ZEBs harvest available energy to meet their electricity and heating or cooling needs. By far the most common way to harvest energy is to use roof-mounted solar photovoltaic panels that turn the sun's light into electricity. Energy can also be harvested with solar thermal collectors (which use the sun's heat to heat water for the building). Heat pumps can also harvest heat and cool from the air (air-sourced) or ground near the building (ground-sourced otherwise known as geothermal). Technically, heat pumps move heat rather than harvest it, but the overall effect in terms of reduced energy use and reduced carbon footprint is similar. In the case of individual houses, various microgeneration technologies may be used to provide heat and electricity to the building, using solar cells or wind turbines for electricity, and biofuels or solar thermal collectors linked to a seasonal thermal energy storage (STES) for space heating. An STES can also be used for summer cooling by storing the cold of winter underground. To cope with fluctuations in demand, zero energy buildings are frequently connected to the electricity grid, export electricity to the grid when there is a surplus, and drawing electricity when not enough electricity is being produced. Other buildings may be fully autonomous.