500 Wh/kg batteries could keep solar drones aloft for months, US study finds

A US-led study involving Harvard and the University of Michigan identifies 500 Wh/kg energy density as the key threshold for solar-powered aircraft to stay aloft for weeks or months in the stratosphere at around 60,000 feet. That is roughly double the ~250 Wh/kg of commercial EV batteries, though experimental cells already reach about 400 Wh/kg.

• 500 Wh/kg target is double the ~250 Wh/kg of EV batteries
• Experimental cells already reach ~400 Wh/kg but remain costly
• Aircraft would operate in the stratosphere at about 60,000 feet
• NASA's ER-2 flies 6–8 hours at tens of thousands of dollars per hour