It's about 200 or 300 kJ/mole. A mole is a measure of molecules (6.022 x 10^23 molecules). You get the same amount out as you put in. There's no free lunch. In fact if you really work it all out, you need more energy one way than the other. This happens if you include activation energy, and entropy into your calculation. Activation energy is this extra bit of energy you need just to get things started, and entropy is this extra energy you need just to keep things going. All this is discussed in virtually every first year university or 12th grade high school chemistry book, usually under chapters on enthalpy or thermodynamics. You can find these numbers in tables of enthalpy and heat of formation in many reference books, or on the web. A classic reference book, which I'm sure is in your town or school library is the Chemical Rubber Book (CRC Handbook). Your librarian is sure to know about this.