UCEsat is the saturation voltage of BJT: when the transistor totally open, this voltage can be measured between emitter and collector. This voltage multiplied by collector current, give the power dissipation of transistor.
See the crop of datasheet: UCEsat typically 0,28V, but the max. (worst case) is 1V. When the collector current 1A, the dissipation 280mW: this is good, but in worst case (1V), the dissipation 1W! The d'sheet give also the max. rating of dissipation:750mW, so the transitor make a smoke.
Why better the FET? The same parameter called RDSon, this is a resistance value, and You can find some types with 0,001ohm value!
Just use the ohm's law.
So, if You have a good 2SD965, just try it.