What is Electric Power and Watt?

Electrical Power is the rate at which energy gets supplied, or used up. Instead of constantly saying joules per second, we have a unit that we use for power, called the watt. 1 watt is equal to 1 joule being transferred per second. So for our example with the motor, 1.8 joules per second means that 1.8 watts of power are being delivered to the motor. If we increase the voltage to 2 volts, more current flows and more energy gets transferred for each unit of charge, so now we are getting 4 joules per second, or 4 watts of power being supplied to the motor. Since more work is being done per second, the motor spins faster. Now in real life, no engineer is going to waste time converting volts to joules per coulomb, and amps into joules per second.

You can instantly calculate power with this very simple shortcut. Voltage x current = power.

Let’s try this formula out with a real life example. This is supposed to be a 40 watt light bulb, but I want to see how much power it’s actually drawing in real life. I’m playing with high voltage mains electricity here, do NOT try to replicate this experiment at home. I’m using a standard north american wall outlet that supplies 120 volts, and my multimeter is showing that we have exactly 119.6 volts going to the light bulb. Now I’m going to switch my multimeter to measure current, and we can see the light bulb is drawing 337 milliamps from the wall outlet. 119.6 volts x 337 milliamps means that this 40 watt light bulb is actually drawing 40.3 watts, so it’s converting 40.3 joules of energy into light and heat every second. Now I want to really emphasize that it is voltage AND current that leads to power.

Voltage and current can be combined in different ways to get the job done. For example you could design an LED lighting system that runs from 12 volts, and make it so that it draws 10 times more current, and you’ll still get the same 40 watt draw. And since LEDs are more efficient you’ll get also get more light from every watt. But not every situation is going to be better off with low voltage. For example, a high powered robot might actually be more efficient with a higher battery voltage. Speaking of LEDs, in my previous video on basic electricity, we had an example with a low current, roughly 20mA, flowing through a resistor and an LED. And the resistor got so hot that it went up in smoke! Now that we know about electrical power, we can explain why this happened. We know that voltage x current = power. And from my previous video about resistance and Ohm’s law, we know that voltage / resistance = current. So let’s substitute the current term in our power equation with Ohm’s law and see what happens! Using a little algebra, you can see that another way to calculate power is voltage x voltage/resistance. Or power = Vsquared/R. This equation will let us find out the amount of power being delivered to a resistor when all we know is the voltage and the resistance. I was powering the system with 140V, and the LED had a 3 volt drop across it. So this meant that the resistor must have had 137V across it. The power being delivered to the resistor, in this case heat, is given by Vsquared divided by R. 137Vx137V / 6800R = 2.76 watts of heat. The resistor I was using was only rated for 1/4 of a watt of heat, so no wonder it went up in smoke! If we wanted something that could handle the 2.76 watts of power, we’d need something a lot bigger, something that could dissipate more heat, like this resistor that is rated for up to 5 watts. Nearly everything in electronics will have a power rating, which basically means how much power can it handle before it goes up in smoke. As you learn more about electronics, you will find that things like diodes and transistors will all have their power ratings listed in a datasheet. And of course, other things will have power ratings! Like light bulbs, electric heaters, microwaves, and this refers to how much energy they are converting per second.