A phone charger used to be a small box that put out the same modest power for every phone. Now chargers are sold by the watt, phones fill to half in a quarter of an hour, and the boxes carry names such as USB Power Delivery, PPS and Super Fast Charging. This guide explains what those words mean, how a charger and a phone agree on a speed, why the cable can hold the whole thing back, and what fast charging means for the battery over the years.
What watts mean for a phone
Power is measured in watts, and for a charger it is a simple sum: volts multiplied by amps. Volts are the pressure pushing the electricity along, and amps are how much of it flows. A charger putting out 5 volts at 2 amps delivers 10 watts. The same charger at 9 volts and 3 amps delivers 27 watts. The more watts reach the battery, the faster it fills.
Plain USB starts at 5 volts. Electrical Safety First, the UK electrical safety charity, puts the older kind of USB charging at around 7 to 10 watts, which is why an old plug from a drawer can take hours to fill a modern phone. Fast chargers start the same way and then, once they have checked what is plugged in, raise the voltage to deliver more power. That conversation between charger and phone is what the names on the box are about.
Reading the output on a charger
Every charger prints its output, usually in small letters on the side or the base, as a list of volts and amps. A fast charger lists several, one line for each step it can supply, such as 5V 3A, 9V 3A, 15V 3A and 20V 3.25A. Multiply each pair to get the watts at that step: 5 times 3 is 15 watts, 9 times 3 is 27 watts, and 20 times 3.25 is 65 watts. The highest of those is the charger's top power, and it is the figure in its name. If a charger with more than one port lists them separately, each port has its own list, and the total for the charger is printed as well. A charger that lists only 5V has no fast charging at all, whatever its box says.
The number on the charger is a ceiling, not a promise
A charger marked 65W can deliver up to 65 watts. Whether it does depends on the phone. The phone decides how much it will accept, and it accepts less as it fills. Apple, for example, says the iPhone gradually reduces the charging current as the battery approaches full. So a phone that takes 25 watts at its fastest will take 25 watts from a 65W charger, and the charger simply holds the rest back. A bigger charger will not harm a phone that takes less; it just will not make that phone charge any faster.
That is also why phone makers quote charging speed as a time to half full rather than a time to full. The first half goes in quickly. The last part goes in slowly on any charger, to protect the battery.
What phones take today
Apple's own figures give a sense of the range. Apple says the iPhone 17, iPhone 17 Pro and iPhone 17 Pro Max can reach 50 per cent in around 20 minutes with a 40W adapter or higher, and the iPhone 17e and iPhone Air can reach 50 per cent in around 30 minutes with a 20W adapter or higher. For earlier iPhones it asks for an 18W or greater USB-C adapter that uses USB Power Delivery. Samsung sells its own Galaxy chargers at 25W and 45W. Many tablets and laptops that charge over USB-C take far more.
So for a phone on its own, a charger in the 20W to 45W range covers fast charging on most recent models. Chargers up to 20W still suit older phones, earbuds, watches and a bedside where speed matters less. Chargers of 45W and over make sense for phones that take it, for tablets, and for charging two things at once, and 65W and over is where chargers start to cope with many laptops as well.
USB Power Delivery
USB Power Delivery, usually shortened to USB PD or just PD, is the shared standard for fast charging over USB-C. It is written by the USB Implementers Forum (USB-IF), the body behind USB itself, and it is used by Apple, Samsung, Google and most other phone makers, as well as by tablets, laptops and power banks.
How it works
When a phone is plugged into a PD charger, the two talk before any fast charging starts. The charger lists the voltages and currents it can supply. The phone picks the one it wants. The charger switches to it. If the phone gets warm, or nears full, it asks for less. The USB-IF describes the aim as letting each device take only the power it requires, and get more when it needs it.
The standard has grown over the years. Before its 3.1 revision, USB PD went up to 100 watts, using 20 volts on a cable rated for 5 amps. Revision 3.1 added fixed steps at 28, 36 and 48 volts, for 140, 180 and 240 watts. Phones use the lower steps; the higher ones are for laptops and bigger kit.
Why one PD charger does so much
Because the phone and charger agree the speed between them, one USB PD charger can charge an iPhone, a Galaxy, a Pixel, a tablet and a pair of earbuds, each at the speed that device asks for. That is the strongest reason to choose a PD charger with USB-C: it is not tied to one make of phone. If a charger's description does not mention USB PD or Power Delivery, it may still charge, but you cannot count on it fast-charging.
One trap is worth knowing. The USB-IF itself points out that USB-C is not the same thing as USB Power Delivery. USB-C is the shape of the plug. A cheap charger can have a USB-C socket and still only put out plain 5-volt power. The words to look for are USB PD or Power Delivery, and the wattage.
PPS and Samsung's fast charging
PPS stands for Programmable Power Supply. It is part of USB Power Delivery, added with version 3.0, and it changes one thing: instead of choosing from a short list of fixed voltages such as 5, 9, 15 or 20 volts, the phone can ask for the voltage to be nudged up or down in small steps of 20 millivolts as the battery fills.
Why that helps
Every time power is converted from one voltage to another, some of it turns into heat. With fixed PD, the phone receives, say, 9 volts and then turns it down inside to what the battery wants, and that turning down warms the phone. With PPS, the charger does more of that work itself, sending something much closer to what the battery needs. Less of the heat ends up inside the phone, and a cooler phone can keep charging fast for longer.
Samsung Super Fast Charging
Samsung calls its fast charging Super Fast Charging, at 25 watts, and Super Fast Charging 2.0, at 45 watts on the Galaxy phones that take it. Both run over USB PD with PPS. Samsung's own 25W adapter lists PD 3.0 PPS at up to 25 watts, and its 45W adapter lists PPS from 5 to 20 volts. So to get a Galaxy's full speed from a charger that is not Samsung's, look for a USB PD charger that says it supports PPS, at 25 watts or 45 watts depending on the phone. Samsung adds that charging speed may vary by device, and its specification pages list which Galaxy models take which speed.
A plain USB PD charger without PPS will still charge a Galaxy, safely, but often more slowly than the phone's top speed. Recent Google Pixel phones also use PPS for their fastest charging, so the same advice applies to them.
iPhones and PPS
iPhones charge from standard USB PD chargers, which is why Apple's advice is given in watts alone. A PPS charger is still a USB PD charger, so it will charge an iPhone at full speed too. A charger that lists both PD and PPS is the safest choice for a home with a mix of phones.
Why the cable matters too
A fast charger and a fast phone can still charge slowly if the cable between them is not up to it. The cable has to carry the current, and cables are built for different amounts.
USB-C to USB-C cables
For USB-C to USB-C cables, there are two power ratings. Most carry up to 3 amps, which at 20 volts is 60 watts. Cables built for more carry up to 5 amps and hold a small chip, called an e-marker, that tells the charger they can take it; these go up to 240 watts under the newest standard. Cables certified by the USB-IF must show a 60W or 240W power logo, so the rating is printed for you.
For a phone on its own, a 60W cable is usually enough. Where it matters is at the top end: Samsung sells its 45W charger with a 5-amp USB-C cable, and a laptop on a 100W charger needs a 5-amp cable to get it. If a cable's description gives no rating, assume it carries no more than 3 amps, or 60 watts.
USB-A and Lightning
USB Power Delivery is built around USB-C, so a USB-A to USB-C cable will charge a phone, but usually not at its fastest. iPhones up to the iPhone 14 use Lightning, and Apple's route to fast charging for them is a USB-C to Lightning cable on a USB-C PD charger. See Lightning cables.
Wear and length
A frayed or kinked cable may charge slowly, cut in and out, or stop. Braided cables resist kinks and fraying better than plain plastic jackets. Length makes less difference than people expect at phone power, but a very long, thin, unrated cable is the one most likely to slow things down. The guide to charging cables goes through plugs, lengths and wear in full.
Charging several things at once
Most homes now have more than one thing to charge: two phones, earbuds, a watch, perhaps a tablet. A charger with two, three or four ports can do them all from one socket, but the watts are shared.
How the power is split
A multi-port charger has one total figure, the most it can deliver across all ports together. When only one thing is plugged in, that one can usually have the lot, up to what its port allows. Plug in a second, and the charger divides the power between them. A 65W charger might give 45 watts to one port and 20 to another, or split differently again with three. Good chargers print the split on the box or the listing, port by port. Read it, because a charger that fast-charges one phone on its own may not fast-charge two at the same time.
A useful rule: add up what each thing takes at its fastest, and choose a charger whose total covers it. Two recent phones at 25 watts each, plus earbuds, wants a charger of 55 to 65 watts to charge everything at full speed together.
Docks and stations
Charging docks and 3-in-1 charging stations do the same job in one tidy unit, with a place for the phone, the earbuds and the watch. Most of these charge the phone wirelessly, which is handy at a bedside but slower than a cable; the guide to wireless and magnetic charging explains why. Many are powered by a USB-C charger of their own, and the station can only share out what that charger gives it, so use the size of charger the station asks for.
In the car and on the move
Car chargers come with one or two ports and the same PD and PPS labels as wall chargers, so the same advice applies. A power bank with USB PD output can fast-charge a phone away from a socket; the power banks guide covers sizes, speeds and taking one on a plane.
Heat, battery health and charging overnight
Phone batteries are lithium-ion, and they age. Each charge cycle wears them a little, and heat wears them faster. Apple puts it plainly: a battery warms up as it charges, which can reduce its lifespan. Fast charging puts more power in, so it can mean more heat, which is why phones manage it carefully.
What the phone does to protect itself
Phones slow their charging as they fill, and they slow it further if they get warm. Apple says iPhone software may hold charging at 80 per cent when the battery is warmer than it should be, and that fast charging may not work when it is too hot or too cold. Apple also advises against using or charging a phone where the air is hotter than 35°C. Samsung's Galaxy phones have a Battery protection setting that can stop charging short of full to reduce wear, with more than one option on recent models.
So fast charging, done through a proper PD or PPS charger, is designed not to cook the battery. The things that do add heat are outside the charger's control.
What you can do
- Charge in the open, not under a pillow, on a duvet or in a bag. Electrical Safety First's advice is to charge uncovered somewhere well ventilated.
- If the phone gets very warm in a thick case while it charges, take the case off for the charge.
- Do not charge in direct sun, on a hot dashboard or beside a radiator.
- Avoid gaming or filming for long periods on a fast charger, which heats the phone from two directions at once.
- Leave your phone's own battery settings on. They are there to slow wear.
Charging overnight
Here the advice differs, so it is worth knowing both sides. Apple says iPhone stops charging when the battery is full, so it is safe to keep an iPhone on a charger overnight, and its Optimised Battery Charging learns your routine over about 14 days and holds the battery at 80 per cent until shortly before you usually unplug. On iPhone 15 and later you can also set an 80 per cent limit. Electrical Safety First's general advice for USB chargers is not to use them overnight or while you are out of the house.
The two are not as far apart as they sound. The phone's own controls stop it overcharging; the charity's concern is the charger and the setting, a faulty or poor charger or a phone left on a bed. If you do charge overnight, use a charger you trust, on a hard surface, with nothing on top. The guide to choosing a safe phone charger covers how to tell a good charger from a poor one.
Questions people ask
Will a bigger charger damage my phone?
No. The phone only takes the power it asks for. A 65W USB PD charger will charge a phone that takes 20 watts at 20 watts.
Can I use one charger for an iPhone and a Galaxy?
Yes. Choose a USB-C charger with USB Power Delivery and PPS. The iPhone uses PD and the Galaxy uses PPS, and the charger supplies whichever the phone asks for.
Why is my phone charging slowly on a fast charger?
Check the cable first, especially if it is old or has a USB-A end. Then check whether the phone is warm, nearly full, or busy, as all three slow charging. A multi-port charger also gives less to each port when several are in use.
Does fast charging wear the battery out faster?
Heat is what wears a battery, and phones slow fast charging as they warm up and as they fill. Keeping the phone cool while it charges, and leaving its battery settings on, does more for the battery than avoiding fast charging.
What does PD 3.0 or PD 3.1 on the box mean?
The version of USB Power Delivery the charger follows. PD 3.0 added PPS; PD 3.1 added higher power for laptops, up to 240 watts. For a phone, the useful things to look for are PD, PPS if you have a Galaxy or Pixel, and the wattage.
Where to go next
To find a charger by power, start with wall chargers, the range for chargers with a UK three-pin plug, or go straight to 20W to 45W chargers for most phones. Pair it with a rated USB-C to USB-C cable, or a short cable for a power bank or the car. Delivery is included in every price and tracked to your door.
- Which charging cable does my phone need?: plugs, lengths and wear
- Choosing a safe phone charger in the UK: the law, the warning signs and recalls
- Power banks explained: sizes, real capacity and flying with one
- Wireless and magnetic charging explained: pads, stands and magnetic chargers
