A Big, Bolded Disclaimer,
this guide assumes that you're playing on a windows 10, but if you know the equivalents, you should be able to effectively do the same thing on a Mac or a Linux OS.

Step 1 | Humble Start
First, get your game to a point that you're getting a decent amount of watts per second, you should do the exploit when you know you'd be getting alot of watts in the future.

Step 2 | Leave the Game
After knowing you'd be getting a decent amount of watts in the future, turn off the game. It should now be in an idle state. Allowing you to do

Step 3 | Confuse the game by adjusting your computers time.
On a windows 10, you can go down to the bottom right where the time is, and right click it. After right clicking, near the top you should find "adjust date/time." Click that button and you'll get sent to the setting app to adjust the time. Turn off "Set time automatically" and wreak havoc by setting your time off by an hour, or even a month off! (I personally haven't done this in so long, so just stick to having a year offset as your maximum for each time you want to exploit the game.)

Step 4 | Profit
After setting your time to a month off, or a year off, get off of the settings app. Run SpacePlan after hopping onto Steam again, and you'll see that you've gained all of the watts that you would gain in the time that you've just hopped.