I see that a complete set should include a key, a fill, a hair and a background light.
What a complete set includes is very subjective and depends a lot on the desired look. Given that this is your first set, I would shy away from buying that many units. Start with one or two and add them as necessary.
You didn't buy the "complete" set of lenses for your camera either, did you?
I learned that fill is usually 1-2 stops less powerful than key. To my great surprise, hair light should be 0.5-2 stops more powerful, depending the hair color. Finally, for background it seems a speedlight is sufficient.
You state this as if it's the one definite way to setup the strobes to take portraits. That's like saying portraits are photographed with an aperture of f/2.0, which is simply not true in general.
I doubt that your goal is to take the same picture over and over again.
There are all kinds of different light setups that include more or fewer strobes, reflectors, flags, etc.
these ratios do not give me a hint on how powerful strobes I should buy
It looks like you are looking for some way to figure out the exact number that you should shop for for that particular light setup that you have in mind. But you should always have some more power that's eaten by the modifiers and the distance. Not shooting at full power usually means faster recycle times, which helps when working with people.
My guess is that any middle of the road strobe kit will work in terms of power.
I want to do portrait photography, both in a home studio and outdoors.
A problem related to power that you can run into is that a strobe is too powerful. Certain looks require the light (modifier) to be extremely close to the subject.If a wide open aperture should be used it can happen that the lowest power setting of a strobe requires a shorter shutter speed than the camera can handle. There's simply too much light and no way to reduce it by means of the camera settings alone and using an ND filter becomes necessary. This however is an edge case that you won't run into much unless you buy very powerful strobes.
I will need to carry them on location, therefore weight -- including that of the necessary stands -- will matter.
There are different types of strobes available.
- Some are a single unit that sits on the stand. This requires a
stronger (heavier) stand because the unit includes everything.
- Others have the "lamps" separate and connect them to a power pack via a cable. The pack is heavy but can sit on the ground, which lowers the requirements for the stands. this seems to be a more advantageous configuration for you.
Then of course there are speedlights, which are the lightest.