Most power banks are designed for phones, and most of them cannot run a laptop at all. The two numbers that decide it — how much energy the power bank holds, and how fast it can push that energy out — are both printed on the box, but rarely in the units that make comparison easy. This guide translates them, adds the airline rule that caps how big a power bank you can fly with, and shows how to estimate real runtime from your own laptop’s discharge rate before you buy.
Check 1: Output wattage — can it feed your laptop?
A laptop needs far more power than a phone. Look for a USB-C port that supports USB Power Delivery (USB-C PD) at a wattage in your laptop’s range: many ultrabooks charge from 45 W or 65 W adapters, and a power bank must offer a PD output in the same territory or the laptop will refuse it, charge painfully slowly, or only charge while asleep. The total-output figure in big print on the box can mislead — it is sometimes the sum across all ports. What matters is the per-port USB-C PD rating. Our Laptop Charger Wattage Guide covers the volts/amps/watts logic in full; the same rules apply here.
Check 2: Capacity in watt-hours (Wh)
Power banks are advertised in mAh, but mAh is measured at the internal cell voltage (around 3.7 V), not at the 20 V your laptop negotiates. To compare with your laptop’s battery — which BatteryInfoView reports in mWh — convert:
- Wh ≈ mAh × 3.7 ÷ 1,000. A “20,000 mAh” power bank therefore holds about 74 Wh.
- Conversion is not free: stepping voltage up and down wastes some energy as heat, so the energy that actually reaches the laptop is noticeably less than the rated Wh. Treat rated Wh as a ceiling, not a promise.
For scale, open BatteryInfoView (from our download page) and read your battery’s Designed Capacity in mWh. If your laptop’s battery is about 50,000 mWh (50 Wh) and the power bank holds 74 Wh, you are looking at roughly one full recharge of the internal battery, minus conversion losses — not the three or four charges the mAh headline might suggest.
Estimate your runtime before buying
BatteryInfoView’s Charge/Discharge Rate field shows, in milliwatts, how fast your laptop is actually draining its battery during your normal work. Unplug, work as usual for a few minutes, and note the (negative) rate. Runtime arithmetic then gets honest: usable watt-hours from the power bank ÷ your average draw in watts = approximate extra hours. A light-work draw makes a mid-size power bank stretch a long way; a heavy draw eats the same bank quickly. The rate field is unpacked in Charge/Discharge Rate Explained, and the Battery Log (F8) can record your drain pattern over a whole day if you want a better average.
Beyond the numbers: ports, cables and pass-through
Once wattage and capacity check out, a few practical details separate a power bank you will actually carry from one that stays in a drawer:
- Simultaneous output. Many banks advertise a total wattage shared across ports. If you plan to charge the laptop and a phone together, confirm the USB-C port still delivers your laptop’s required wattage while the other port is in use — the per-port table in the specifications is where this is stated.
- The cable in the box. A short USB-C cable rated for high current is often included; a random thin cable from a drawer can cap charging speed or fail to start charging at all. If you buy a separate cable, it needs the same current rating as the bank’s output.
- Pass-through charging. Some banks can charge themselves while charging the laptop, which turns them into a mini power hub at a desk or in a hotel room. Handy, but it generates heat in both directions — treat it as an occasional convenience rather than your permanent setup.
- Recharge speed of the bank itself. A large bank refilled through a low-wattage input can take most of a day to recharge. If you travel back-to-back days, the bank’s own input wattage matters almost as much as its output.
- Weight. Capacity is mass. A bank sized for a full laptop recharge is a noticeable brick in a bag; the runtime estimate above helps you avoid carrying capacity you will never use.
Check 3: The airline 100 Wh rule
If you fly with your gear, capacity has a legal ceiling. The general rule used by airlines worldwide: lithium batteries up to 100 Wh are allowed in carry-on luggage without special approval; larger batteries (typically up to 160 Wh) need airline approval, and anything bigger is not permitted for passengers. Spare batteries and power banks belong in carry-on, never in checked bags. Rules vary slightly by airline and country, so check your carrier before travelling — but if you want a power bank that flies anywhere without paperwork, stay at or under 100 Wh, which in the marketing units on the box is roughly a 27,000 mAh ceiling.
Where to buy
Some links below are affiliate links — see our Affiliate Disclosure.
- Anker publishes clear per-port PD wattages and Wh figures across its laptop-class power banks; browse via our Anker affiliate link.
- Amazon — search laptop power banks with USB-C PD on Amazon (affiliate link) and filter by the per-port wattage from Check 1, not the headline capacity.
More accessory guidance — chargers, replacement batteries and backup power — is gathered in our buying guides.
FAQ
My power bank has USB-C. Why won’t my laptop charge from it?
USB-C is only the plug shape. The power bank must support USB Power Delivery at a wattage your laptop accepts, and the cable must be rated for it. A USB-C port that tops out at phone-level wattage will not start a laptop charge no matter how large the power bank’s capacity is.
Is a bigger power bank always better?
Only up to the point where it stops being portable — or legal to fly with. Above 100 Wh you gain airline-approval hassles, and weight climbs quickly. For most laptop users, a sub-100 Wh bank sized against their real discharge rate (Check 2 and the runtime estimate) is the sweet spot.
Can a power bank damage my laptop battery?
A standards-compliant USB-C PD power bank negotiates voltage with the laptop before delivering power, which is the mechanism designed to prevent mismatches. Stick to reputable brands, verify the PD wattage matches your laptop’s needs, and avoid unbranded units whose ratings cannot be verified — the same caution as with wall chargers.