Previously the generator was just a fixed timed event. You put a fuel cell in, it ran for 60 seconds and gave a flat amount of power per second. That worked fine, but was incredibly inflexible. Now, fuel cells have a fuel value, the generator has fuel storage, and burns an amount of fuel per second. This allows for:
- Items having different amounts of fuel
- Adding multiple fuel items to the generator at once and letting it run for longer
- Upgrading the generator to burn fuel faster
- Upgrading the generator to burn fuel more efficiently
- Upgrading the generator to hold more fuel
Not seeing the time remaining is a known drawback. There is a resolution to it planned, but there wasn't time to implement it yet.