Commercial solar planes: why we’re not there yet—even after Solar Impulse 2
If a single-seat solar aircraft could circle the planet, why can’t we book Delhi–Mumbai on one? Short answer: physics is rude, batteries are heavy, and passenger aviation demands far more power than sunshine can deliver on a wing.
In 2015–16, Solar Impulse 2 (SI2) flew 40,000 km around the world—yes, including stops in Ahmedabad and Varanasi—without a drop of fuel. That wasn’t a prototype for commercial service; it was a proof-of-possibility for clean tech. And it worked, brilliantly. But scaling that stunt to a 150-seat airliner is like asking a bicycle to tow a train.
Let’s unpack the mismatch.
What Solar Impulse 2 actually proved
SI2’s accomplishment was historic: a 72-m wingspan, ~2.3-ton carbon-fibre bird, ~17,000 solar cells, and four small electric motors. It hopscotched from Abu Dhabi to India, across the Pacific in a five-day leg, then across the Atlantic, returning to Abu Dhabi in 2016. Average speeds were roughly 60–100 km/h, with a single pilot in an unpressurised cockpit and meticulous weather choreography. (Solar Impulse)
The point: sunlight plus ultra-light structure plus patient flying can sustain flight. The catch: only for one person, very slowly, with generous margins of time and sky.
This was never pitched as a template for commercial operations—and for good reasons.
The power and energy math: sunshine vs megawatts
Start with the raw solar budget. Above the atmosphere, the Sun delivers ~1,361 W/m²; at Earth’s surface on a clear day, the practical figure hovers ~1,000 W/m². Even with excellent cells (~20–25% efficiency), you net ~200–250 W per square metre. With roughly 270 m² of solar area, SI2 could make on the order of 50–70 kW in full sun—exactly what its specs show. That’s fine for a slow, feather-light craft; it’s pocket change for an airliner. (Wikipedia)
By comparison, a single-aisle jet’s total power is measured in tens of megawatts. Even “just” the electrical systems on a modern long-haul jet can run into megawatts; the Boeing 787’s electrical architecture is around 1.5 MW, and A380 cabin systems alone can draw hundreds of kilowatts depending on phase of flight. Propulsion power dwarfs that. (WIRED)
There’s no elegant way to collect tens of megawatts from sunlight on a flying vehicle without obscene wing areas and structural penalties. The energy density problem makes it worse.
Batteries vs jet fuel: a harsh ratio
Jet fuel stores ~43 MJ/kg (≈12 kWh/kg). State-of-the-art lithium batteries are in the low-hundreds of Wh/kg—over an order of magnitude less. That means to carry the same energy, you’d need a battery mass many times heavier than the fuel it replaces. Great for cars; brutal for airplanes that must lift their own energy store. (eaglepubs.erau.edu)
Yes, battery-electric planes are coming for short hops: Eviation’s Alice (9–11 seats) and Heart Aerospace’s ES-30 (30 seats, hybrid-electric, targeted 2029) aim at 200–800 km regional ranges. But note: neither is solar-powered for cruise—they rely on ground charging and, in Heart’s case, hybrid backup. That’s telling. (Wikipedia)
Aerodynamics and payload: the ugly trade-offs
To sip power, SI2 used an enormous, fragile wing with very low wing loading. Great for endurance; terrible for payload and weather tolerance. Add passengers, seats, galley, cargo, emergency equipment, pressurisation, and you multiply mass fast. The slow cruise speeds (often under 100 km/h) and tight weather windows that SI2 accepted would be operationally impossible for commercial schedules. (Solar Impulse)
Even high-end solar craft that live in the stratosphere, like Airbus’s Zephyr, carry payloads of just a few kilos—useful for telecoms and surveillance, not people. (Airbus)
Safety and certification: lithium in the sky is not trivial
Aviation regulators are understandably strict with high-energy batteries after past incidents. The 2013 787 grounding over lithium-ion battery fires was a sobering case study. Any concept stacking megawatt-hours of batteries into a pressurised passenger cabin faces tough certification pathways for thermal runaway detection, containment, and emergency procedures. That adds weight, complexity, and time. (WIRED)
Why not mix sun + batteries + hybrid and call it a day?
You can (and some designs do) use solar as a trickle-charger—to run avionics on the ground, top up during cruise, or lengthen endurance for drones. But the math still says sunlight is a rounding error at airliner scale. Even if perovskite-silicon cells keep setting lab/module records in the mid-20s to high-20s percent, you don’t escape the square-metre limit on a wing. You’d need orders-of-magnitude better areal power or a radically different airframe class. (Reuters)
India lens: lots of sun, little help at 35,000 feet
India has excellent solar resource—GHI of ~4–6 kWh/m²/day across many states. That’s fantastic for ground-mounted PV and airports installing rooftop arrays to offset ground energy use. But at cruise altitude, panel area is still the choke point. For commercial ops, India’s nearer-term electric action is on eVTOL/regional hybrids, where DGCA has begun publishing guidance for certifying piloted eVTOLs (≤5,700 kg). That signals a regulatory pathway for short-range electric mobility—not solar airliners. (Global Solar Atlas)
Pragmatically, the decarbonisation levers for Indian airlines this decade are: newer airframes/engines, better air traffic management, sustainable aviation fuel (SAF) blending, and, later, regional hybrid-electric. Solar contributes indirectly—powering airport ops, charging ground fleets, and cutting airline Scope 2 emissions.
In other words: India’s sunshine helps aviation—just not by pushing a 180-seater through the sky.
Risks and unknowns
· Tech leaps: Dramatic increases in battery specific energy or structural solar cells embedded into load-bearing skins could shift assumptions—but timelines are unknown.
· Certification pace: India’s eVTOL guidance is a start, but certifying large electric transports (battery or hybrid) remains a long road.
· Operational economics: Even if solar could contribute a few percent of cruise energy, added weight/complexity may erase savings.
· Weather volatility: Monsoon dynamics and cloud cover complicate any design that relies on real-time insolation.
Commercial solar-only passenger planes aren’t blocked by a single killer problem; they’re limited by a stack of stubborn ones. Until energy density jumps by multiples and we can harvest far more watts per square metre on an airframe, “solar-assisted” will remain a niche endurance trick—not a mass-transport solution.
For India, the smarter bet is combining efficient jets, SAF, and near-term electric/hybrid regional aircraft, while letting the Sun do what it already does best here: power the ground.