how to take wattage of anything?

Did you ever wonder how many watts your equipment is using? Whether it is a PC with monitor, or a whole UD cluster setup with 100's of machines this article will let you know how to easily find out. Or perhaps you just want to know how much power you would need to build one of those clusters. You can estimate yourself since some appliances such as monitors show the rating on the back but some other things such as computer power supplies don't. And just because it's a 300W power supply does not mean it uses 300W of power. It all depends on how much of that power the components are using. Watts are usually the best measurement to know because it does not change. But volts and amps do. If you plug in a vacuum cleaner in an 80volt power source it will be using way more amps then if you plug it in a 120V power source.

Watts is equal to volts times amps. So basically the higher the voltage, the fewer amps being drawed. Amps are also the actual current and what decides on the power of the circuit. What I mean by that is, if there's not enough amps, you can't run much, even if the voltage is high. A static shock can have millions of volts passing through you but yet it's not enough to power a light bulb, because there are very little amps. Amps also cause wires to heat up, the more amps, the bigger wires you need since if they are too small they will melt. This explains why power cords for large equipment are thicker, while a small lamp is just a cheap cord that you can cut with a pair of scissors, since one uses more amps but same voltage, and this results in more watts as well. Because of this heat though, there is power loss, so this is why when power is delivered to far distances, it is sent in high voltages, so they can use smaller cabling (cheaper) and there's less loss of energy from heat. So it's more efficient. But just because of what I said about the static electricity, don't think you can go climb one of those poles and get a shock that tickles.

Enough theory, let's get started.

First this is what you will need:

A multi meter: One that can measure AC amps and AC voltages and should have a high enough rating. These can be purchased at a Radio Shack or similar store for about $100 and are very useful around the house. Crocodile wires or other wires easy to manipulate and plug. An old power cord with both ends open to connect the crocodile clips to it easily. (or equivalent, basically any thing that can easly be plugged and unplugged from the socket) Calculator (everyone has at least one around the house) Safety common sense (this can be hard to find in some households) Like mentioned earlier, Watts = Volts X Amps. So we need to know how much volts we are giving to the equipment, we know it's near 120 because that's the normal house voltage. But we want to be more exact, so set your meter to measure volts AC and put both ends in a socket that is on the same circuit as the equipment. For safety purposes, try to find a socket that is easy to access and has plenty of room to move quickly in case something happens. But if you have everything set correctly and don't short it out somehow, you're safe. Just make sure it's set to Volts AC and if your meter has different ranges, set the right range so you don't overload it. But most good meters will auto sense the range for you. Because my plug has too many wires near it at the PC, for this picture I'm measuring a different circuit but if your house is wired like mine there should only be 2 main circuits (two separate AC phases) so they should both be near anyway. I get the same reading here and in my room. The voltage tends to move a bit. In my case it was around 119 and 121 so let's just say 120.  this article is copied from icetecks.com