Based on what I read in IRS Publication 946, page 29 at https://www.irs.gov/pub/irs-pdf/p946.pdf it's 5 year property. But if it is "a physical part of" the structure, there's nothing wrong with classifying it as residential rental real estate and depreciating it over 27.5 years. I myself would prefer the 27.5 year route, since that keeps the depreciation I"m required to take each year, significantly lower.
Additionally, be aware that you do not qualify for the "home energy tax credit" on this, since those panels are not on your primary residence.
