On 13/05/2018 11:33, Andrew wrote:

Go round the house and unplug everything, maybe except broadband,
and switch on something like a 1kw fan heater or if you have them
a collection of 60w or 100w bulbs for exactly one hour.

Is that heater and collection of bulbs exactly a 1KW load or do they
have a tolerance inaccuracy _much_ greater than that of the meter
measurement.

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