Tesla's Wardenclyffe Tower could be powered with an Earthly power source (Tesla just lacked funds). But, when The Doctor powered it with TARDIS in the recent episode "Nikola Tesla's Night of Terror", she had to turn off the force field. Maybe, to destroy the spaceship, Wardenclyffe Tower needed more power. But, earlier in the series, the TARDIS:
almost destroyed the universe.
kept Earth warm for billions of years (during "Total Event Collapse" before Big Bang 2.0).
dragged Earth from one part to another part of the universe.
In the episode "Journey to the Center of TARDIS", The Doctor said this about TARDIS' power source:
The Eye of Harmony. Exploding star in the act of becoming a black hole. Time Lord engineering. You rip the star from its orbit, suspend it in a permanent state of decay.
This seems an infinite power source. Why, then, couldn't the TARDIS sustain a force field after powering Tesla's Wardenclyffe Tower?