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Litres is the defacto unit for a volume of liquid. In this case - coolant. If you have a gauge showing coolant, it would be in terms of how much coolant is contained in the system - which is the provider and would be in litres. You wouldn't, for example, have a fuel tank leak in a car and say that millions of joules leaked out onto the road. You would say that litres or gallons of fuel leaked out. Equally, loss of coolant would be measured in litres. And so the amount contained within the system must be monitored in litres.
You are completely right about the fuel. That's why I said you could either use energy or volume as measurement. The problem here is, that there is an essential difference between the functionality of fuel and the functionality of coolant:
Fuel gets burned/used/fusioned/whatever and is gone or at least useless after a one-time use. Having more or less fuel on board only affects the maximum amount of energy you can gain from it, not the overall functionality of the system.
The coolant in contrast (always assuming I understand the cooling system right) doesn't get burned/used/fusioned/whatever. It only fows inside tubes and changes its temperature from time to time. You either have enough of it to have the cooling system running or you don't. It doesn't get less during use. Therefore the sheer amount of coolant is not a good measure for the effectivity of your cooling grid.
To stick with the car analogons: The coolant is like motor oil. Oil flows inside tubes and through certain parts of the engine and absorbs the emerging heat. It then gets cooled down by cold air coming through the grill of your car (or something like this) and flows back in the circuit. You won't get better cooling if you fill more oil into your car, but you get a problem if there is not enough. Again this "enough" doesn't depend on how much heat your motor generates but on how much volume cable you have to fill. Sure, you have to fill in more oil from time to time, but this is only for technical losses and has nothing to do with the oil getting consumed by the engine or something.
tl;dr: the amount of coolant isn't suitable to describe cooling effiency because doesn't get less during the cooling process.
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Voltage is a derivative measurement of current and resistance. It only represents a mathematical equation and not a physical substance. Current actually represents the electrons - which are the provider.
Yes, current describes how many electrons pass through during a certain amount of time (A mechanical analogon would be speed). But no, voltage isn't just a mathematical construct. It describes (over some detour) the difference in potential before and after a resistance. A mechanical analogon here is a bit more difficult, but I would go with pressure. Trust me, I'm a physicist. (And wikipedia also says that voltage has a physical meaning:
https://en.wikipedia.org/wiki/Voltage