Tim+ expressed precisely :
http://www.bbc.co.uk/news/uk-39169313

"A system error in smart meters has led to customers being quoted thousands
of pounds for a day's usage of gas and electricity.
Confused customers posted the strange readings from their SSE smart meters
on social media, with one display showing almost £20,000 for a single day."

Tim

The meters don't calculate the cost, the in home display does that,
using consumption and costs fed to it from the meter(s).

My two, as near as I can tell, have been spot on since they were fitted
last June (then on OVO)- that despite not functioning as SM's since I
switched to First Utility. FU have emailed to say they will be
re-enabling both SM's as SM's, with remote reading in 28days.