With solar panels on your roof you can generate your own energy. However most of that energy you won’t use yourself. The sun shines during the day when most people are at work. Their solar systems produce energy that’s exported back to the grid. Home energy storage systems allow people to use the energy they produce themselves. Japan is introducing hydrogen fuel cells to power homes. Tesla’s Elon Musk recently launched the Powerwall, a battery system for energy storage at home. A London startup offers a smaller and cheaper alternative: the Powervault. The unit is smaller than an average washing machine and stores enough energy to wash two to four loads or to watch television for 14 to 28 hours.