> put that same $40,000 in an index fund that got the stock market's historical 7% average, and waited 30 years, I'd have $308,000.
stock market isn't a very safe investment (that is, risk is much higher than putting in a bank), you can't make a comparison against buying some equipment, which is zero risk. A closer comparison is putting the $40,000 in a bank that guarentees interest (or treasury bonds, which is also pretty much risk free like a bank) - you'd get $99,457, which is $59,457 over your initial investment.
> you can't make a comparison against buying some equipment, which is zero risk.
Equipment isn't zero risk. It could fail and need replacement, or need dramatic repairs. Or the price of electricity could drop dramatically and your investment won't be saving you as much in electricity bills...
There is always risk. Maybe the price of electricity changes, or the laws change, so you can't recover costs? (Look at what happened in Nevada.)
Maybe Tesla goes out of business or drops support for this product? Maybe they aren't as durable as planned? Also, a roof is part of a house which is a very illiquid investment, and there are a lot of other factors affecting the price of a house.
But, presumably people aren't buying this just as an investment. If you're going to put a lot of money into a house anyway, maybe a fancy roof isn't so bad.
Well investing in solar panels isn't either, 30 years is a lot of time for such (currently not very efficient) technology. Price of the electricity could go down significantly as current renewable or nuclear generators get improved over time, or perhaps someone will finally figure out how to do the fusion efficiently?
stock market isn't a very safe investment (that is, risk is much higher than putting in a bank), you can't make a comparison against buying some equipment, which is zero risk. A closer comparison is putting the $40,000 in a bank that guarentees interest (or treasury bonds, which is also pretty much risk free like a bank) - you'd get $99,457, which is $59,457 over your initial investment.