A laptop battery that lasts a working day at a desk can run out surprisingly quickly on a train, in an airport or in a car park between meetings. The answer is to carry your own power: a power bank big enough to charge a laptop, a charger that runs from the car, or both. This guide explains what a laptop needs from either, how to read the numbers on a power bank, the rules for taking one on a plane, and how to look after the laptop's battery while you are away from home.
What a laptop needs from a power bank
Most power banks are made for phones. They hold enough to fill a phone two or three times, and they give it out slowly, because a phone only needs a little power at a time. A laptop needs much more power at once, and at a higher voltage. Plug a laptop into a small phone power bank and, at best, the battery level creeps up very slowly or simply falls more slowly than it would have done. At worst, nothing happens at all.
A power bank that can charge a laptop has three things a phone power bank may not: enough power going out of one port, a way of agreeing that power with the laptop, and enough stored energy to make the trip worthwhile. Laptop power banks are made with all three in mind.
A USB-C port with Power Delivery
Most laptops made in recent years charge through a USB-C port. When you plug a USB-C laptop into a charger or power bank, the two talk to each other before any real power flows. The language they use is called USB Power Delivery, often shortened to USB PD or just PD. The laptop asks for what it needs, the power bank says what it can offer, and they settle on a voltage. This is what lets the same USB-C socket charge a pair of earbuds gently and a laptop quickly.
For a laptop, the power bank's USB-C port needs to support Power Delivery at a high enough level. The USB standard's normal range goes up to 100 watts. A newer extended range, added in 2021, goes up to 240 watts for the largest laptops. Most power banks sit well inside the normal range, which is enough for most laptops that charge over USB-C.
How many watts your laptop wants
The easiest way to find out what your laptop needs is to look at the charger that came with it. The watts are usually printed on it, often in small grey letters on the side, as a figure followed by W. If you only see volts and amps, multiply them: 20 volts at 3.25 amps is 65 watts.
A power bank that gives the same watts as your laptop's own charger will charge it at about the usual speed. One that gives less can still help, but more slowly. Apple says that a Mac will show the words "Slow Charger" in its battery menu when the charger gives less power than that model needs, and that the charger and cable together should deliver at least the recommended wattage for the model. A laptop that is working hard, with a bright screen and a busy processor, may use almost everything a small power bank can give and charge barely at all, so a low-power bank is more useful with the lid closed.
The cable matters too
Ordinary USB-C cables are made to carry up to 60 watts. To go above that, the cable has to be rated for a higher current: these are sold as 100-watt or 240-watt cables. If your power bank and laptop can agree on 100 watts but the cable can only carry 60, you will get 60. A short USB-C to USB-C laptop cable rated for the watts you need is worth keeping with the power bank, so the two always travel together.
Laptops with a round charging plug
Older laptops, and some larger ones today, charge through a round barrel plug rather than USB-C. A USB-C power bank cannot charge these directly. There are two ways round it. Some power banks have a DC output with a set of tips, like the ones on universal laptop chargers; you choose the tip that fits and make sure the voltage matches the laptop's own charger. Others have a mains-style socket built in, so you can plug in your normal charger. Those are larger and heavier, and some energy is lost turning the battery's power into mains power and back again.
Ports, size and weight
Look at how the power is shared. A power bank might give 100 watts from one USB-C port on its own, but split that between two or three ports when you charge a phone at the same time. The product description usually gives the most from each port and the most in total. Check the figure for the port you will use for the laptop.
Laptop power banks are heavier than phone ones, because more stored energy means more battery cells. If you carry one every day, a medium size you will actually take with you is more use than a large one left at home.
Capacity, watts and what the numbers mean
Two numbers on a power bank matter most, and they mean different things. The watts tell you how fast it can give out power. The capacity tells you how much it holds. A power bank can be fast and small, or slow and large, so you need to read both.
Milliamp hours and watt-hours
Capacity is usually printed in milliamp hours, written mAh, such as 20,000mAh. That figure is measured at the voltage of the battery cells inside the power bank, which for the lithium cells used in most power banks is about 3.6 or 3.7 volts. Laptop batteries, though, are described in watt-hours, written Wh. To compare the two, you need the power bank's capacity in watt-hours as well.
Many power banks print the watt-hours on the case. If yours does not, the UK Civil Aviation Authority gives the sum: multiply the voltage by the milliamp hours and divide by 1,000. With 3.7-volt cells, that works out like this:
| Printed capacity | Watt-hours at 3.7 volts |
|---|---|
| 10,000mAh | 37Wh |
| 20,000mAh | 74Wh |
| 26,800mAh | about 99Wh |
The watt-hour figure is the energy stored in the power bank, not the energy that reaches the laptop. Some is lost as heat when the power bank turns its cells' low voltage into the higher voltage a laptop charges at, and the laptop loses a little more as it charges. So expect a power bank to give a laptop noticeably less than its label suggests.
How far a power bank will go
To get a feel for what a power bank will do, compare its watt-hours with the size of your laptop's battery. The laptop maker's specifications page usually gives the battery in watt-hours. Two examples from Apple's own specifications: the 13-inch MacBook Air with the M4 chip has a 53.8Wh battery, and the 16-inch MacBook Pro from 2024 has a 100Wh battery. A 74Wh power bank would give the first about one full charge once the losses are taken off, and the second well under one. Many thin Windows laptops and Chromebooks have batteries in the same range as the smaller MacBook.
Charging the power bank itself
A large power bank also takes a while to refill. The input figure, again in watts, tells you how fast it can be charged. A bank that takes a high input through USB-C can be refilled from a laptop charger in a few hours; one with a low input may need all night. Charge it at home from a charger with a UK three-pin plug, and the guide to laptop chargers, watts and UK plugs explains why that matters. A fast charger from the USB-C laptop chargers range can then do both jobs: the laptop at your desk, and the power bank overnight.
Flying with a power bank
The rules for power banks on aircraft were tightened from 27 March 2026, following a change agreed internationally. The UK Civil Aviation Authority sets out what they mean for passengers, and its points are below. Airlines can add rules of their own, so check with your airline before you fly, and check again if you change airline for the return journey.
Hand luggage, never the hold
Power banks and spare lithium batteries must go in your hand luggage, never in a suitcase that goes into the hold. The reason is simple: if a battery overheats in the cabin, the crew can see it and deal with it quickly. A fire in the hold is far harder to reach. If your cabin bag is taken from you at the gate to go in the hold, because the overhead lockers are full, take the power bank and any spare batteries out first and keep them with you. Phones, tablets and laptops should normally travel in the cabin too.
How many, and how big
- No more than two power banks per person.
- Power banks up to 100Wh are allowed without asking the airline.
- Power banks over 100Wh and up to 160Wh need the airline's approval before you fly.
- Over 160Wh is not allowed in passenger baggage at all.
That is why so many laptop power banks are made just under 100Wh: it is the largest size you can take without asking. If the watt-hours are not printed on your power bank, you can work them out with the sum in the section above, but a power bank with its rating clearly marked is easier for everyone at security.
On board
Under the 2026 rules, a power bank must not be recharged on the aircraft, and should not be used to charge other devices on board either. That means you cannot sit at your seat charging your laptop from your power bank, even on a long flight. Charge the laptop fully before you travel and use the seat power if the aircraft has it. When the power bank is not in use, each one should be protected on its own: in its own pouch or a small bag, with the ports covered, so nothing metal can touch them. Cabin crew may ask you to keep power banks in the seat pocket in front of you rather than in the overhead locker, where a problem would be hard to spot.
Charge your devices before you go
The government's guidance on hand luggage says to make sure your electronic devices are charged before you travel. Security staff can ask you to switch a device on, and if it does not switch on, you will not be allowed to take it onto the aircraft. A laptop that ran flat in the departure lounge is a real risk here, which is one more reason to charge it to full before you leave home.
Damaged batteries stay at home
The Civil Aviation Authority is clear that batteries showing signs of damage, overheating or swelling should not be used, and should never be carried on an aircraft. If a power bank's case has split, bulged, been crushed or gets hot when it is not charging anything, stop using it and take it to a recycling point that accepts batteries. Never put a damaged power bank in the household bin.
Charging in the car
A car is a good place to top a laptop up between jobs, as long as it is done with the right charger and some care for the car's own battery.
Car laptop chargers
Car laptop chargers plug into the car's 12-volt socket, the round one that used to hold a cigarette lighter. The most useful kind for a modern laptop has a USB-C port with Power Delivery at laptop power, so it charges a USB-C laptop exactly as a wall charger does, through a USB-C cable rated for the watts. For a laptop with a round plug, there are car chargers with a set of tips, like a universal charger. As with a power bank, check that the charger's watts are close to the laptop's own charger, and that the tip and voltage match a round-plug laptop.
Vans and lorries sometimes have 24-volt sockets rather than 12-volt ones. If you drive one, choose a charger marked for both 12 and 24 volts.
Inverters
An inverter turns the car's 12 volts into mains power with a three-pin socket, so you can use your laptop's normal charger. It works with almost any laptop, but it is the least efficient way to do the job: the power is turned up to mains voltage by the inverter, then down again by the laptop charger, losing some each time. A direct car charger is usually the neater answer.
Look after the car battery
In many cars the 12-volt socket only works when the ignition is on. In others it stays live with the engine off, and a laptop charging from it for a long time can flatten the car's battery. If you charge with the engine off, keep it short. Your car's handbook says whether the socket stays live and what it can supply.
Safely in the car
- Plug the laptop in and set it up before you set off, and never handle a laptop or its cable while driving.
- Run the cable where it cannot catch the pedals, the gear lever or the handbrake.
- Do not leave a laptop or a power bank in a parked car on a warm day, or on the dashboard in the sun. A car heats up quickly, and heat is the main enemy of a lithium battery.
Trains, coaches and cafés
Many trains and coaches now have power at the seat. A three-pin socket takes your normal laptop charger, which is the simplest thing to carry. A USB socket on a seat or table may not give enough power for a laptop, even if it charges a phone well, so treat it as a bonus rather than a plan. A compact charger in a tech pouch with the right cable means you can use whichever socket you find.
Looking after the battery on the road
Travelling is harder on a laptop battery than sitting on a desk, mainly because of heat, cold and long spells on charge or completely flat. A few habits keep the battery in good health for longer.
Heat and cold
Apple says its MacBooks work best at room temperatures between 10 and 35 degrees Celsius, and that using a device somewhere hotter than 35 degrees can permanently reduce how much the battery holds. Charging in heat makes it worse. Other makers give similar advice. So keep the laptop out of direct sun, out of hot cars and away from radiators, and if it feels hot while charging, take it out of any snug case and let it cool.
Cold is less harmful. In very cold weather you may see the battery run down faster than usual, but Apple says this is temporary and the battery returns to normal once it warms up. If the laptop has been in a freezing car, let it come up to room temperature before you charge it.
Give it air
Laptops push their heat out through vents, usually on the underside or at the back. Hotel beds, sofas and laps block them. Apple's safety guidance is to use a laptop on a stable surface that lets air move under and round it, never to put it or its charger under a blanket or pillow while it is plugged in, and to move it to a firm surface if it gets uncomfortably warm on your lap. A tray, a book or a folding aluminium stand gives it a firm base in a hotel room. The guide to keeping a laptop cool goes further.
Not full, not flat, for long
Lithium batteries are happiest in the middle. Leaving a laptop plugged in at 100 per cent for days on end, or running it completely flat and leaving it that way, both wear the battery faster. Many laptops now have a setting that holds the charge below full when they are plugged in most of the time; it is worth turning on if yours has one. If you are putting a laptop or a power bank away for a long trip or for months, Apple's advice is to leave it at about half charge.
Look after the power bank
Power banks need the same care. Keep them out of the sun and away from heat, do not leave them completely flat for months, and carry them in a pouch or pocket where keys and coins cannot touch the ports. If a power bank swells, cracks, smells odd or gets hot when it is not working hard, stop using it at once.
Questions people ask
Can any power bank charge my laptop?
No. It needs a USB-C port with Power Delivery at enough watts for your laptop, or the right DC tip for a laptop with a round plug. A small phone power bank will charge a laptop very slowly, if at all. Check the watts on your laptop's own charger and choose a power bank that gives close to that from one port.
What is the biggest power bank I can take on a plane?
Up to 100Wh without asking the airline. Between 100Wh and 160Wh you need the airline's approval before you fly. Over 160Wh is not allowed in passenger baggage. You can carry no more than two, and only in your hand luggage.
Can I charge my laptop from my power bank during the flight?
Under the rules in force since March 2026, a power bank should not be used to charge other devices on board, and must not be recharged on board. Charge the laptop fully before you fly and use the seat power if there is any.
Can I pack a power bank in my suitcase?
Not in a suitcase that goes in the hold. Power banks and spare batteries go in your hand luggage. If your cabin bag is taken to the hold at the gate, take them out first.
Will a car charger flatten my car battery?
Only if the socket stays live with the engine off and you charge for a long time without driving. With the engine running, or in a car whose socket switches off with the ignition, it will not.
Where to go next
To charge on the move, start with laptop power banks and car laptop chargers, with a USB-C laptop cable rated for the watts. The accessories for travellers bring together the other things that make a laptop easier to take away, and shopping by charging power lets you choose by the watts your laptop needs. For the rest of the subject:
- Laptop chargers explained: watts, USB-C charging and UK plugs
- USB-C explained: ports, cables, power and video
- How to keep a laptop cool
- How to choose a laptop sleeve or bag for the journey
