alt Hacker NewsNit: When they did the actual flight I did the math on the $5 worth of power claim, and it just doesn’t pencil out even at the lowest power rates in North America. Just to illustrate: at $.05 per kWh (which is less than half the price of electricity where they flew), you get 100kwh for $5. 100kw is what it takes to takeoff a 1200 pound Cessna. Probably about 75kw for an efficient cruise in that same plane.
Still an incredible feat, but no idea where the $5 comes from. Maybe that’s just the power they use to get from the threshold of the runway to wheels up.
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The takeoff roll is only a minute or so to lift-off, so it's possible. But also somewhat meaningless, as an airplane can take off for free in high-enough headwinds.
"hybrid system (electric + conventional turbo prop)" there's your answer
so for an hour flight it costs $5?
You can't convert between kWh and kW. 100 kWh = 6 MW for one minute... so there is plenty of power if the flight is short enough. They said the plane has 4 Tesla's worth of battery power, let's assume they are 80KWh packs then that's 320kWh that they could use for a short flight. That's a fair amount of juice, they definitely won't be flying long in a 25 ton aircraft without starting up the auxiliary motor but I have no doubt it will be able to get off the ground on $5 worth of electricity.
To be fair though it is more likely to be 50 kWh just for the take-off, so that's probably where their $5 figure came from (at $0.10 / kWh). For comparison: a single gallon of Jet-A = ~150MJ. 320 kWh = 320,000 W for one hour so 3600 * 320,000 = 1150MJ, or about 40 gallons and I suspect that these electric motors are quite efficient.
So it does pencil out, I think. Or maybe my pencil is broken and no doubt HN will correct my math.