That’s exactly what will happen - plus there might well be a phase shift due to the regulator too that will cause further errors.
How long is your piece of string?
That’s anyone’s guess. If the PV is feeding in at the legal maximum of 254 V and your house is regulated at 220 V and you’ve not got a phase shift introduced by the regulator, then it’s out by 256/220, or 16%. I wouldn’t expect that to happen often, but it could.
I can’t accept these claims in the documentation I’ve looked at:
All domestic electrical equipment is designed to work within a range of 207V and
253V if it complies with European Conformity CE But, critically; the optimum and most
efficient supply voltage for these appliances is 220V.
When the supply voltage exceeds 220V wasted power can be generated, as both
heat and vibration; this could significantly reduce the life span and efficiency of the
If anything you own was designed in the UK and before the UK joined the EU, it will be designed for 240 V ± 6%, so running that at 220 V might use less energy but it could actually decrease the efficiency of the appliance.
I can see that you haven’t. If there’s room for another MCB in the box bottom right, you could add a 6 A MCB (that’s plenty, but you could use a 16 A) in there and feed a double socket designated for the emonPi only, one for the d.c. power and one for the a.c. adapter.