My opinions about solar on investment properties have "evolved" since I first posted on BP about the topic.
TLDR: Probably not worth it for rental properties, even with tax breaks, because of the cost/payback period, and the complication for tenants. Unless you are doing it for ethical/environmental reasons, then go for it.
MORE: I have 2 separate solar systems that offset 100% of the electricity for each unit of a duplex I own. Systems were "free" to install, but it's a lease, so I still pay the company (SolarCity/Tesla) for use of the panels, just at a lower rate per kWh than what the power company would charge. Essentially, it's the same monthly rate as if I'd purchased the systems and financed them over 20 years.
When the solar was installed in 2016, I lived in one unit and rented out the other. It made sense for me, since I have an electric/gas hybrid car and, well, I was living there. After I moved out, I still paid the solar lease, but I also had to compete for tenants with surrounding rentals that didn't have the added expense of the solar.
I figured out a way to structure tenant leases so that I wasn't simply paying for my tenant's electricity, but . . . it's complicated and tenants don't really understand it at first. You have to sell them on the idea. I keep the electricity bill with the power company in my name (because net metering erases all annual solar credits whenever the bill changes names) and put the monthly connection fee ($10) on autopay. The solar covers all the tenants' electricity (UNLESS the tenants start a grow op or try to get around gas heating costs by using space heaters. More on that later.)
To be marketable in my rental listings, I list the price I'd charge if I weren't paying for electricity, state that tenants pay for electricity, and in the description write, "ask us how the solar works." Then when they've seen the (beautiful!) property in person, I can explain, before solar was installed, the electricity averaged about $75/month, but now solar offsets 100% of their electricity needs. I tell them that I have a solar lease that I pay each month, so their cost for power (e.g. $65/month) reimburses me for that lease and is less expensive than what they'd otherwise have to pay the power company. I tell them, it's a flat rate each month, but if they use more electricity than the panels can produce, they are responsible and will get billed for the grid power ("but that's never happened to any tenant in the past, and unless you have a grow-op in the garage, haha, it's not likely to happen to you" <smile>). That disincentivizes them from wasting energy. Or growing weed.
Anyway, I'm also paying SolarCity each month, so solar isn't exactly saving me any money at this point. There are also laws in Oregon that if you pay for utilities, you can't charge tenants more than what you pay. (This is a gray area with a solar lease, however, since I'm paying a lease, not for power.)
Oh, and yes, I still am eligible for the state and federal tax credits (no more new Oregon state credits for solar effective 2018), but I haven't been able to claim them fully because I didn't have enough taxable income (and the state credits can only carry forward one extra year). So that's something to think about too, if you have a good accountant. ;)
All that said, if you want to save the planet, and your roof isn't shaded by trees, please get solar.