rabbut
I don't bite, all that often...
I forgot to mention the what if's because as Tolak has said, overstocking is fine while everything works, but as soon as something stops working, things start to go wrong very quickly. An alternative to a Generator to keep the juice flowing is a UPS power supply like the type used on computer systems.
UPS systems are great for short (less that 1hour) power cuts, but they aren't a solution to longer cutts, as they are designed to run only for a few minuites with low power draining appliences, and heating and filtration in a tank will quickly drain the power cell of these devices. IMO, a generator is the best way to go with tanks over 60l, but you may get away with it on smaller systems

All the best
Rabbut