I am trying to calculate the MTTF of our electromechanical system after gathering field failure data over the years.

The systems are being installed gradually so it could be 1000 installs in year 1, 1500 in year 2, etc... This means that the population is growing steadily and the failures are below 0.5% per year.

I have the install date of each system and I have the failure date of the failed units, in this case I will consider that once a system has failed its non-repairable. The current population is 5000 which are still alive and going, and 200 which have failed.

My questions are:

Will MTTF make sense in this case, or will it be just a confusing metric?

For calculating MTTF, do I take the operational hours of the whole population up to today, or only the population that failed and the amount of failures in that sample?

Thank you in advance for any help.