This is a physics textbook describing, at a college level, the physics and technology needed to provide sustainable long-term energy, past the era of fossil fuels. A summary is given of global power generation and consumption, with estimates of times until conventional fuels will deplete. Sustainable power sources, largely those coming from the Sun directly or indirectly, are described. As sustainable energy must preserve the Earth’s atmosphere and climate, key elements of these topics are included. Key energy technologies in this book include photovoltaics, wind turbines and the electric power grid, for which the underlying physics is developed. Nuclear fusion is described in the context of the Sun’s energy generation, in a brief description of tokamak fusion reactors, and also to introduce ideas of quantum physics needed for adequate treatment of photovoltaic devices. Energy flow in and out of the Earth’s atmosphere is discussed, including the role of greenhouse gas impurities arising from fossil fuel burning as trapping heat and raising the Earth’s temperature. Discussion is included of the Earth’s climatic history and future. Exercises are included for each chapter.