The United States uses approximately 100 exajoules (100 x 1018 or 100,000,000,000,000,000,000 joules) of energy per year for all sources — about one-fifth of all energy usage in the world. About one-third of that amount is equally divided between commercial/residential usage (heating, cooling, hot water, and electricity), industrial usage, and transportation usage. In terms of sustainability concerns, energy generation contributes primarily to resource depletion of energy products (such as fossil fuels) and air pollution issues. Sustainable energy generation implies near-zero lifecycle emissions, and also a renewable energy source one where the fuel is not used up any faster than it is renewed (sustainable biomass) or comes from an essentially infinite source (the sun, deep geothermal sources, and nuclear fusion). The two most often discussed green energy generation options are direct solar and wind energy. In addition, we group hydroelectric, geothermal, and biomass in this category. Many other alternative energy generation methods are being studied, such as ocean thermal, wave, and tide systems and nuclear fusion research.