It appears you can get a number that's at least related to it.

https://lua-api.factorio.com/latest/classes/LuaEntityPrototype.html#max_energy_usage

This thing definitely scales with the max_power entry in locomotive prototypes, but I haven't done any other testing to see what all affects it.

Interesting, thanks for looking into it. Will have a look how it is affected by mods, when I have time.

Nice, it turned out to scale almost linearly with the hardcoded values I made up.
Had to tweak the linear relation to make powerful engines stand out more.

powerFactor = max_energy_usage / 10K -- 10K means 600kW
if isShip(....) then powerFactor /= 3.33
powerFactor = math.pow(powerFactor, 1.25)

Excellent