Loyd Case, the editor of the soon-to-be shuttered computer-enthusiast website Extreme Tech, recently wrote up his experience “going solar” one year into his experience. He spent around $38,000 to saves $3,000 a year in electricity costs. He mistakenly concludes that this the payback period is 12.5 years. That’s because he doesn’t include the the time value of his money.

If instead of paying for the solar panel up front, financed them with a loan, making payments of $250 a month, it would take 20 years at 5% interest to pay back his loan (if electricity costs and use remain constant). That is a long payback period, especially because solar panels will reduce in efficiency in the future.

Also, Case received a California rebate and Federal Tax Credit because he installed these solar panels. The California rebate probably amounted to $10,000. The real cost of his solar panels were nearly $50,000, making the investment in solar panel a bad deal.