What does a 200MW capacity power plant mean?
It sounds like you're misunderstanding power.

Power is a rate of energy generation or consumption. If my circuit is using 1 W of power, it needs 1 J of energy to run for 1 s, 10 J to run for 10 s, etc... but the power consumption is still a constant 1 W.

In your case, you have a power plant that can generate 200 MW. It can do this for 1 s (generating 200 MJ of energy), 10 s (2000 MJ), or any greater length of time - but it cannot create more than 200 MW of power.