There are different manifestations of energy - in particular, electromagnetic energy, work or heat - and as many different forms and sub-forms (eg gasoline, natural gas, fuel oil) of energy currency.

Each of these will vary in price by reference to a standard unit of energy, both relative to each other, and also with the relative locations of currency issuer and user and value judgement of the user.

"The future is already here -- it's just not very evenly distributed" William Gibson