Why do we speak of watts (and not amps) when we refer, for example, to the "7.5W Wireless Charger for iPhones," but when we talk about cord-based USB charging, the specs typically refer to amps (and not watts). For example, one page explains that the "USB spec is 500 mA maximum current. The charger provided with your iPhone can provide up to 1000 mA."

The difference between amps, watts, and volts is something I have struggled to understand since high school, which was a long time ago for me. I've re-read your standard references like Wikipedia, but I'm still not getting it.

Can someone help me understand these concepts by demonstrating how I would apply them to understand how to compare the difference, in performance, between a 7.5W wireless iPhone charger and a 1000 mA corded USB charger?

Thanks in advance for your help and patience.

  • \$\begingroup\$ The difference between amps, watts, and volts those are units of measurement ..... i think that you are asking about the physical phenomena and how they relate to each other .... you have to include resistance .... The relationship between current, resistance, voltage and power \$\endgroup\$ – jsotola Jan 3 '19 at 23:54

The reason for the difference is that it's kind of apples and oranges.

With a wireless charger it's (hopefully!) safe to assume that the whole chain is reasonably efficient, and in the end what you care about is how much energy is stuffed into your battery, not how many electrons have passed through. So the volts and amps don't matter as much, and in the case of the power that's being transferred wirelessly, aren't even in play.

For USB, you can fully specify the power by specifying the current. USB is (or should be) a fixed 5 volt system. So a 1000mA charger will supply up to 1000mA, which means up to 5000mW (or 5W, but I didn't want to make you do all the conversions until the end).

I'm not sure how to help you with the amps/watts/volts problem -- watts = volts * amps if you're talking electricity, but because energy is a much more fundamental physical property than anything electric, watts = force * speed, or torque * rotational speed, or any number of other physical quantities that multiply out to power.

  • \$\begingroup\$ Thanks; I hadn’t previously understood that USB is (or should be) a fixed 5 volt system. That explains a lot. \$\endgroup\$ – LaissezPasser Jan 4 '19 at 7:18

It is really very simple. You cannot have watts without amps flowing at some voltage level. Then:

watts = volts * amps

Don't make it more complicated. If something is rated to deliver 7.5watts then that power is delivered at some voltage. Divide the 7.5 by that voltage and know the corresponding amperage capability.


The Amps only is useful when V is constant. However W is useful always regardless if V or A or both change.

USB has constant 5V thus 1.5A is needed for 7.5W and thus a 1000 mA USB charger cannot drive that.


When you talk about Watts it's because you want to know roughly how long the device will take to charge another device or in the case of a battery device how long it will last. Talking about Watts gives you an idea of how much punch it can deliver or withstand depending on the type of device.

In the case of Amps you talk about them because you want to impply in a way the mechanical strength or resistance of the device and its wires when conducting current. That's because a 40 AWG wire, for example, can withstand as much as 1.5A According to this, however, in theory it could transmit hundreds of volts as long as 1.5 Amps are not reached. And Remember Watts = Amps (times) volts. So if you talk about just watts you restrict to a defined voltage but if you talk about Amps its more about the resistance and in a way duration of a device.


The difference between amps, watts, and volts

You cannot compare the three because they are all measurement units for different physical phenomena.

An Amp is a unit of measurement of electric current.
A Watt is a unit of measurement of electrical power.
A Volt is a unit of measurement of voltage.

Here is how they relate.

Imagine a fire hydrant with two attached hoses, each having a nozzle at the end.

One is a fire hose and the other is a garden hose.

The fire hydrant provides water at a certain pressure, let's say 100psi.

That pressure can be thought of as voltage.

Water flowing out of the fire hydrant can be thought of as current.

If you open the fire hydrant valve without an attached hose, you will get a lot of water flowing out of the fire hydrant because there is very little resistance to water flow when there is no hose attached.

If you use a fire hose, you get somewhat less flow because the fire hose restricts the water flow a bit (small resistance).

If you use a garden hose, then you get very little flow compared to the fire hose (high resistance).

The water pressure at the fire hydrant cannot push a lot of water(current) through the small diameter garden hose (high resistance).

Pressure(voltage) remains constant. Water flow(current) decreases when resistance to flow increases.

Now to explain power, somewhat.

The water flowing out of a fire hose has enough power to peel bark from a tree, because the pressure at the nozzle is still quite high and a lot of water is flowing.

A garden hose spray may be able to knock some leaves from a tree, because the water pressure at the end of the hose is quite low and the rate of water flow is much less.


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