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# Please explain WATTS to me! Answered

I know watts equal amps times volts. But why? What is the benefit?

Hypothetically, I wanted charge a laptop with some batteries ( instead of with the charger cord). The charger cord says 65 watts or some such, but what does that tell me?  Is 65 watts the same as 13 volts at 5 amps? Or 13 amps with 5 volts? what voltage/amperage would I need to charge this laptop?

Thanks!

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## 3 Replies

It's a measure of energy per unit time (Joules per second).

5A at 13V is 64W, as is 13A at 5V, but the right charger for the laptop or the laptop manual should tell you what you need (which is the right charger for the laptop).

L

kelseymh (author)2010-04-14

The watt is a unit of power, which is energy per unit time.  In SI, watt = joule/s.

The ampere is a unit of electrical current, which is electric charge per unit time.  In SI, ampere = coulomb/s.

The volt is a base unit of electric potential.

Electrical potential energy is charge times potential:  coulomb*volt = joule.

Put all those things together and you discocver (coulomb/s)*volt = joule/s = watt.

The arithmetic of multiplcation is trivial:  5*13 = 13*5 = 65.  However, your laptop battery has physical limits on what minimum voltage it needs in order to charge (the input voltage must be larger than it's natural output voltage), and on the maximum input current it can handle.

As Lemonie noted, read the (blank) manual.

steveastrouk (author)2010-04-13

+1