In a lot of places, peak supply is during the day now. Overnight conditions are more about a lack of demand than a surplus of supply (although they're sort of equivalent), but really the most effective use would be charge while the sun shines, dump the battery onto the grid when you get home, and then charge slowly overnight after the end of peak. Assuming there's no downsides from cycling your battery and you and the system have perfect knowledge of your evening plans.

Depending on the renewables mix the grid settles on, selling some back during the night might make sense. Both solar (obviously) and wind (less obviously, at least to me) decrease during the night so we may find we need to supplement the generation with battery reserves during the night for demand peaks.

Nuclear and traditional dammed hydro prefer to be setup to provide extremely consistent power too so there's a gap in the power generation if we completely eliminate fossil fuels. Even if we don't completely eliminate them, peaker plants are extremely expensive to operate because their base costs and ideally low usage means their cost per megawatt is pretty high.

I there is a surplus of electricity at night, then why have batteries? There seems to be no problem at all with just spot market demand in that case!

I thought the issue was that solar panels are generating electricity during the day only. So as someone with solar, you want to sell during the day and buy at night. No?

My point is, rather than using batteries to store huge amounts of energy and lose much of it in the process, find efficiencies and use credits instead.

Typically the sell back time is in the evening, after solar production is done but before people go to bed. Also some in the early morning, albeit to a smaller degree.