I think the solar panels and accumulators are way too cheap and too strong.
Thats why in my game i changed the efficiency multipliers to x2 per generation
i changed the crafting costs to the following:
2 panels or accumulators of the previous generation
panel mk2: 25 green circuits
panel mk3: 20 red circuits
panel mk4: 20 processors
panel mk5: 50 science packs + 15 processors
accumulator mk2: 20 green circuits
accumulator mk3: 15 red circuits
accumulator mk4: 15 processors
accumulator mk5: 25 science packs + 15 processors
like that it feels way more balanced. maybe i would nerf it even more. i feel like solar power should be something that takes a lot of space.
i also changed the graphics to make the accumulators more distinguishable.
most of them are stolen from another mod, but the mk5 i colored myself. (so at least you have my permission to use those)
https://www.dropbox.com/s/ihkazpmlfqudcgj/graphics.zip?raw=1

"i feel like solar power should be something that takes a lot of space." -- well, you see, the exact point of this mod is -- I DON'T like it that way :) That's the whole purpose of this mod: solar power should take LESS space, with a little overhead price. Initial multiplier was 10, by the way ;)

maybe, but all the solar power mods are about that. they all give you free micro solar power plants that you have to do nothing for.
i thought you were more interested in balance since your multiplier is just x4 instead of x10 per generation like the others.
the solar power field is already super small when you have just 8 or 16 solar panels in one (my 250 mw field currently consists of mk2 panels (x2))

i think you should also make a new subgroup for solar panels
just change items.lua:
- new panels into subgroup = "solarpower"
- data:extend({ {
type = "item-subgroup",
name = "solarpower",
group = "production",
order = "b",
}, ...
- and
data.raw.item["solar-panel"].subgroup = "solarpower"
data.raw.item["accumulator"].subgroup = "solarpower"