Warren Buffett is synonymous with his hometown of Omaha, Neb., but for a glimpse into the future of his investment empire, look east…to Iowa.

In the neighboring Hawkeye State, the Berkshire Hathaway Inc. chairman has sunk billions into wind-farm projects, part of a big gambit on renewable energy by a utility company he acquired in 2000 and has built into one of the country’s largest power suppliers.

Through...