Where that "cost" over time is highly likely to be negative, aka free money, and it's only the coordination problems that prevents people from doing it.

(See also lease of EVs, if you were paying monthly anyway then the lower running costs of EV can be factored in for a fairer comparison that reflects better on EVs than the up front cost.)

If there was really “free money” in installing solar panels everyone would be doing it already.

How does that apply here? In fact, the opposite would seem to be the case; the fewer people that install solar panels, the more expensive electricity is to buy off the grid, meaning the faster the payoff time for a home solar system.

If you build a new house with solar integrated, the person who buys the house (and society generally) benefits over time, but they need to pay up front for the costs. If you add it, then people will buy the other "cheaper" house that will cost them more in the long term and you'll go out of business. This is called a "Race to the bottom".

Pretty much every nation has a whole bunch of regulations about what you can build and sell, and what kind of houses is a popular specific example everyone is familiar with, not because they're all secret authoritarians, but because in the past, people did a whole bunch of stupid, short-sighted things to hit a lower price point and the businesses that didn't do stupid, short sighted things couldn't compete with them and were forced into doing stuff they knew was stupid or dangerous and people ended up living in dangerous, hard to heat homes.

This is the core problem of climate change. We'd all be better off dealing with it, but if someone else goes for the short term profit they ruin it for everyone.

It's a time-value-of-money problem. Over the long run, solar saves money. The payback period of our panels was about 9 years, but our utility raised their rates, so I think it's more like 7 now. Problem is, most people don't have 7 years worth of utility bills available to spend now, and if they did, they'd rather spend it on other things.

You could say the same about investing, and yet a ton of people still don't do that.

there's 'free money' in exercising and eating healthy but most people don't do it enough.

People don't always make the most economically rational decisions.

It's economically rational in a world filled with the moral hazard provided by universal healthcare and insurance.

Why eat right or exercise when 99.8% of my $500k quadruple bypass surgery will be paid by other people?