You invest your savings, and own the solar PV panels. You take all the financial benefit from avoiding buying grid electricity from your electricity supplier; get government incentives for green energy; and export any spare electricity back into the grid for an income. Typically over 20 years, it is like getting a return of say 6% interest per annum from the savings bank. Typically payback period is around 7 to 9 (this is merely is simplified illustration)
You don’t have savings to invest. You could get a solar supplier to lease the roof space. They would own the solar PV panels and get the financial benefit (because they are using their money to fund the project). You would avoid buying some grid electricity from your electricity supplier. Reducing you overall electricity bill, and doing the right thing for the environment. Remember electricity prices have been rising by around 6% per annum over the last 10 years.