There are two ways to get this cheevo depending on your style.
One way is to just play. You will get a million easily enough along with several other cheevos. This one just comes along with playing and having fun.
If effort is not your thing, then it comes down to letting the game run.
Start a new save on easy and sell everything in your inventory and take out the max loan. Purchase three solar panels and place where you'd like. Crank the game speed to 120X and let it do its thing. If you leave just the three it takes around 18 hours.
If you want to cut down on the time you can purchase a new solar panel once you have enough money saved. I stopped at 7 panels which took about 13ish total hours to complete everything. That's about 5ish hours to get all the panels and then another 8 hours to run unattended and generate the million.