What Size Power Bank Do You Actually Need?
Dmitry Orlov, Editor-in-chief · phones, charging, networking · · 9 min read
For most phones, 10,000 mAh is enough for one to two full charges, and 20,000 mAh covers a full day of heavy use or two phones. For a laptop, match watt-hours (Wh) to your battery's Wh rating and check that the bank supports enough USB-C PD wattage, not just a big mAh number.
The short answer: match capacity to how you actually charge
Power bank sizing comes down to two numbers most shoppers never look at: watt-hours (Wh), which is the real measure of stored energy, and USB-C Power Delivery (PD) wattage, which determines how fast that energy can leave the bank. The mAh figure printed on the box is a marketing number based on the internal battery cell, not on what actually reaches your phone or laptop.
As a starting point: 10,000 mAh is the everyday-carry size for one phone and a bit of top-up. 20,000 mAh is the "all day, maybe two devices" size. Anything meant to also charge a laptop needs to be judged by Wh and PD wattage, not mAh at all, and anything meant to run a CPAP machine or several devices overnight is really a portable power station's job, not a power bank's.
mAh vs Wh: what the number on the box really means
Lithium-ion cells inside a power bank run at a nominal 3.7V, even though the USB-C port outputs 5V, 9V, 12V, or 20V. The rated mAh figure is measured at that internal 3.7V, so to get watt-hours you multiply mAh by 3.7V and divide by 1,000. A 20,000 mAh power bank, for example, stores about 74Wh at the cell level.
That is not what reaches your device, though. Converting 3.7V into a higher USB output voltage costs energy at two points: stepping the voltage up (a boost converter that is typically 85-90% efficient) and internal resistance losses in the cells and circuitry. Independent measurements and manufacturer explainers put the real-world usable output at roughly 60-70% of the rated mAh figure, depending on the bank's quality and which output voltage you're drawing.
How many phone charges do you actually get?
Because phone batteries are also rated in mAh at a similar nominal voltage, comparing effective output mAh to a phone's battery capacity is the simplified method reviewers use, and it's accurate enough for shopping decisions. Using current flagship batteries as a reference: the base iPhone 17 uses a roughly 3,692 mAh battery, and Samsung has confirmed the Galaxy S26 uses a 4,300 mAh battery.
| Power bank (rated) | Effective usable output (~65%) | iPhone 17 (3,692 mAh) | Galaxy S26 (4,300 mAh) |
|---|---|---|---|
| 10,000 mAh | ~6,500 mAh | ~1.7 full charges | ~1.5 full charges |
| 20,000 mAh | ~13,000 mAh | ~3.4 full charges | ~3.0 full charges |
| 26,800-27,000 mAh | ~17,000 mAh | ~4.5 full charges | ~4.0 full charges |
Real-world numbers will run a little lower than this if you fast-charge (higher voltages generally lose a bit more to heat) or use the bank while it's cold, and a little higher if your phone is only ever topped up rather than charged from empty. Treat these as planning numbers, not lab guarantees, since neither Apple nor Samsung has published third-party charge-cycle testing against specific power banks as of this writing.
Charging a laptop: think watts and Wh, not mAh
Laptops need two things a phone-focused power bank often doesn't have: enough Wh to matter against a 50-100Wh laptop battery, and enough USB-C PD wattage to actually charge while the laptop is in use. A thin 13-inch laptop typically needs 45-65W to charge at a reasonable pace; a 16-inch or gaming laptop can need 100W, and some high-performance machines want 140W with an Extended Power Range (EPR) cable.
Because of the same 60-70% efficiency loss described above, a power bank needs meaningfully more rated Wh than the laptop battery you're trying to refill. A 20,000 mAh bank rated around 74Wh will realistically hand a 13-inch laptop with a 50-55Wh battery something close to one full charge, not more, assuming the bank actually supports 65W+ PD output. A bigger laptop, or a full top-up plus phone charging on the same trip, calls for a 25,000-27,000 mAh bank (roughly 96-100Wh, the practical ceiling before airline rules kick in) with 100W PD support.
Camping, travel, and multi-day trips
For a weekend where you can't recharge daily, plan for total device battery capacity times the number of days, then add the usual 35-40% efficiency buffer. Charging one phone and a pair of true wireless earbuds for three days off-grid typically means a 20,000-27,000 mAh power bank. Add a camera, a GPS watch, or a second phone and you're looking at either a second bank or a small solar-rechargeable power station instead, since most USB-C power banks top out around the 100Wh mark anyway (see the airline section below for why).
CPAP machines and other medical devices
CPAP machines are a common reason people go shopping for "the biggest power bank," and it's usually the wrong category. A CPAP without a heated humidifier draws roughly 30-40W continuously; with the heater running, that can climb past 60-80W. Over a full night that adds up to 250-500+ Wh, which is several times more than even a large 100Wh-class power bank can supply, and it needs to be delivered as stable AC or a compatible DC output, which most phone-style power banks don't provide.
For CPAP use, reviewers and manufacturers both point toward dedicated CPAP batteries or portable power stations in the 150-300Wh range with a pure sine wave inverter (some CPAP power supplies specifically require pure sine wave AC rather than modified sine wave). Always check your CPAP manufacturer's power accessory guidance before relying on a generic power bank or power station overnight.
Airline rules: the 100Wh and 160Wh limits
Under FAA and TSA rules, spare lithium-ion batteries and power banks must travel in carry-on baggage only, never in checked luggage. Capacity determines what's allowed: banks rated at 100Wh or less need no special approval, banks rated between 101-160Wh need airline approval before you fly, and anything above 160Wh is not permitted on passenger aircraft at all.
Airlines have also tightened their own rules on top of the federal baseline through 2026. Several major US carriers, including American, Delta, and Southwest, now cap passengers to one or two power banks per person, ban charging devices from a power bank stored in an overhead bin, and prohibit recharging a power bank from the seat's power outlet during the flight. Policies vary by airline and change periodically, so check your specific carrier's current battery policy before a trip, especially if you're carrying a larger 20,000-27,000 mAh bank.
Power bank or portable power station? Where the line is
There's no official cutoff, but in practice the market draws the line close to the same 100Wh mark that matters for air travel. Below that, you're shopping USB-C power banks: pocketable, USB-only or USB-plus-small-DC-output, generally under 27,000 mAh. Above it, products shift into portable power station territory: AC wall outlets, pure sine wave inverters, wheels or handles on the larger ones, and capacities measured in hundreds or thousands of Wh rather than tens.
Get a power bank if you're charging phones, earbuds, a tablet, or occasionally topping up a laptop. Get a portable power station if you need AC power, need to run a CPAP or small appliance for hours, or regularly need more than about 100Wh between charging opportunities.
Here's the sizing breakdown in one place:
| Use case | Recommended capacity | Why |
|---|---|---|
| Daily phone top-up | 5,000-10,000 mAh | Covers one to two charges; small and pocketable |
| Heavy phone use, all-day travel | 10,000-20,000 mAh | Roughly 1.5-3 full phone charges depending on model |
| Two phones or a phone plus earbuds/watch, multi-day | 20,000-27,000 mAh | Stays under the 100Wh airline threshold |
| Occasional laptop top-up (13-14 inch) | 20,000-27,000 mAh with 65-100W PD | Needs both enough Wh and enough PD wattage |
| Laptop as primary use, or 16-inch/gaming laptop | 25,000-27,000 mAh with 100-140W PD | Matches high-capacity laptop batteries and fast charging |
| CPAP, appliances, camping base power | 150Wh+ portable power station | Power banks can't sustain CPAP-level continuous draw overnight |
Common mistakes when sizing a power bank
- Buying on mAh alone. A bigger mAh number with weak PD output won't charge a laptop meaningfully faster, or at all if the wattage is too low.
- Assuming mAh converts 1:1 to phone charges. Efficiency losses mean a 20,000 mAh bank behaves like roughly 13,000 mAh at the device.
- Ignoring the cable. A basic USB-C cable caps out at 60W regardless of what the power bank and laptop can handle.
- Packing a large bank in checked luggage. Power banks and spare batteries must stay in carry-on baggage under FAA/TSA rules.
- Trying to run a CPAP off a phone-sized power bank. The continuous overnight draw is far beyond what a 100Wh-class bank can sustain.
Frequently asked questions
How many mAh do I need for a full day of phone use?
Why does my power bank charge my phone fewer times than the mAh suggests?
Is a 20,000 mAh power bank enough to charge a laptop?
Can I bring a power bank on a plane?
What size power bank do I need for camping?
Do I need a power bank or a portable power station?
Sources: TSA — Lithium batteries with more than 100 watt hours, FAA — Airline Passengers and Batteries, MacRumors — Apple Lists iPhone 17, iPhone Air, and iPhone 17 Pro Battery Capacities, PhoneArena — Samsung Galaxy S26 battery and charging: everything you need to know, HP — What Watt Charger Do You Need? Laptop Charging Guide (45W vs 65W vs 110W), Anker — Converting Wh to mAh: A Beginner's Guide.
How we write: we compare manufacturer specifications, independent lab measurements and verified owner reports. Prices are typical US street prices as of September 2026 and change often.
