You have to factor in two important things:
"$28k cost after credit."
That $28,000 would either be a loan which you pay interest on or cash which you lose investment interest on.
"Estimated solar savings over 25 yrs = $109K or $4360/yr."
That's the most confusing since $4,360/year=$363.33/month. I've never had a $363/month electric bill in my life. I've never had a $200 electric bill. The highest we've ever had was a few months ago having six heaters, dehumidifiers and fans running 24/7 and my bill was $150 which my insurance company paid from a water leak. Our electric bill averages around $100/month. Even if it was $150/month that's $45,000 in 25 years. In 25 years I'll be 95.
$28,000 invested in the S&P 500 if it follows the last 25 years at 7.98% would be $190,690.12. And I wouldn't have unsightly panels, maintenance or damage. As it is now the power company is responsible for everything up to my meter base.
Living here 43 years our total loss of power has been a few days total, most of that loss was a major ice storm and we were in Florida and missed it. Ten years ago I bought a 9kW generator which has less than 3 hours of run time.