TBH we would avoid sites with those sort of meters as the customer generally loses i.e. you need to top up the meter on the morning before you leave and do not use up what you paid for and no way of getting a refund. Secondly not every one carries a bunch of coins with them to constantly top up a meter. Thirdly these meters should be checked annually and also calibrated at the same time. It will take a long time for them to recoup the outlay for fitting the sub meters.
We use a commercial site where you pay a fixed rate of £3.50 a day. If at the end of your stay you are in credit, this is refunded back to your card. Excellent way of dealing with increases in electric and if they can do it, why can't other sites do it.
No objection to paying the extra for electric, but wary about the way some go about it to charge the extra.