Not everything from what you said is clear for me. Does it mean If I have 48v 500w in my bike but will use only part of its power it is bad for the motor? Whats optimal engine load for BBS02 500w 48 and for 48V 250W TSDZ2? Explaeqin it as simple as you can please.
I gave you information on the BBS02. If you don't derestrict your bike, you only use about 100W out of 800W that the BBS02 is capable of. You will not damage the motor. You will of course need more than 100W to climb hills, that temporary increase in power is permitted legally as long as you ride under 15.5mph. The kit will heat up more but it's only for a short duration while climbing hills so the risk of damaging the motor remains small.
The problem arises when you derestrict the bike. That's when I cannot anticipate how you are going to use it.
Most motors are damaged because the riders don't realise that they are in the wrong gear when climbing (too low RPM), or
ride at maximum speed for a long time, or pull off at full power from a stop or hit a large pothole causing a spark at the battery connector.