Hello, I would like to know how to use the mod, and especially if we can have satisfaction in the combinator, because I would like my nuclear power plant to be activated if the energy satisfaction is in yellow, which I can't do with the mod

Hm, yeah, that's actually quite easy to measure, just has to be done in a different way, you might want a different mod for that.
(can be done by checking stored energy on any power-consuming entity, as that gets refilled proportionally to satisfaction, at least within that power consumer priority level)

You can get something similar with this one, if your power production is fairly steady - just Ctrl+C and select your power producers on the map to count them, put an arithmetic combinator to divide W value by whatever they produce and you got something like % of satisfaction.
Or if you want to gradually enable reactors one-by-one (not sure if good idea, iirc they take forever to startup), just have e.g. second one turn on at 38MW (about the limit of 1 reactor), third one at 155 MW (limit of two adjacent ones), etc etc.

Though again, note that if it's power satisfaction value you're after, there might be a better mod for that, or at least it's possible to measure rather easily in lua afaik, just in a different way from what this mod does.

i.e. to check the energy stored on any consuming entities, on assembly machines for example? To divide the value W, I have to separate my nuclear power plant from the general electricity grid to avoid having the solar panels taken into account?

I did Ctrl+C, which gives me 840 steam turbine, so I multiplied 840 by the maximum output of a steam engine which is 5.8 MW which gives 4 872, so I divided by W by this value which gives me a value around 100 and 200, did I proceed well
here are 3 screens
https://www.cjoint.com/c/IGorAWTt8AY
https://www.cjoint.com/c/IGorBt4bnwY
https://www.cjoint.com/c/IGorBIwUbWY

i.e. to check the energy stored on any consuming entities, on assembly machines for example?

Yeah, anything that consumes energy at a steady rate should do.
Don't think assembler is best candidate, as it can stop/start arbitrarily... maybe an idle inserter, or a special low-power lamp that doesn't have on/off times-of-day.
When your power satisfaction is e.g. 60%, then its stored energy will be at the same 60%, as it can't all be replenished.

Note however that iirc prototypes have power priorities, so high-prio things will probably get 100% while low-prio should get less.
Not sure whether it's significant though, or how these are applied, but might be a good idea to check which protos have that at non-normal and check how power shortage affects such entities compared to others.

to avoid having the solar panels taken into account

If you have enough batteries with solars, I think you can just grab average production value for these from the wiki and ignore the sun entirely for the purposes of such calculations, but as you're talking about power shortage, guess batteries don't play any role there.

You can also subtract solar panel production from W total to not take their production into account.
Given that heating new reactor up will take like a day anyway, maybe there's no point adjusting output for that noise, as by the time exchangers get to 500C, solar panel output will be entirely different anyway, but not sure if maybe it makes more sense with other mods that you might have.

did I proceed well

Idk, figure it out yourself :)

It should be clear what kind values that measuring combinator provides, so it's totally up to you what to do with these, while I'm not sure what are you trying to do and don't really want to deprive you of the joy of playing the game yourself.