So, yes, I could probably get a higher return if I invested that capital elsewhere, but, apart from the diversification, I get benefits beyond the raw financial return.
Firstly, earlier this year, we had a cable coming into the house fail. By the time the electrician and city had done all they needed to do to sort it out, almost 4 days had passed. We would have been without power for that time. As it happens we ran completely on solar for that period - freezers stayed frozen, could run the laundry, etc. Some stuff was limited (no oven, no hot water) but the impact was minimal compared to what it might have been.
Secondly, during the day at least, I'm not really fussed about electricity usage. If lights are on, or AC is on or whatever. So there's less "hey, that light is costing money" etc. So we end up using more electricity, but the marginal cost (during the day) is 0. My next car is electric (already on order) and that can charge at home as well (during the day, I work from home) and so that just increases the return (utilization of available power goes up.)
From a financial point of view, for me, it's a no-brainer. Obviously ymmv - everyone's numbers are different. For me the payback is in the 5-6 year range (probably under 5 once the car comes online.)