The 860 panels are capable of producing 210,000kWh of electricity each year – more than 10 per cent of the building’s annual electricity use – all of which will be used on site.
South West Water says this is part of its “ongoing commitment” to increase its renewable energy generation.
The company has already installed solar panels at 32 operational sites across Devon and Cornwall, and generated more than two million kilowatt hours of solar energy to date.
The savings from the reduction in electricity purchased from the national grid and the income from the feed-in tariff at some sites will help keep energy costs down.
These costs are “ultimately passed on to customers”, according to the company.
Project manager James Pearce said: “We installed our first batch of solar panels at six sites in December 2011 and until now all our arrays have been at water and sewage treatment works so it’s great that we have been able to install panels at our head office.
“Adding more solar panels is part of our drive to reduce the company’s carbon footprint and help combat climate change.”
The water company also operates hydro-electric and biogas plants and a wind turbine.
South West Water uses approximately 250 Gigawatt hours (GWh) of electricity each year while it generates 19GWh of renewable electricity.
It has a target of generating 30GWh of renewable electricity per year by the end of 2015 and 50GWh by the end of 2020.