I am a bit confused about the difference between current and power. I am hoping someone can explain the difference to me maybe using an analogy or something.
I do understand that current at a point is a measure for how/much charge (in coulomb) is passing through the point (per second). I think I understand Ohm's law which says that over some component with fixed resistance R and voltage drop V there will be a current of \$V \over R\$.
I understand that power is much energy in joules is "deposited" in the component per second.
Some examples of where my confusion comes from: Usually when one buys appliances they have a wattage rating. So, my microwave oven uses (up to) 800 W. So it seems that wattage is the important thing to understand in many cases.
A power supply is (from what I understand) usually given with a (max) voltage and a (max) current. Why is the wattage not given here?
If I have an LED that can handle maximum 3V and I also have a 9V battery, I would need a resistor. But to find the size of the needed resistor I need to know how much current the LED uses. I have been told that one should find this in the datasheet of the LED, but that LEDs rarely come with a datasheet when you buy them. I have been told that it is common to assume that the LED uses about 20 mA. So I can understand how I can calculate the needed resistor. Where does the wattage come into to this situation? Why isn't the wattage listed?. Also, if one has a complex circuit that can handle only 5V, but one only has a 9v power source, then how would one know the current?
Maybe my confusion is that it seems that a certain current should always give a certain wattage.