I normally build one power plant on every planet. If the planet isn't extremely tiny, I also build a factory. If the planet is of reasonable size and the layout works, I'll build more factories in a formation around the power plant.
Q: Why a power planet?
A: Long term the power plant has the best +% for a single hex of space, topping out at +100% while a factory is only +50%.
Q: What do I do with my manufacturing capability?
A: Due to how painful coercion is when you force your populace to dedicate themselves too much in one direction or another, I normally run my empire with a 44-44-12 manufacturing-research-money split. This means that (for me) every planet will be dedicating a good chunk of it's production towards manufacturing indefinitely. For a long time this mostly goes into constructing buildings, but when there are no more available that 44% instead gets dedicated towards a shipyard instead. Small planets with only moderate production can still produce constructors fairly easily.
Hope this helps.
- Manii Names