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I have very basic electrical knowledge and I'm struggling to know what to do.
I have recently bought a house and there is an electrical box that has been covered up. It is higher up on the wall so I'm guessing it was a light switch.
I've taken the cover off and below is what I'm dealing with. I've also attempted to draw out how it's all connected.
Is it possible to wire in a standard power receptacle/plug outlet to this?
Yes, I see a big problem with crossed neutrals. If the blacks are separated for some reason, the neutrals MUST also be separated.
Nobody cares about the grounds being all together. Unfortunately some people get carnal knowledge that all neutrals and grounds are in direct contact in the main panel, and that makes them tend to think of neutrals and grounds as the same thing. That's not true at all. Ground is a safety shield and power flows only during fault conditions. Current flows in loops and neutral is the other half of the loop!
Thus, as Scott mentions, where the blacks are separated, the whites must be separated likewise.
Could you put a receptacle there? Yes, but only if you also add a "box extension" there as well. A typical box extension would be a Legrand Wiremold Surface Conduit Starter Kit.
Since you have separated the hots and neutrals into 2 groups, place the receptacle on only one of those groups (hot and neutral pair).
The short answer is NO! Legally you cannot add anything to that box because it is at capacity. While you could physically cram more stuff in that box NEC (National Electric Code) limits the number of conductors in a box to prevent overheating and damage due to crushing everything in it. You currently have 9 conductors in that box (all the grounds only count as 1) So 4 black + 4 white + 1 ground = 9. A device counts as 2 so that would make the total 11. Check out the chart below and you can figure out the capacity of your pictured box (best guess its capacity is 9). This assumes that all the wires are 14 gauge.
You have a second possible problem depending on the number of circuits that are running into that box. you have 2 distinct black circuits, but all the neutrals are tied together. IF you do have 2 circuits, you have crossed neutrals, which requires a whole new question and a lot more information from your side, and may be out of your comfort zone.
