This is a question that has been bugging me for some time, especially since the questions about methanol synthesis revived my interest in energy storage. Say I have a wind turbine or solar panel or whatever, that produces 1000 Wh per day. If I use that electricity to electrolyse water, store the generated hydrogen at roughly atmospheric pressure in an upside-down water butt or a big gas-tight bag in my shed (don't worry, I'm not going to actually do this) and then feed it into a generator converted to run on H2, what percentage of that initial energy input would I get back out? 20%? 5%? 1%? The follow-up questions to this are a) How does that compare against a battery bank? What about a similar DIY-style pumped water storage system? b) What one component of the system should be improved to raise the overall system efficiency? Electrolyser, storage, generator? c) Are there any other DIY-friendly methods for storing intermittently generated electricity that I'm not thinking of? And, I suppose, d) Does doing this and providing 5-10x your overall power requirements in wind turbines work out cheaper than spending thousands on batteries?