It will cut an estimated £63,000 from the centre's electricity bills over the next 20 years and save 9.8 tonnes of carbon (CO2e) from entering the atmosphere every year.
The centre would have had to pay £33,000 to fund the capital costs of the installing the system without REI's funding model.
Owners Brian and Lynn Lewis will buy zero carbon electricity from renewable energy investor and developer REI at a heavily discounted rate under its Solar Power Purchase Scheme. They will then take ownership of the system entirely in 20 years.
The roof-mounted solar 27-kilowatt peak (kWp) system will generate 24,5000 kWh of clean green energy per annum - nearly half of the centre's annual 56,000 kWh usage.
That is forecasted to cut an estimated £63,000 from the business's electricity bills over the next 20 years - as well as save about 9.8 tonnes of carbon (CO2e) from entering the atmosphere every year.
The centre, known for its giant geraniums, would have had to pay £33,000 to fund the capital costs of the installing the system without REI's funding model.
Brian Lewis, who started the 0.8ha garden centre near Gloucester 35 years ago, said: "Although well established as a business, we have always moved with the times.
"We started from scratch and over the years we've made many changes, including the addition of this solar PV system.
"It's a win-win situation as our investment was absolutely nothing and there was no risk as far as I am concerned."
Staunton Garden Centre, which has a gift shop, English wine centre and licensed restaurant on its site, employs 10 staff.
Its solar PV system was fitted on the roof of the centre's main building in a week and will meet an estimated 59 per cent of the centre's demand over a year.
REI arranged the system with one of its accredited installation partners.
Zara Thorn, a business development manager at REI, which is based in Ferndown, Dorset, said: "We are finding increasing numbers of garden centres taking advantage of their large roof spaces.
"Because they don't incur any costs under our Solar Power Purchase Scheme, they can preserve their capital to spend on their core business.
"Brian and Lynn at Staunton Garden Centre have also already started to making significant savings from their energy costs, also reduce their carbon footprint and contribute to battle against climate change."
REI is rolling out a large number of systems nationwide - including recent installations at the Hendy motoring group in Chandler's Ford, Hampshire, and the Hampshire Golf Club in Andover.
PV panels convert sunlight into electricity.