At the current rate of oil consumption, the world will run out of it in twenty years. The Netherlands is investing somewhat into wind energy since that's one of the few renewable resources around in our country. Now the obvious problem with wind energy is that it is unreliable, so people are looking at manners in which to store that energy. The most popular solution is to store overspill energy into a lake, a gigantic gravity battery, by pumping water into, sometimes out of, it, and harvesting that energy at a later stage with generators. A proposal is to construct a massive lake, 40 metres deep, in the sea. It sounds wasteful to me. What about storing energy directly at your house by lifting a mass? If you use lead instead of water, you need less volume/height, and a mechanical solution should have a better efficiency than a 'hydraulic' solution. So I arrived at the following basic physics question: Suppose that you store energy by lifting matter, and suppose that system runs at 90% efficiency, and suppose the mass is lifted (at most) 5 metres. How big does the mass need to be to store the equivalent of one day of electric energy for one household? Any idea?