There was a lot of coverage of Tesla’s new home battery earlier in the year and Powerwalls will start shipping soon.
Since one kilowatt hour is about 3.6 million joules, pumping 360,000 litres of water into a tank raised 10 m above the ground stores something close to 10 kWh (E = m g h and a litre of water has a mass of about 1 kg). 360,000 litres is 360 m3, which means that you’d need a tank that measures 10 m x 10 m x 3.6 m, or so. This is bigger and heavier than two double-decker buses and it stores just 10 kWh! For home use, the Tesla Powerwall’s energy density means that it wins against the tank of water.