Not really Ian. A Watt is the measure of energy transfer over a unit of time. Remember the old electrical equation …..
Watts = Volts x Amps
For the same amount of energy transferred (Watts), Amperage (the rate at which electron flow is running through a system) goes up as voltage (the pressure allowing electrons to flow) goes down. The fuses’ job is to limit that rate because electron flow causes heat, and too much heat (electron flow) will damage the cabling.
For example, a 60 W bulb on a 240 V (UK) circuit needs to be protected by a 0.25 A fuse. The same bulb, on a 12 V circuit, needs 5 A fuse protection.
In Chris’ 12 V application a 40 A fuse is the equivalent of having a 2 A fuse on a household 240 V circuit. If a 40 A fuse was fitted into a UK residential 240 V electrical system, and the wiring was rated high enough, it could drive approximately a 10,000 W power application.
It’s a bit like an airbrush system. If you think of the wattage as the amount of paint moved, voltage as the compressor pressure, and the airbrush needle as the fuse in amps, lower pressure (volts) needs a bigger needle (amps) to move the same amount of paint (watts).
Well that woke me up LOL…..fifty year old physics knowledge coming creaking into daylight….