Travelers know how hard it can be to find a plug at the airport to charge a phone or laptop. Now airlines are facing a similar challenge as they electrify their ground and air operations.

Why it matters: Even the largest airports don't have enough juice to quickly charge all the electrified planes, flying taxis and ground equipment that airlines intend to roll out over the coming years.

• Stationary power storage — meaning, gigantic industrial batteries — will help fill the gap.

Driving the news: United Airlines this week bought an undisclosed stake in energy storage company Natron Energy, making it the first airline to invest directly in a battery manufacturer.

• Natron makes large sodium-ion batteries that can charge electric airport ground vehicles, such as baggage tractors and pushback tugs.
• These batteries can recharge vehicles more quickly than today's lithium-ion batteries — an advantage at busy airports, where vehicle uptime is key.
• Longer term, United expects such batteries to recharge short-range electric aircraft, such as regional planes and urban air taxis.

Where it stands: United has electrified about 30% of its ground equipment, but is running into power constraints at some hubs.

• "These airports weren't built for electrifying at this scale," says Mike Leskinen, president of United's investment arm, United Airlines Ventures (UAV).
• Airports will eventually build a more robust infrastructure using renewable energy, he predicts. "But when the wind is blowing and when the sun is shining isn't always exactly when you need the peak load."
• Natron's battery storage system helps solve that problem.

Zoom out: Aviation accounted for more than 2% of global energy-related CO2 emissions in 2021, and its share is growing faster than road, rail or shipping, according to the International Energy Agency.

• Many big airlines have set ambitious sustainability goals to achieve net zero emissions by 2050. But traditional carbon offsets, such as planting trees, won't be enough to meet those targets.

Most are looking to replace jet fuel with sustainable aviation fuel (SAF) made from agricultural or forest waste, algae or even used cooking oil.

• UAV, which invests in sustainability and customer experience startups, has stakes in or has agreed to purchase fuel from several SAF producers.
• United was also the first airline to invest in a biofuel refinery, NEXT, which plans to open a facility in Oregon in 2026.

Yes, but: SAF is still a long way from widespread commercial availability — which is why airlines are looking for other ways to meet their goals, such as electrifying smaller planes and ground equipment.

What to watch: United has also invested in newfangled aircraft companies like Boom Supersonic, which is designing super-fast planes meant to run on SAF, and Heart Aerospace, which is developing a 30-seat hybrid electric aircraft to serve smaller markets within 200 miles of United's hubs.

• And it has stakes in a couple of electric air taxi companies: Archer Aviation and Eve Air Mobility, a spinoff of Embraer.

The bottom line: Electrifying ground vehicles is a heck of a lot easier than building a cleaner airplane — so airlines like United are starting on terra firma.

Editor's note: This story has been corrected to clarify Natron's batteries are large but not trailer-sized, and that United plans to eventually use them to recharge, not power, electric aircraft.