What is the return on investment on your car, clothes, computers, etcetera? 

All of the items are considered expenses by an accountant and the Internal Revenue Service. These items are bought without a care as to how much financial gain or loss. We buy them because there is a use. The reasons are many and unimportant.

Solar is not an investment. Neither is electricity, water, natural gas, or gasoline.

Definition of ‘Investment’ (from http://www.investopedia.com)

“An asset or item that is purchased with the hope that it will generate income or appreciate in the future. In an economic sense, an investment is the purchase of goods that are not consumed today but are used in the future to create wealth. In finance, an investment is a monetary asset purchased with the idea that the asset will provide income in the future or appreciate and be sold at a higher price.”

Using this definition, buying a solar is not an investment.

Solar is an expense. What buy solar power equipment?

Cars are expensive. Why buy a car?