Note also that efficiency cannot exceed 100%. If you're in a system with a dim red dwarf, you need to be really close in, or have a really huge array, to get a decent amount of power.
There should actually be two different types of solar power arrays:
1. The huge, fragile arrays that are possible for a space station that never has to stow away it's power array.
2. The smaller, sturdier arrays that can be stowed inside a ship during maneuvers more energetic than mere station keeping.*
For sizes and costs, if the Mongoose numbers just don't make sense, look at another edition for more reasonable numbers. (Just don't use the classic ANNIC NOVA as an example; it's wacky.) I would look at the GURPS Traveller numbers, because they were designed by engineers and gearhead gamers. I somehow don't have Fire, Fusion, and Steel, but it also has a reputation for more thorough engineering.
As for what you can power with solar arrays, I would include these kinds of things:
- batteries
- life support (with expendables resupplied in annual maintenance at worst, ideally more often so crew don't get tired of food made from recycled sludge)
- routine electronics
- laser weapons, missile launchers, sandcasters, repulsors, and screens
- station keeping drives limited to about 1/1000 G (built as a maneuver drive 1/1000 the size of a 1 G drive, but subject to the same minimum size, so probably much larger)
Some systems could operate briefly on solar power:
- particle and meson weapons are shooting very small quantities of highly energetic matter, so I would give them a fairly generous number of shots before the weapons need an injection of matter from gunners (who will complain a lot, and need a skill roll), such as the hydrogen in a bucket of water from the nearest fresher
- plasma and fusion weapons shoot larger quantities of energetic matter, so I would limit them to a fairly small number of shots, comparable to the number a missile launcher can fire without an extended magazine
Note that these weapons require so much power that it's probably a really bad idea to design a ship with them that depends on solar power anyway.
Some systems are off limits with solar arrays, but only one comes to mind right away.:
- no use of maneuver drives for anything more than the 1/1000 G of station keeping; to run maneuver drives at 1 G (or even 1/100 G), the solar arrays need to be stowed, and the drive run on batteries or fusion (or at lower technology, fission or even chemical power)
So what about the really large, fragile arrays for space stations? What if you need to stow them even though they're not meant for that? Then, instead of pushing the "retract solar panels" button, with a couple of maintenance people outside in vacc suits to kick the inevitable stuck joints, you need a full space manufacturing team on the job for days or weeks to dismantle the arrays, then you need them all over to reinstall them at the new location.
* These could also be used for starships, but that strikes me as bad design. The only reason to use solar arrays is because you'll be in a system for so long that it's inefficient to use a fusion power plant that needs to be refueled regularly. With a starship, you need massive fuel loads anyway, to power the jump drive. If you're going to be on station for a long time anyway, power your ship with fusion from the jump fuel tanks, and refuel when it's time to jump. If you'll be in the same system for a really long time, it makes more sense to use a non-starship and hire a jump tug to get there and eventually leave.