The power increases with development work throughout the season. At the start of 2004 most of the top engines were in the 900 to 940 bhp bracket. By mid summer they were up to 940 to 970. The info is relatively freely available if you know engineers working for the F1 teams but it won't be divulged in writing of course or even over an F1 team phone line which could get someone sacked if it's monitored. Over a few beers in the pub is the best way.

Quite amusing looking at the "guesstimates" earlier in this thread though