I think that either

a. The portal maintains itself via energy supplied remotely by the portal gun or

b. Once created, a portal does not require energy to exist. It's simply an anomaly in space-time. A strange loophole whereby one location is closer to another location that it's supposed to be or

c. The portal runs on chuckle-power generated by Cave Johnson's witticisms

I think the idea that the portal generates infinite energy is too complicating and problematic to be assumed.