Powerhouse: chiefly US : someone or something that is full of a particular thing
http://www.learnersdictionary.com/definition/powerhouse
(Merriam Webster Learner's Dictionary)
According to this thread growth-powerhouse -- meaning?
As Adam has indicated in his comment, powerhouse is often used metaphorically to indicate something which is known to be strong or powerful in some respect.
Take this example from Merriam Webster Learner's Dictionary
He is a powerhouse of ideas.
I find this sentence still hard to understand. Does it mean he can produce a lot of ideas (a source of ideas)? Or does it mean he is a source of inspiration (he inspires others to have ideas)?