I believe it's in comparison to normal Zinc carbon batteries
They say 'extra power'. That is different from 'capacity'. Should be about 2.25AH, the Ultras being over 2.5AH and standard 1.9AH. Discharge standard Duracell at 500mah and the capacity becomes paltry, this likely being where Duracell derive the word 'power from'.
"Power Plus: up to 40% more power than standard Duracell batteries" Or maybe +50% It's think-of-a-number time! Has anyone ever seen the evidence for Duracell's marketing puff?
That’s a very good price - I paid just under 50p battery for the same brand/type