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I have a simple question about watts... Answered

Watts = amps times volts, correct?

So if I had a laptop that charged with a 60 watt power supply, could I:
-Charge it with 10 amps at 6 volts? (that is the same as 60 watts)
-Charge it with 6 amps at 10 volts? (that is ALSO the same as 60 watts)
-Charge it with 5 amps at 12 volts? (that is ALSO the same as 60 watts)

Watts is just how much 'energy' passes a single point every second (or whatever), so it doesn't matter how you get the watts, watts is still watts?

I'm still trying to grasp the whole concept of watts, so yours answers are infinitely appreciated!!!



Best Answer 8 years ago

1) Yes.

(two-part answer follows)
2a) No.
Most devices, such as a laptop charger, have an operating voltage. If you change that voltage, you will damage the device.

Because devices have a predicable load (think resistance, though it is somewhat different) at that operating voltage, they draw a specific amount of current (amps). And therefore a specific wattage.

In the case of a laptop, the load will vary because the battery will change it's state of charge. So the load can increase, but the charging circuit will compensate to prevent damage. The wattage will increase when the battery needs charging.

2b) Yes.
In theory you are correct--for a theoretical device that wouldn't be damaged by different voltages.

A simple high-wattage resistor, for instance. For a change of voltage, there will be a change of current drawn, so the wattage will also change.

3) Yes. Watts is watts--a measurement of energy consumption.

One more little piece of info: Why are watts used as a unit of power?

Because they allow a single figure "universal" comparison for devices of different voltages and current draw.

Otherwise both the amperage and the voltage must be used in the comparison...

You are confused because you are trying to understand a concept (watts), without FIRST understanding the simple relationships of VOLTS, OHMS, and AMPERES as found in the ohms law equation.  I will explain this so you dont need to go through hundreds of ohms law problems.  If you take a KNOWN load-resistance such as a 10 ohm LIGHT BULB.  Place 120 volts on the bulb and it lights up brightly and uses 12 amps... amps equals volts divided by ohms.  Now turn the voltage down to 12 volts.  The bulb only uses ONE amp now.  Notice the bulbs resistance remains the same, while voltage differences change the amperes used.  You can also calculate the WATTS the bulb uses by multiplying AMPS times VOLTS.  12A x 120 volts = 1440 watts..... and the 2nd example 12V x 1amp = 12 watts.  

This light bulb is a 120 volt lightbulb.  It will not light up very bright at 12 volts.

In this same way,... your chargers will not work ANYWHERE just because they use the same WATTAGE!!!   The charger must put out the correct voltage and amperage for the item being charged.  

Watts = power.

But no, just because you can get to 60 W by 1V x 60A or 60V x 1A, they are not equivalent, except for measuring the amount of power used (or required)

devices not only require power, but they require electromotive force (EMF, or more commonly, Voltage) and flux or amperage.

imagine this

you try to push N amps thru a diode, which has a forward voltage drop of .6 V. Well, let me tell you, if you have an EMF of 0.1 V to drive that current, i don't give a hoot if you have a million amps available, it ain't gonna pass that diode, since it can't get high enough to jump over the .6V hurdle...

Didn't you already ask this question? You did, I remember giving an answer.

Get the correct charger for the laptop. Or find out what the battery should be charged with, then ask how to build.


Funnily enough, "Groundhog Day" has just finished on the telly.


I just needed to clarify some things

Why didn't you read the answers the last time you asked this?  We explained that the battery does care about the current and voltage separately, not just the energy.

Unfortunately, it DOES matter. The circuit requires a specific voltage, and will need at least the rated number of amps.  If you have a power supply that reaches 60 watts as the product of two values other than these, it won't work and may damage the circuit and/or the battery you're trying to charge.

"Watts is watts" only in the sense that you can convert from 60W at one voltage to almost 60W at another voltage by using a transformer (for AC) or a DC-to-DC converter (for DC) -- "almost" because you will lose a bit of power in the process -- and that you can get the same amount of work out of 60W however it's supplied IF the circuit which uses that power is designed for that voltage.