Aloha All,
As in all investments you have to understand your market. Solar on rentals can be a great idea if your market calls for it, but electricity does not cost the same in every state.
We here in Hawaii pay the most for electricity, 33.5 cents per kilowatt hour (kwh), which is twice as much as #2 Alaska. The national average is around 11 cents/ kwh. (Most SFH bills are around $350/ month, though I've seen electric bills as high as 1700/ month!)
Just as electricity costs vary from market to market as do photovoltaic systems (PV). Different areas get different amounts of sun each year, which means you may require more or less panels depending on where you live- and this can vary from neighborhood to neighborhood, not just state to state.
For example say a family uses 700kwh a month, in order to build a PV system that will produce that much, it may take ~12 panels in an area like Ewa Beach here on Oahu (the highest sun zone in the country) but it may take ~30 of the same type panels in a place like Seattle. You've more than doubled the cost of the system and, depending on your electric bill, may have lost the value of seeing a ROI for a quite a while.
There are several other factors when pricing a PV system like the size/pitch of your roof, how many stories your home is, what type of roof you have, the quality of PV panels you use, any trees/ buildings that may block the sun etc.
When considering installing PV in a rental unit, you have to understand your market. Personally I love having PV on my rental because now I can hike up the rent ~$250/month and include electricity at no cost to me.
It's all about running your numbers and making sure you come out in the green.
