Most "stock" ship designs are underpowered and will need an extra generator adding to them.
Basic Gerax drive = 78 power usage for max speed.
Basic fisson gen = 60 power generated.
All the starting designs for frieghters, escorts, frigates and mining ships have a single generator so will have reduced speed unless you add more power yourself.
Even without considering weapons (which you should) all your starting designs should be updated with extra powerplants simply for the extra speed alone.
Pay attention to the energy collection at the top right, it is significant.
Not for ships it isn't.
Energy collection only works on stationary objects, and thus is irrelevant when determining the power loadout of ships which always move when in combat - except for deployed supply ships because they are special snowflakes.
(You will still want to have collectors on your ships though.)
Generally combat ships will want at least 2 reactors just to meet the power demand, one for the hyperdrive to ensure max speed, and two for the weapons and engines. A little redundancy doesn't hurt either.
Civilian ships will want the smallest number of reactors they can fit while still achieving max speed. Early game this will be 2.
True, all ships move - to some extent - when in combat. Although at standoff mode many will move, stop, then shoot, but i agree i would design for nor count on the collector energy when in combat. Where this does help though is combat dmg. Even with reactor shot out (or offline till repair bot can repair), an extra collector can keep weapons or some of them firing while ship fires from standoff.
Out of combat howvever, only escort and frigates classes are always generally on the move - DD, CA and up often have 'wait at' or protect planet/station missions which keep them stationary for long periods.
You noted above ships should always have collectors so this isnt disagreement or anything - just further expansion that collectors are useful on all ships, but especially useful on DD class and higher due to the frequent immobile missions they do when automated.