Works when placed between Lava and Water (or Ice). Place lava under it and water on all four sides, and have a Blue Alloy Wire coming out of the top.
Using a Voltmeter it is possible to read the power output of a Thermopile.
Connecting one Thermopile (with a lava block under it and four surrounding water blocks) to a Battery Box through a Blue Alloy Wire will produce an output between 0.46A and 0.50A
Bear in mind that each Blue Alloy Wire between the Thermopile and the Battery Box will reduce the output by ~0.01A.