On Friday morning, Susan Millar was readying her home in Madison, Wis., for an open house. But she wasn’t trying to sell it. Instead, Ms. Millar opened her doors to a dozen strangers curious to see her heat pump, solar panels, battery storage and electric induction stove.

It was an early peek at one of more than 450 events taking place as part of “Sun Day,” a nationwide demonstration of solar power and renewable energy. Most of the events spread across nearly all 50 states, including E.V. parades, church sermons and house tours, are set to take place on Sunday, the day before the Autumn solar equinox. Ms. Millar was getting a jump on the action.

“You can live in a 90-year-old home like this one without burning anything,” she said. “Now all those fossil fuels are gone.”

Bill McKibben, the climate activist who also writes for The New Yorker, got the idea for Sun Day a few years ago as the cost of renewables began to drop. Solar power has become the world’s cheapest form of energy, with costs declining by nearly 90 percent in the last 15 years, according to Our World in Data.