This new source of energy has a large market

This new source of energy has a large market
 

-This new source of energy has (a) large market potential.

Is the indefinite article optional? Is my sentence natural?

The indefinite article is required. Your sentence is natural.

Thank you~

The indefinite article before the direct object is not required.

What does this new source of energy have? It has potential.

Here are a few other examples:
This investment has little growth potential.
Smith has great potential as a sales manager.

THank you~

I concur with Rusty's examples of ways one can use the word 'potential' where we do not use the indefinite article.

Thank you~