Start by working your fields, earning your money, and storing it until a sale appears, then sell for even more money than regular sales. Once you have enough money, purchase your first Solar Panel, and let it start accumulating money whilst you keep working fields and selling for profit. Keep buying a Solar Panel every time you can afford one, until you have roughly 10 (I have this many). This brings in $3,800 every hour, and with the profits from Demand Sales too, you shouldn't have too much trouble accumulating the Money.