The EU-Reader takes measurements over a period of time and provides the average EU per tick. To use, right click on any cable or storage device. The message "Starting new measurement" will appear on screen. Then right click again on the same cable or storage device and a message will appear with the "Measured power". It does not use EU for each use so it can be used as many times as you want.
If you right click on a cable, then right click on a different cable, the EU-Reader will begin a "new measurement". Only if you right click on the same block twice will it give a measurement.
Note: EU must be running through the cable or storage unit being measured in order for the EU-Reader to work. For instance, if an MFSU is completely full, any cable supplying the MFSU will show 0 EU/t from the EU-Reader since no EU is actually running through the cable.