If you are like me, you need to measure electrical current (Amperes/Amps/A) so you can figure out the power used by your electronic circuits (Watts/W - or Volt-Amps/VA) - not to mention the power used by your toaster (a surprising 1 KW).
You need a Clamp Meter! I currently use the Agilent U1211A.
When I first got a Clamp Meter from Fluke Inc., I could not get the thing to work, I put it around every electrical wire I could find - no effect. Only later did I realize/remember that the two AC lines were cancelling each other out - I needed a line splitter.
Therefore, to measure AC and indirectly DC current you plug in the line splitter into your 120V ac outlet and clamp the Clamp Meter around one of the 1x or 10x split lines. Your clamp meter will experience a magnetic field proportional to the current and display the Amps for the 120v line.
You take the Amp reading (say 5.56A) and multiply by the Voltage (usually 120V) and get your Wattage (W = AV) or 657W. Since Physics 102 I remember this as WAV or PIE (as in power = current * electromotive force).
In the photos, I am running 4 PC's, 2 monitors, and XMOS processor and a Parallax Propeller based obrienlabs LED display board.
The amp reading comes in as 5.5A on the Agilent clamp meter and 3.8A on the Sperry clamp meter - why different readings? True RMS - more about this later.
The Agilent U1211A True RMS Clamp Meter is excellent and comes with a calibration certificate - therefore I trust it's reading.
The Agilent meter (formerly part of Hewlett Packard - HP) can measure up to 1MW, it is a little overkill but worth the price. One observation is the fact that for low wattages like 100W it takes about 5-15 sec to stabilize the Amp reading due to the small magnetic field on the clamp.