Solar, hydropower, and wind energyWind energy systems are considered; each has characteristic power as a function of area, height, and wind speed, respectively. The efficiency of solar photovoltaicsPhotovoltaics (PV) has increased, across many photovoltaicPhotovoltaics (PV) technologies. Solar and wind energyWind energy are both intermittent; storage can smooth out the variation. The costs of solar photovoltaicsPhotovoltaics (PV), wind, and batteries have fallen significantly in recent years. Storage includes batteries, pumped hydroPumped hydrostorage, and other technologies. Lithium-ion battery costs have fallen substantially.