BMW may not race KERS in Melbourne
The German squad have been one of the main supporters of the new system and were among the first to run it in their car last year.
BMW were also against the idea of delaying the introduction of KERS, while many other teams thought it was still too early to race it.
Theissen said KERS remains an exciting challenge for the team, but he conceded the system is not yet ready and may not be for the season-opening Australian Grand Prix.
"KERS is still the most challenging and exciting part of the new package to me," said Theissen during the launch of the new car in Valencia.
"Looking back to when we started more than a year ago on KERS, that time was pure research. Then we went through a stage I would call pre-development. Now we are in the development stage.
"We are still not ready to race, but if I look at what progress we have made in last 12 months, it's amazing. We have learned so much.
"We are still pushing hard. We are not ready yet. I am sure we will be ready at some point, I don't know whether we will be ready for Melbourne.
"That is the character of innovation, you take risks and you don't know when they will pay off. I'm pretty sure it will pay off at some point in the season and that it might become the crucial factor."