Jan Olieslagers Energy density does not necessarily = danger...
However, as most clearly realise, with conventional electric transportation it's the energy density of the supply that's the limiting factor at present.
What brings things into perspective is a comparison of the best portable electric source (let's say a Lithium-ion battery) against a unit of petroleum. If we use Joules per kilogram we find that 1kg of petrol has ~44MJ/kg, a Li-ion battery has ~0.6MJ/kg, give or take. Thus conventional fuel typically contains around 70 times the energy per kilo than electric.
This is offset to some extent by the relative efficiency of converting this energy into power that will propel your 'plane. It's a complex field, but for the purposes of a simple comparison let's assign an efficiency of 90% to electric, and 30% to a combustion motor.
However, while the electric system has much greater efficiency, from a practical perspective it clearly remains overwhelmed by the vastly superior energy density of petroleum.
So, there have been significant advances in (electric) energy storage over the past 20-30 years but, unless there is a 'eureka' moment, I feel it will be quite a long time before it becomes fully viable and comparable to 'conventional' fuel-powered aircraft.
FP.