More is lost if you coast than if you regen since drag increases with the square of speed. I did a bar napkin calculation to show the difference:
A P100D is on flat ground doing 60mph at 280 wh/mile. It arrives at a 3% downhill grade. The driver can keep TACC at 60mph, which will result in ~5kw regen downhill, or let the car accelerate to 70mph which will result in 0kw regen. The coasting car will initially have an energy advantage (kinetic plus battery) of about 30kJ. But at 70mph, the coasting car's drag is 36% higher than the drag at 60mph. That 30kJ total energy advantage from coasting will be lost in less than 10 seconds due to higher drag.
If you have a very short/steep hill, coast it out. Under most conditions regen is better.
..make it simpler to demonstrate laws of physics (conservation of energy) and imperfect energy conversion of modern day electrical systems.
Flat ground. Any speed, but 60 mph is good. Get in any Tesla and drive it.
Sample 1: standard regen.
At point X lift foot off pedal completely.
Begin computing battery charge power. (hard to do but say you could measure this precisely)
Car comes to rest at point Y.
Using only the power returned to battery from this decel cycle, resume driving
in any fashion you choose under the car's own power, until that charge amount is depleted. Let's assume (best case) once the energy is depleted, allow the car to roll out rather than halting abruptly.
Car comes to rest at point Z.
Sample 2: zero regen.
Re-do the run arriving at point X at exactly the same speed as first sample. Car can be at any charge level at the beginning of the run.
Flick the car to neutral. No charge is returned to battery.
Car rolls past point Z. Every time.
Repeat at any speed.
Repeat at low regen setting instead of standard.
Sample 2 always surpasses point Z.