Drying out walls - how is electricty use metered ?

Something like this would do the job.

https://www.amazon.co.uk/dp/B07H1ZFVFM/ref=cm_sw_r_cp_tai_n24CDbTT1YGVA

This might also be handy.

https://www.ukpower.co.uk/tools/running_costs_electricity

Edit to add: I changed the first link because the original was an overly fancy one that needed a a smart hub.

Is this an insured water escape type loss/claim? Are they being run 24/7?

I'd expect the dehumidifiers to have hour meters on them - you'll see them if you look on the control panels. The insurers contractors will take start and end readings to calculate the total power used by them. Alternatively they'll use the estimated running time (N days at Z Watts x 24) to work out how much they have used assuming they are on 24/7.

That's what our insurers did for the units we had drying the bits. Only the large fan to move the air in the garage had no hour meter.

We had to supply the unit rate via our electricity bill for them to reimburse us.