Many calcs of breakeven fail to include the "opportunity" cost of capital. This means:
- Loan interest charges, if you are borrowing the money, or
- Alternative Income earned, if you have the cash and invest it elsewhere
- Throw in the extra cost of managing, building and operating your own power company
All these things will lengthen your breakeven time and reduce the attractiveness of your DIY solar project.
If you know this and still want to do it anyways, that's fine but you should recognize that it's more of a hobby project and not really a pure investment decision.
Sorry to rain on your parade, but let's be realistic here.