Where were they getting power from?

From imagination.

Wasn't explained in the movie, so imagination as Geff suggests is all we got.

but we do have some clues:
-We know the father was a tinkerer or sorts.
-We know that they lived near running water,

so we can guess that their power system was set up to be self sufficient prior to day 0.

a) hydro (my best guess)
b) solar
c) wind

we did not see any wind turbines, solar panels, hydro dam or energy storage location, but then we didn't see all that much outside the farm and their path to the river/town.

Prolly the solar panels the movie clearly showed.