Recently I bought a power meter at the Bladel radio flea market. To make it easier to use this power meter I needed a measurement adapter. I could buy this, but it is more fun to build one.
I made this schematic to connect everything. The resistor on the schematic is internal in the power meter. The switch is to place the voltage measurement before or after the current shunt. This is done because the voltage measurement draws power.
For a high power DUT (Device Under Test) you want the voltage measurement on the DUT side because there is a voltage drop across the shunt.
For a low power DUT you want to have the voltage measurement on the mains side to prevent the power from the voltage measurement being added to the result.
This small circuit I build in a remote controlled socket enclosure.
Finished it looks like this: (I know I still need to replace the banana plugs by a safety model, they are on order).
Installed on the safety transformer:
Dutch 
