What to do with the excess of energy from a solar off grid system?
Large buildings often run chillers during off-peak to make ice, then use the ice for cooling during the day. That’s a bit opposite of what you want, but it explains the principle of using phase change to store energy.

The pump idea isn’t so dumb at all. It’s been used at larger scale in hydro dam projects to store energy during off-peak.

Simplest thing? Use the excess PV energy to heat water. It’s 100% efficient, and with a well-insulated tank the losses are low.