Depending on your current tenants, have you considered letting the them install the system themselves in exchange for rental reduction ?
 
I believe tenants can be eligible for the subsidy (?)  so assuming the electricity savings are great enough an  offer of a rental reduction equal to the tenants out of pocket expenses could be a win / win situation. 

Tenant saves money on electricity, sense of 'investment'  may help retain long term. You increase property value interest free ?