My back-of-the-envelope is:
- Assuming 0.4 kWh/kg for batteries, and they have to run for 4 hours, then the total mass per kW is 10 kg (batteries) and 0.08 kg (motor).
- A 1% increase in motor efficiency could eliminate 0.1 kg of batteries, which would let you double the weight of the motor.
- (My analysis is invalid if you need much higher peak power than cruise power.)
I m curious how you optimize the entire system for such trade-offs. This is an excellent question. For narrow body aircraft we ve studied, they require high propulsive power during the takeoff and climb phases, and a fraction of the peak propulsive power during the cruise phase. One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise.