How do I turn on the Gamepad's IR without turning on the Wii U itself?

I use it to play PC games with a Wiimote, but my main concern is with power usage. I don't like the idea of having both my PC and Wii U taking up electricity.

Short answer:

You can't.

Long answer:

You could mod the gamepad to only turn on the gamepad's Sensor Bar, but if you could do that, you'd have significantly better options to choose from.

The "Sensor" Bar, unlike its name implies, doesn't have any sensors at all. It's simply made up of 2 IR lamps. That's it.

The actual sensors are in the Wiimote. The Wiimote senses the IR lights emitted by the Sensor Bar, and uses position, size, and distance of the lights to calculate where the cursor should be pointing, its rotation, and distance to the TV.

The Sensor Bar is so simple, in fact, that anybody could simply place two candles in front of the Wiimote, and it would work. Alternatively, two regular lamps would work as well, although they'd have to radiate enough heat for the Wiimote's sensor to pick up.

A better solution would be to use two IR lamps and create your own sensor bar. You can harvest IR lamps from TV remote controls. Or you could buy a Wireless Sensor Bar instead, which can be turned on even with the Wii U turned off.