There are already companies making thermal storage systems to store excess energy. They heat sand up to about 500 degrees when there's excess power and then convert it back to electricity or just use the heat directly for heating water or living spaces.

There's also companies (googles do nothing but link to YouTube videos) working on scaling this down to about the size of a water heater.

What about storing energy as potential energy using some high density objects. This may work.. Right?? gravitricity Check out their work.

I did some work at a place called The hollow mountain that does this. But seeing as it looked like an underground James Bond bad guy base and I was a rope access mook in a boiler suit, I felt like I could die at any moment by tuxedo clad hero.
It wasn't solar they used to power pump the water back up though. They just, hmm I want to say, bought cheap electricity when no one was using it.

With an energy conversion efficiency of usually 75 to 80 % they are really efficient and don't have as much energy loss as other types of energy storage. It's a simple, but powerful concept and I find it beautiful. However, there is some concern regarding their impact on the local ecosystem. Not only do they need huge water reservoirs, which are artificially created and therefore might impact nearby rivers and even fish migration, but the way they are sealed with concrete or asphalt also disallows the development of riparian vegetation. From an ecological perspective they are basically dead zones.

Still, considering several alternatives, I think it's one of the better options. Although it's not cheap to build those, which is a problem in our current capitalitic society