On Sunday, 13 May 2018 09:11:38 UTC+1, Harry Bloomfield wrote:
Rough check - turn everything off, except for one 1Kw heater. Run it
for 60 minutes and the meter should clock up exactly 1Kilowatt hour.

Or a 3 kW immersion heater for 20 mins, a 10 kW shower for 6 minutes, etc.

Owain