Metered electricity lead

I have one of these commonly used leads for my bateau when plugged in -

View attachment 69904

and wondered, how I can check its working correctly ? Is it measuring Kwh and what does the 1/10 actually mean ? Thanks in anticipation

I’m guessing the final column records tenths of units. If you plugged in a 1kw fan heater, for example, the 3 in the final column should change to 4 in six minutes - a bit rough and ready, perhaps :)
 
The nice people at our marina come round and check the calibration every year. Have an ask at your marina, they may well have the kit and will check for you
 
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