As a solar panel was raised onto the roof of their mud-brick home in a Tanzanian village in sight of Mount Kilimanjaro, Akida Saidi and his wife felt giddy at the prospect of entering a new era. In a place where most residents make do with pit latrines instead of toilets and till their fields of maize and pigeon peas with hoes, suddenly having electricity would catapult them into the 21st century. With the flick of a switch, they’d light their kitchen without fear of kerosene fires and charge their phones without trekking to town.

The couple’s unexpected journey to solar power began one day in 2015, when a fleet of motorbikes buzzed into the village of Gedamar, carrying salesmen from Zola Electric, which counts Tesla Inc. as one of its biggest backers. The agents offered Saidi and other residents a way to improve their life while saving money. For a small down payment, followed by a monthly fee less than the cost of fuel, they could have three lightbulbs, a phone-charging port and a solar panel. In two years the kit would be theirs to keep, the salesmen promised, with free electricity coursing through their home in perpetuity.

"When we got the solar power equipment, we thought our lives would change and we would live a modern life, just like those in the big cities,” says Saidi’s wife, Mwasiti Waziri, who recalls that her neighbors were so dazzled by the lights that many of them signed up, too.