Making sure you’re not overloading your UPS or misc power source
By Red Squirrel
So turn off and unplug the device you want to test, in this case a monitor. Connect everything like shown in this picture. The arrows indicate the flow of electricity to make it easier to follow the circuit with all these wires all over. Note that this flow of electricity changes 60 times a second (50 times in some regions, the UK I believe) so this is why AC is different than DC.
So the white wire is plugged into the mains, and the black is the plug for the monitor. Completely ignore the ground for testing, DO NOT PLUG ANYTHING TO IT! In a permanent setup you would want to ground it, but for testing purposes it's ok to have it ungrounded for a bit, just be careful and if you feel safer ground a crocodile clip and connected to the grounding pin, but be careful since there's more chance of a wire slipping and shorting out. In the picture it's hard to see but the two wires connect to the two pins of the plug, so think of the two pins as the + and - of a device that needs power and the two wires as the + and - of a power source, since this is what it is, but with AC so the + and - order does not matter, but make sure the two wires don't contact each other since that's a short and you can blow the meter.
On the amp meter, the terminals used are the unfused 10A max and the COM. The fuse in the meter has a max of 500mA so it's best not to take a chance at blowing it, when we're measuring this much amps, and chances are you won't measure anything more then 10Amps. Never measure an entire power strip, but each item individually, to make sure of this.
When setting up the wires, make extra sure to unplug the cable that is feeding the power (the white one in this case) Once completed let part of the circuit broken like in thes picture below, plug in the cable then take both wires (the red crocodile clip and black meter cable in this case) and tick them together quickly just to be sure you won't cause some kind of short. Again, this is only to make sure you did not mess anything up such as badly setting the meter, or if some wires are touching somewhere that should not. Then hook them up together solidly and turn on the device you are measuring and you will get a reading. Lot of equipment have transformers and stuff which use up a bit of amps even when turned off, so this is why you will sometimes get a reading even when it's turned off. If you calculate all the devices in your house like this, you may be using a few amps from items just sitting there turned off!
The following pic shows the circuit unplugged and ready to go.
Take note of the reading and repeat this process for each item. So when done you should have something like this:
House voltage: 120V
PC full load: 1.2A
PC without fans, lights and UD: 0.8A
Once you have all the readings, just do Volts times Amps. So...
120V X 1.2A = 144W
120V X 0.8A = 96W
120V X 0.5A = 63W
Since the first 2 readings are the same device you can decide on if you just want the average, or the most widely used. In my case I put the most widely used, since it's very rare that I turn off UD (0.2Amps) or the lights. Sometimes I may turn off a set of fans but those don't make a big difference. So I'll stick with 144W.
So now we simply add 144 + 63 which gives us 207 Watts. So now I know that my PC and monitor which are plugged into my UPS use up about 207Watts.
Also note that this is only the base of the concept and because this is AC and not DC, it get's more complicated. Amps are sometimes not in sync with the volts and unless you are using a RMS (root means square) meter, you may get inaccurate results. Some meters don't even tell you if they are RMS or not, so I'm not even sure if the one I used is or not but according to what some say these readings seem about right for the equipment used. Basically, try to not go too near your UPSes max wattage and you should be safe. Some UPSes also don't even tell you how many watts it can handle, but rather VA (volt amps) which are slightly different.
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