I've never used a supercap before, though I have had one kicking about
here for a long time, a 5v 1 Farad. I have an old code entry lock,
which I am looking to install, which runs on 5v and needs a battery to
maintain its code memory at 5uA.

I don't expect the mains supply to ever be off for long, but with no
supply at all available to the lock, it reverts to a not very secure
entry code of '0'.

Would the cap do the job? Anyone any experience of using them?