If you have the DLC, head to the Isle of Armor, and find Digging Pa. He can give upwards of hundreds of thousands of Watts, and his services are quite cheap.
If you don't have the DLC, you can participate in the Rotom Rally. Alternatively, traverse the Wild Area's dens, and pick up all the Watts from the glowing red ones. After beating the game, they yield around 2,000 Watts each if there's a Pokemon inside, and 200 if there is not.
You can also find and capture/knock out Brilliant Pokemon. They yield a couple hundred Watts each time you capture/knock one out, but it's a decidedly ineffective way to get Watts.
A glitch that allows you to get many Watts is also able to be performed, detailed here.
Hope I helped!