I got my roof mounted solar by saving for 3 years, getting multiple quotes, and going with what I thought was sensible. At current rates it will pay itself off in 9 years. It's not a great investment but also good for the environment. I wouldn't recommend borrowing for it. Just save for it if you can
I have solar on all of my properties (primary, secondaries, rentals), but there is a lot of peril in the residential solar install market. Below are some of my observations on the topic from my experience.
You can install on asphalt shingle roofing, but I don't recommend it; unless aligned with your roof replacement cycle, you're going to be paying thousands to remove and reinstall the system (racking and panels) to reroof when the time comes. Install solar on your roof if the roofing material will outlive the initial solar lifetime (~25-30 years). I love standing seam metal roofs for this, as there are friction mounting systems where you do not need to penetrate the roof to mount the racking and PV panels; $$$ but worth it imho. If your roof is coming up for replacement, do not install until replacement has occurred.
Installers are fly by night (lots of sales->subcontractor to install relationships) and it is very likely they won't be around to maintain the system; the only mitigation for this is to find a vendor who is also a roofer or some other business that can buffer from the cyclic nature of residential solar (cyclic from PV panel commodity costs and government incentives). If you're capable, get certified by the inverter manufacturer to get installer access to your system (Enphase University [1] or the like).
Get multiple quotes from installers [2] (a coop buying org is also a great option if available in your area [3] [4]), compare on equipment and price per watt. Research what your interconnect agreement with your utility looks like. 1:1 net metering is best, everything else less so. Net metering vs something else will govern system sizing and panel layout/direction. No batteries unless your grid is unreliable and you have the economic tolerance for it, or there are generous incentives from your state and utility. No PPAs or leases, they are economically inferior with the cost of systems having fallen so much and can make a property transfer transaction more difficult. As you mention, borrowing for this is economically inefficient unless you can get a long duration loan at something like ~3%.
It is a great equivalent of an after tax bond return as Retric mentions, but you must do your homework [3]; it is not turn key unfortunately.
