How to Choose a Power Bank for Your Laptop

In short: what actually matters

A standard smartphone power bank almost never charges a laptop properly. Before buying, check four things in order:

• Wattage (W): it should match or exceed your laptop's original charger, not a phone charger.
• Capacity in watt-hours (Wh), not mAh: it's the only unit that lets you compare a power bank to your laptop's real battery life.
• USB-C Power Delivery (PD) support (PD 3.0 up to 100W, PD 3.1 up to 240W) and the exact port on your laptop that accepts it.
• Real-world behaviour on your specific model: some laptops refuse charging on certain ports, or even charge the power bank instead of the other way round.

Why mAh barely means anything for a laptop

Manufacturers advertise capacity in milliamp-hours (mAh) because it's the number that looks most impressive on the box. The problem: mAh alone says nothing about the energy actually stored, because it depends on the internal cell voltage of the power bank (usually 3.6 to 3.7V), which is almost never the voltage used to charge a laptop (typically 15 to 20V over USB-C PD).

The right unit for comparison is the watt-hour (Wh), which factors in voltage: Wh = (mAh / 1000) x V. A 20,000mAh power bank rated at 3.7V holds roughly 74Wh, not 20,000 of anything. That Wh figure is what you should compare against your laptop's own battery capacity, usually listed in Wh in the manufacturer's specs, most often between 40 and 99.9Wh.

The right move: start from your original charger

The most reliable approach isn't to look at the power bank first, it's to look at the charger that came with your laptop. Flip the power brick over: it always lists its output wattage (W), sometimes alongside voltage and current (for example “20V, 3.25A”, which is 65W). That's the number to match, or beat, on the power bank side.

Without the charger on hand, the manufacturer's specifications (product page, user manual) list the wattage of the original AC adapter. A few market benchmarks, for reference:

USB Power Delivery took a major step forward with revision 3.1: it reaches up to 240W (at 28V, 36V and 48V), a ceiling only the most powerful gaming laptops actually use. See the official USB-IF page on USB Power Delivery.

A power bank rated below your original charger's wattage won't damage anything, but it will charge slowly, or not at all if the laptop detects insufficient power to start charging.

Universal USB-C or proprietary charging: the Dell, HP, Lenovo trap

Not every USB-C PD power bank behaves the same way once plugged into a laptop, and the laptop brand makes a real difference.

• Not every USB-C port charges. On many Dell and Lenovo laptops, only one port, often the Thunderbolt port, actually accepts power; the other USB-C ports are data or video only. Check your model's manual or spec sheet before blaming the power bank.
• The laptop can end up charging the power bank instead. Reported by several Dell Latitude owners, this happens when Power Delivery negotiation fails: the power bank gets identified as too weak or unrecognised, and the laptop switches roles to become the “charger”.
• A fully drained laptop may refuse to restart from a power bank. Some laptops need a minimum amount of power to negotiate the PD protocol before accepting a charge; at 0%, they can't negotiate anything and need their original power brick, with its own dedicated connector, to come back to life.

In the United States, there is no federal mandate requiring USB-C charging on laptops the way the EU now enforces it: each manufacturer stays free to pick its own charging port and connector. That makes checking your exact model's own spec sheet, not a general rule, the only reliable way to know which port and wattage will actually work.

Calculating how many full charges you'll get

A simple formula estimates how many full charges a power bank will give your laptop:

Concrete example: a 27,000mAh power bank at 3.7V holds roughly 99.9Wh. Charging a 57Wh ultrabook battery at 88% efficiency: 99.9 x 0.88 / 57 ≈ 1.5 full charges. For a 99Wh gaming laptop battery, the same power bank covers only about 0.9 of a charge, and only if the laptop accepts charging at the wattage supplied.

This estimate stays theoretical: a laptop that keeps running while charging (screen on, background tasks) draws power at the same time, which reduces how much energy actually ends up stored in its battery.

Your laptop refuses to charge: causes and fixes

Several signs point to USB-C PD charging not kicking in correctly:

• The laptop's charging light stays off, or blinks without ever switching to “charging”.
• The laptop's battery level keeps dropping despite being plugged in.
• The power bank loses charge faster than expected: the laptop identified it as something to recharge, not the other way round.

Before assuming the power bank is faulty, four checks in order:

1. Try another port on the laptop: test every USB-C port, since sometimes only one truly accepts power.
2. Try another cable: an uncertified or 60W-capped USB-C cable throttles charging even if the power bank can supply more; use an “e-marked” cable that explicitly states its maximum wattage.
3. Check the BIOS “USB-C charging” setting if your model has one: some manufacturers deliberately cap input current by default.
4. Never start from 0% relying solely on a power bank: see the pro tip above.

Safety: the certifications to check before buying

A power bank feeding a laptop pushes noticeably more power than a phone charger, sometimes 100W and more through a single port. A few things worth checking before buying:

• Electronic protections: overcharge, overvoltage, short-circuit and over-temperature protection should all be explicitly listed by the manufacturer.
• Safety certification: in the United States, the UL 2056 standard specifically covers power banks, beyond the generic tests applied to lithium-ion cells. In the EU, CE marking remains the baseline to check.
• An identifiable brand with real support: if something goes wrong at high power, a reachable, identifiable manufacturer makes all the difference.

Which wattage for which laptop: the recap table

To sum up, here's the wattage to aim for by laptop type, keeping in mind that your original charger's own rating remains the most reliable reference:

For a selection of models that can deliver 100W and more over USB-C PD, our detailed comparison is the resource to check: the best fast-charging power banks (100W and above).