My solar project was an investment with an expected return. Here are the numbers as of today.
My solar panels cost was $17,000; I researched solar for 2 years and talked with dozens of companies. I got a really good deal with state of the art materials. My cost was $11,900 net of tax credit. That was March 2018. I have been paying about $9 per month for electricity for 64 months.
Electricity around here is between $200 and $300 per month. So $12,800 to $19,200 for 64 months. I am pretty frugal so I would err towards the lower figure.
I have about 15K on the Model 3; the pandemic has cut annual mileage way back due to working from home.
My GS gets about 22 miles per gallon, so about 680 gallons @ $5 per gallon = $3,400. The Tundra gets 16 mpg...
Do the arithmetic. The solar project will easily show a ROI of over $100K over the 25 year lifetime. That assumes energy costs remain constant, which is not likely.