Look at the back of any charger and you'll find three numbers: voltage (V), amperage (A), and wattage (W). They appear in every product description, on every label, and in every device spec sheet. Knowing what each one actually means saves you from blown devices, slow charging, and the wrong purchase.
This guide covers the three units, how they relate, and how to read any charger label in two seconds.
The water analogy that actually helps
Electricity is hard to visualize because we can't see it. The standard analogy is water flowing through a pipe:
- Voltage is the pressure pushing the water through the pipe. Higher pressure means more force per unit of flow.
- Amperage is the rate of water flow. How many liters per second pass a given point.
- Wattage is the total power being delivered. It's voltage times amperage, the equivalent of pressure times flow rate.
A high-pressure narrow pipe delivers the same wattage as a low-pressure wide pipe if the product of pressure and flow is the same.
For mains electricity:
- US 120 V mains: low pressure, more flow needed for the same power
- European 230 V mains: high pressure, less flow needed for the same power
- Both deliver similar power but through different combinations of pressure and flow
Reading a charger label
A typical charger label looks something like:
INPUT: 100-240V ~ 50/60Hz 1.5A
OUTPUT: 20.0V == 4.5A 90W
Breaking this down:
Input
The voltage range and amperage the charger pulls from the wall.
- 100-240V: dual voltage, works on any country's mains
- 50/60Hz: handles both common AC frequencies
- 1.5A: the maximum current the charger draws from the wall at full load
The input amperage tells you the charger's wall-side appetite. At 120 V and 1.5 A, the charger pulls up to 180 W from the wall. At 240 V and 1.5 A, it pulls 360 W. The actual power drawn depends on the device's current need.
Output
The voltage and amperage the charger delivers to your device, plus the resulting wattage.
- 20.0V: the DC voltage the device receives
- 4.5A: the maximum current the charger can deliver to the device
- 90W: total output capacity (20 × 4.5)
The output spec tells you what laptops or devices the charger can support. A 90 W charger handles most thin-and-light laptops. A 30 W charger handles phones, tablets, and ultra-thin laptops only.
Why each number matters for travelers
Voltage
The voltage range matters for international travel. If both 100 V and 240 V are in the range, the charger works in any country with just a plug-shape adapter. If the range covers only one voltage, you need a converter or a replacement.
Amperage
The amperage matters for two scenarios:
-
Cable compatibility: cheap cables rated 3 A can't carry the 5 A that a 100 W USB-C PD charger wants to deliver. Use cables rated for at least the wattage you expect.
-
Circuit protection: a UK Type G plug has a fuse rated for the appliance's amperage. A device that draws too much amperage will blow the fuse before damaging the device.
Wattage
Wattage tells you whether the charger can handle the device:
- Phone: 5-30 W
- Tablet: 10-30 W
- USB-C laptop (thin and light): 30-65 W
- USB-C laptop (workstation): 65-140 W
- Hair dryer: 1,000-2,000 W (way too much for any travel charger, plug into wall directly)
Common charger ratings
| Device class | Typical output |
|---|---|
| iPhone Lightning charger (old) | 5 V × 1 A = 5 W |
| iPhone USB-C charger (new) | 5 V × 3 A = 15 W (PD 20 W mode) |
| iPad charger | 5-9 V × 2-3 A = up to 20 W |
| MacBook Air | 30 W USB-C PD |
| MacBook Pro 14-inch | 67-96 W USB-C PD |
| MacBook Pro 16-inch | 96-140 W USB-C PD |
| Dell XPS laptop | 65-130 W |
| Most Windows business laptops | 65-100 W |
| Modern gaming laptops | 100-300 W (usually a dedicated brick, not USB-C) |
For travel purposes, a 65-100 W GaN USB-C PD charger handles 90% of laptops you might carry.
Why amperage matters for cables
USB-C cables look identical from the outside but have very different current ratings. A cheap USB-C cable might be rated:
- 5 V × 3 A = 15 W maximum
- 20 V × 5 A = 100 W maximum
The cable's rating limits the total power flowing through it. If you connect a 100 W laptop charger to a laptop via a 3 A cable rated 15 W, the cable becomes the bottleneck. The laptop charges slowly, or worse, the cable heats up and melts.
For laptop charging, use USB-C cables rated at least 100 W. The Apple-supplied cables that come with new MacBook Pros are rated 240 W. Anker, Belkin, and other major brands sell USB-C cables clearly labeled with their wattage rating.
The math you actually need
If you remember one formula: Watts = Volts × Amps.
This lets you:
- Calculate how many amps a 100 W device draws at 120 V: 100 ÷ 120 = 0.83 A
- Calculate how many amps a 100 W device draws at 230 V: 100 ÷ 230 = 0.43 A
- Compare two chargers: a 5 V 2.4 A charger (12 W) vs a 20 V 1 A charger (20 W). The second is faster.
- Verify a circuit can handle a device: a 15 A US circuit can handle up to 15 × 120 = 1,800 W
What about kilowatt-hours?
Power banks and EV batteries are rated in milliamp-hours (mAh) or watt-hours (Wh). These measure energy capacity, not power output.
- A 20,000 mAh power bank at 3.7 V cell voltage stores 74 Wh of energy.
- A 100 Wh battery can deliver 100 W continuously for 1 hour, or 50 W for 2 hours.
- The 100 Wh airline carry-on limit for lithium batteries is based on this rating.
Energy (Wh) is different from power (W). A power bank that stores 100 Wh might only be able to deliver 20 W at any moment. Check the output rating separately.
The bottom line
Three numbers on every charger:
- Voltage tells you what mains it accepts (100-240V for dual voltage)
- Amperage tells you the maximum current it can deliver
- Wattage tells you the total power capacity (V times A)
For international travel, the voltage range is the safety check. For laptop charging, the wattage is the speed check. For cable selection, the amperage rating is the compatibility check.
Read the label. Match the numbers. Pack accordingly.