Green Energy

Teach a man to fish, and you feed him for a lifetime.

Convince a man to let you build a fishing pier with exclusive fishing rights in his pond in exchange for giving him a fish every week, and you've got a customer for life.

In reality, China doesn't need to be so crass as to demand exclusive rights. The mere fact that these are Chinese panels with Chinese hardware means Cuba is going to be dependent on China for as long as they aren't willing to throw out a massive chunk of their infrastructure. As long as this move comes with some change to the material situation that makes Cuba pay China back and then some, China gets richer from it.

Capitalist corporations and nations have done the same for centuries. European colonial empires kept advanced manufacturing in Europe so the colonies couldn't become independent without a massive reduction in industrial capacity. Because of this many former colonies are still economically dependent on Europe and/or the US. China is simply coming in and offering them a better deal.