@FabreGAS : There’s several way to measure the difference.
The simplest/cheapest is to turn everything else off, take a meter reading, turn on (say just the shower), use it, take another meter reading and turn everything back on.
Then you can step up to things like the ‘Kill A Watt’ style electricity usage monitors/power meters/meter sockets which plug into sockets and the item (such as the fridge) plugs into that. The meter will then either display (or send to your phone) the usage of that socket. However, this won’t work with ‘hard wired’ devices (such as showers and some electric ovens) as there is no plug to put the meter into.
The next step up I guess would be to either get the ‘clamp meters’ and clamp the power lines going into sockets/devices (such as the cables leading to the shower: finding them, as they’ll be hidden in walls - will the ‘fun’ job, but you may be able to clamp around them coming out of the distribution unit) or see if you can get circuit breakers with built in displays and replace those in the distribution unit (which will give the power usage per ‘circuit’ - but ovens/showers tend to be on their own circuits anyway).
I just work on the ‘I’ll try and keep my main [heavy duty] PC off for most of the month and see what my bill is at the end of it’ basis (turns out it’s about £10/pm saving!).