being at a similar angle to the sun, and not being masked by shadow. Between 2010 and 2020, solar power modules have declined in cost by 90% and continue to drop by 13–15% per year. Solar cell efficiency has also risen substantially, from 2% in 1955 to 20% in 1985, and some experimental systems now exceed 44%. However, most of the technologies at these high efficiencies have only been possible under laboratory settings and not at full scale production level. The free availability of sunlight makes solar power attractive for high-altitude, long-endurance applications, where the cold and reduced atmospheric interference make them significantly more efficient than on the ground. The drop in the dry-air temperature as altitude increases, called the environmental lapse rate (ELR), averages 6.49 °C/km (memorized in pilot training as 1.98 °C/1,000 ft or 3.56 °F/1,000 feet) so that temperature for a typical airliner's cruising altitude of around will be substantially lower than at ground level. Night flying, such for endurance flights and with aircraft providing 24 hour coverage over an area typically require a backup storage system, which is charged during the day from surplus power, and supplies power during the hours of darkness. Microwaves Power beaming of electromagnetic energy such as microwaves relies on a ground-based power source. However, compared to using a power cable, power beaming allows the aircraft to move laterally and carries a much lower weight penalty, particularly as altitude increases. The technology has only been demonstrated on small models and awaits practical development at larger scales. External power cables For powered vehicles replacing tethered aerostats, an electrical power cable can be connected to a ground-based supply, such as an electric generator or the local power grid. At low altitudes this avoids having to lift batteries, and was used by the experimental Petróczy-Kármán-Žurovec PKZ-1 observation vehicle of 1917. However the higher it flies, the heavier the length of cable it lifts becomes. Power storage Mechanisms for storing the necessary electricity include: Batteries which use a chemical reaction to generate electricity which is reversed when recharged. Fuel cells consume fuel and an oxidizer in a chemical reaction to generate electricity, they need to be refueled, typically with hydrogen. Batteries Batteries are the most common onboard energy storage component of electric aircraft, due to their relatively high storage capacity. Batteries first powered airships in the nineteenth century but the lead-acid batteries were very heavy and