Is policy making in the area of energy an expressed power a concurrent power or a reserved power?

The correct answer is "concurrent power".

Concurrent powers are powers shared by both the Federal Government and the State Governments. These powers may be exercised simultaneously by the Federal and State governments in relation to a particular state.

In regards to energy, policies lowering state taxes on renewable energy projects may be enacted by state legislatures. However, if the Federal government deems it necessary, it might simultaneously raise taxes for overall energy companies. This evidently includes renewable energy.