They can capture 100% of Sun energy, but can't be 100% efficient.
- 1-10% is feasible number for wide variety of applications.
Mass budget is a bit tight, about 1kW of sun output per 1kg of material to work with.
A lot depends on the uses of energy, in which way it is used, for what, because different energy uses will require/have different mass per energy requirements.
For general machinery the number 1kg/kW of available materials which composition we can't choose that's a poor number and in a more typical case, where we can pick and choose materials, this number is around 5-10kg/kW
However, heating and cooling processes, which we use on regular basis for many applications in our technological activity, in microgravity(!) and in vacuum(!) can easily have a better mass to energy performance, a few micron thick foils are enough to operate (direct or redirect it to different places, where we want it to be) with the incoming energy flow.
- as an example, as electricity we use only 1/6(do not recall exact number, but order of things) of the total energy we consume.
So if there is fine balance between cheap means to redirect energy (reflectors in microgravity of vacuum) for its used in appropriate tech processe, and between less mass efficient energy consumers(electronics as an example - no cpu can crunch numbers using 1kg of materials and 1kW of energy, if it is not one bit, lol) then it can be 100% capure use minus efficiency.
But yeah, the mass for it all is a little too low, but still, there is no hard number as a percent they can or cannot capture and use - they can 100% capture use, but ways the energy will be used, how and for, will be limited but maybe sufficient for what they want (as an example to mine the sun, huff and puff way).
If we take stuff for computation it will depend on efficiency of mass use, as average number(which also will depend on technologies they use as an example regular solar pannel or specifically designed for vacuum-microgravity will have significant differences in mass of stuff required)
Things have to be specific enough for us to say what they can or can not do, it all will depend on technologies they have at hands at that moment and their specific goals of what they want to do.
However 1 to 10% of output capturing (10 to 100kg of mass per kW of flow) seems feasible and reasonable number for wide enough spectrum of technological activities, but 100% is not impossible for more narrow set of goals and uses.
And combining those 2 together gives us again 100% utilisation, it just we use 95% of sun energy for sun "mining" and 5% to crunch the numbers and there is not enough mass to change this proportion to increase our number crunching capacity, but as a whole it 100%
- minus efficiencies ofcourse
All in all 1 to 10% is reasonable number, but not the limit, and it all minus efficiency, never drop that one out.
And this number can be further expanded by extracting heavy elements which can be used in construction of things and energy consumers, and sun has it enough, yes it is a fraction of a percent, but it the fraction of the biggest mass in the system, and there are ways to extract that stuff. However gas giants are easier target.