So there is an unofficial summary of the Dragon Ball Super Broly movie from apparently someone who watched it in Japan in an exclusive event, and there it claims Broly when he was a kid had a power level of 920 (stated in the movie). But in the Dragon Ball wikia, they state the new Broly (the Broly from the new movie) had a power level of 10,000 when he was born (as stated by the old non canon movie IIRC). Which was the power level of Broly when he was a kid, 10,000 or 920?
I have not watched the new movie, but if in the new movie they have stated that his power is 920 when he was a kid, then that is his "true" power level.
The old DBZ movies were never considered "canon", since we can not fit them anywhere in the storyline. The Broly movie was not even directed by Toriyama as you can see here, so it is kind of a "spin off" series, events that happened here had no impact on the DBZ storyline.
Regarding Super, things are a different. The first two movies "Battle of Gods" and "Resurrection of Freiza" were showed in the anime itself, although there were some differences between the anime and the movie. The Broly movie is direct continuation to the anime story line, since it picks up after the conclusion of The Tournament of Power. Furthermore the movie is written by Toriyama which kind of establishes it as part of the canon now.
So to answer the question, his power level is 920. The Broly in the new movie is a new character (which is based on the popularity of the old one) that is part of the canon storyline.
When Paragus scans Broly's power level, it is shown to be 920. However, there is supposedly a scene in the movie where a Saiyan Personnel's on attempting to scan Broly's power level receives abnormal readings and one of the scouter even breaks due to his fluctuating power level. You can read about it in the plot summary here.
You should completely disregard all information with regard to Broly from any of the previous series while watching the new movie as Broly has been written as a completely different character and this character is canon to the original series.