What is the energy output when obtaining hydrogen from water using electrolysis?

Let's say one uses 100 kilowatt generating capacity to produce hydrogen from water using electrolysis, and runs the process for one hour. If he then turns around and uses the hydrogen to generate electricity, how much can he generate? Can he then produce 25 kilowatts for one hour? 50 kw? In other words, is the return from electrolysis 25%? Is it 50%? I am interested in the storage capacity from hydrogen produced with renewable energy. Thanks.