In the UK, domestic mains voltage used to be standardised at 240V +6% -6% so a range of 225.6V to 254.4V, in 1994 it was decided to harmonise with Europe at 230V, but with a fudge of -6% +10% so a range of 216.2V to 253V, but still nominally 240V. Europe changed from 220V to a nominal 230V.
247V at the top, 244V at the bottom, 0.5V per Div, Horizontal scale 30Sec per Div. Change of 2.5V over a matter of minutes, makes it hard to calibrate to 0.1V.
Tasmota energy monitoring has 2 commands to set voltage, VoltageSet<x> Calibrate voltage to a target value in V with x denoting which channel if more than 1. & VoltageCal<x> range of 1000 to 32000 (Default 1950) VoltageSet changes VoltageCal to suit the target Voltage.
Several ways to calibrate the voltage, using a calibrated voltmeter.
Buy a mains Voltage calibrator (several £k)
Use a pure sinewave UPS or inverter (several £100)
Setup an audio generator to 50Hz, plug into an audio amplifier, connect to a low voltage transformer used in reverse, rated so no distortion, rough guess for about 50W is a 20V secondary (used as primary) transformer, transformer rated about 100W, allows enough to power the device under test. (Several £10)(This appeals to the geek in me)
Last is setup a Voltmeter & the device, feed with mains Voltage, repeatedly set the voltage until near enough, this is what I did, but it rankles that its not as accurate as it could be, especially when you want to get several devices to agree with each other.
