You're mixing up two different sets of numbers here. $6,000 is the extra amount that Jug would have paid after being sold the solar installation as a way to sav

You're mixing up two different sets of numbers here. $6,000 is the extra amount that Jug would have paid after being sold the solar installation as a way to save a lot of money over the next 20 years. The author, who was willing to pay a little more to save the environment, would have been paying an extra $30/month from the start, before the annual increases started widening the gap. Their bills without the solar started at $30/month, so this would start out doubling their bill and increase from there.