How does EmonTX calculate mains voltage?

My EmonTX is telling me that the mains voltage is 260.61v
My Invertor says 250.11v

How does the EmonTx work it out and If its via the ac/ac adaptor does that mean its likely
that the ac/ac is getting to the end of its life?

No, it simply means that your emonTx has not been calibrated. All components are subject to normal manufacturing tolerances, and details of that, how they affect the voltage, current and power measurements, and how to perform the calibration, are all in Resources > Building Blocks.

Is that tolerance likely to change over time? Asking as the voltage does change during the day.
ie solar panels start working & the voltage lowers, end of the day and it rises again…

The specified tolerance cannot change, but the component’s value within the band might, but not substantially. The divider resistors might show a very small change due to ageing, but unless you have some serious test instruments, I doubt whether you would have the capability to measure it. They will also show a small change due to temperature, but hopefully in each case both resistors will change by a similar percentage so the effect on the ratio will be minimal.

You’ll probably find that the daily voltage variation has more to do with the working day and when capacity is put onto and taken off the grid in order to match generation with demand. Whenever I’ve watched voltage (and frequency) late into the evening, voltage and frequency both rise gradually as people go to bed and demand falls, then both quickly fall as a generating set is taken off, and then both slowly rise again as demand continues to fall.

Ok thanks for the info.
Sounds like i need to do a calibration !
Think I’ll start everything from clean with a new SD image and calibrate emontx.

The SD image has nothing to do with the emonTx, it only receives the data from it.

If you have a programmer, the ‘cleanest’ is to calibrate your emonTx. If you don’t, it’s quite easy to tweak the multipliers in the “scales = …” line in emonHub to achieve almost the same thing. (“Almost” because you can’t calibrate the phase error that way.)

My SD image is also waaaaaay old & I want to bring it up to date - ie its just “another” one for the list.
I have a programmer, so I’ll calibrate the image.
There probably is a newer image for the EmonTx as mine hasn’t been updated since I bought it about 2 years ago!
Doing the calibration should be “fun” !
Thanks for the help