A Math Problem for Your Kids

There are two next door neighbors in identical houses in Florida.

they both use 2002 kWh/mo, on average.

Mark finances an 18kW solar system with a 25 year warranty for $54,000 at 7.85% interest

Rebecca keeps paying her electric bill.

Mark's loan payment is $409 for the first 8 months.

Rebecca's electric bill is $268.27 (13.4 cents/kWh) + interconnection fees, etc

The following April, Mark gets a tax credit for 30% of the solar system's cost and feeds the extra money he didn't pay in taxes into the loan, reducing his monthly payment to about $286

Rebecca's electric bill goes up 5% every year, forever.

Question:

• by year 10, how much have each of them spent?

Bonus Question:

• in year 25, when Mark's warranty expires, how much have each of them spent?