Windsor Castle will be powered by hydro-electricity, the royal household has said.
The castle will be supplied by Southeast Power Engineering Ltd (SEPEL), after former renewable energy plans were halted in 2009.
Two hydro-electric turbines at Romney Weir on the River Thames will be lifted into position by a crane in late September, the firm said.
The 40-tonne Archimedes Screw turbines will be ready to operate from November.
SEPEL director David Dechambeau said he was "over-the-moon" to have the Queen as a customer.
SEPEL is working in partnership with the Environment Agency, which is leasing the weir.
A Buckingham Palace spokeswoman said: "I can confirm that the royal household now has an agreement in place to purchase the energy generated by the hydro scheme, implemented by SEPEL."