laptop's draw into hours. We are also building Waystone, an offline assistant for laptops that is still in development, so battery arithmetic matters to us too.
Common questions
How many times can a 20,000 mAh power bank charge a laptop?
Usually less than once. At 3.7 V the bank holds 74 Wh, and at Anker's 0.65 you get about 48.1 Wh out. That is 0.91 of a 13-inch MacBook Air battery and 0.48 of a 16-inch MacBook Pro.
Can a 10,000 mAh power bank charge a laptop?
Only partly. It holds 37 Wh, about 24.05 Wh after losses, which is 0.46 of a 13-inch MacBook Air charge. It is enough for a top-up, not a full charge.
Can a 50,000 mAh power bank charge a laptop fully?
Yes, more than once for a thin laptop: 2.29 charges of a 13-inch MacBook Air and 1.20 of a 16-inch Pro, by our sums. At 185 Wh it is also too large for the FAA's 160 Wh range for carry-on batteries.
How do I convert mAh to Wh?
Multiply the milliamp-hours by the voltage and divide by 1,000. Anker lists lithium-ion voltage as 3.6V to 3.7V, so a 20,000 mAh bank is 72 to 74 Wh. If the pack prints a Wh rating, use that.
How much of a power bank's capacity can I actually use?
Anker says energy loss during charging causes a 30% to 45% capacity reduction, and its formula uses 0.65. Other brands may differ, and we found no other manufacturer that states a figure.