Fasten your seat belts, ensure tables are stowed—and check the batteries for charge. At least, that’s what the preflight checklist might be for a pair of plucky new startups promising to build passenger electric airplanes inside a decade.

Zunum Aero, backed by both Boeing and JetBlue, today announced plans to build a fleet of electric planes that could each ferry 10 to 50 people as far as 700 miles. Zunum plans to make use of underutilized airports in the U.S. in order to provide more efficient regional travel, a kind of bus service in the air. It's aiming to start operating flights by the early 2020s.

Zunum joins Wright Electric, fresh out of Y Combinator, which last month described its own plans to build electric airplanes. Wright's aircraft would be larger, capable of carrying 150 people on journeys of up to 300 miles—enough to allow it to fly trips between London and Paris. It hopes to be flying within 10 years.

They’ll find themselves up against competition from larger established players. In the past, Boeing has experimented with fuel-cell light aircraft and, more recently, Airbus has been testing an entirely battery-powered two-seater called E-Fan.

Their goals are laudable. The aviation industry accounts for greenhouse gas emissions similar to those produced by the whole of Germany, so even cutting emissions from just regional flights would be a start on helping clean up the planet. And electric airplanes make less noise than their regular counterparts, so residents of the relatively quiet regional airports being targeted by Zunum shouldn’t find themselves disturbed too much.

Both Zunum and Wright are gambling on improving battery technology to make their business proposals truly fly. As we’ve reported in the past, the technology required to fly small two- or four-seater electric airplanes has advanced, and the idea certainly isn’t as farfetched as it once seemed. But hauling 50 people or more is a much bigger challenge, one that will require much larger aircraft and far more power. While solar panels may have helped a lightweight airplane to circumnavigate the world, they’re not a practical option for larger craft.

Realistically, that means better batteries will be required in order for all-electric commercial aviation to take off. But big advances in commercial battery technology have proved hard to come by, and it’s not clear how long it might be before the next one. If a battery breakthrough isn't forthcoming, Zunum and Wright may be forced to use hybrid technologies—which could deliver some, but not all, of the advantages the companies are promising—to get their businesses off the ground.

Read more: BBC, VentureBeat, “Once a Joke, Battery-Powered Airplanes Are Nearing Reality,” “Why We Still Don’t Have Better Batteries”)