The payback on Solar PV is very poor.
There are better things to spend your money on, such as insulation and draughtproofing, that cost less and have greater benefits.
If you have extra money sloshing around, after you've paid off all your debts, including your mortgage, and have fully funded your pension and built up a rainy day nest-egg, then you might consider investing what's left over. Of all the investments you might consider, solar is not a very good one. It also lacks marketability. I.e. if you want to withdraw some or all of the money you "invested" can you get it out? No.
A sensible investment should realistically be worth more in ten or twenty years than you put into it today. Does Solar meet this criterion? No.