Solar power in Illinois has been increasing, as the cost of photovoltaics has decreased. Illinois adopted a net metering rule which allows customers generating up to 40 kW to use net metering, with the kilowatt hour surplus rolled over each month, and lost at the end of either April or October, as selected by the customer. In 2011, the limit was raised to 2 MW, but is not net metering, as the term is commonly known, as it uses two meters for systems larger than 40 kW.
Illinois ranks 18th nationally in cumulative installed solar capacity. There is enough solar energy installed in the state to power 6,000 homes.
In 2012, $27 million was invested in Illinois to install solar on homes and businesses. This represents a 259% increase over the previous year, and is expected to grow again this year.
A 2012 estimate suggests that a typical 5 kW system will pay for itself in about 9 years.
In 2010 the country's largest urban solar array, 10 MW, was installed in West Pullman, on Chicago's south side. In 2012, IKEA installed solar PV on its two stores in Bolingbrook and Schaumburg totaling almost 2 MW.
The first experimental solar power plant was in 1902, in Olney, Illinois, by H.E. Willsie and John Boyle, and was based on a design by Charles Tellier. In 1904 they set up the Willsie Sun company in St. Louis, and build a 6 horsepower motor.