Honestly I think even my number is a little optimistic because I doubt his battery lasts 2 years. Most lead acid batteries are only good for 100-200 100% cycles. I also doubt that the PWM controller or the inverter last 30 years. I've seen failure rates as high as 50% for cheap PWM controllers like that after 10 years. The inverter is likely similar. Sure you can buy stuff that will last 30 years but that will be much more expensive. 8 years seems like an appropriate amortization schedule which puts my $100 at 6.5 years as a little generous.