An electric aircraft is an aircraft powered by electricity. Electric aircraft are seen as a way to reduce the environmental effects of aviation, providing zero emissions and quieter flights. Electricity may be supplied by a variety of methods, the most common being batteries. Most have electric motors driving propellers or turbines. Crewed flights in an electrically powered airship go back to the 19th century, and to 1917 for a tethered helicopter. Electrically powered model aircraft have been flown at least since the 1970s, preceding the small unmanned aerial vehicles (UAV) or drones used today. Small UAS could be used for parcel deliveries, and larger ones for long-endurance applications: aerial imagery, surveillance, telecommunications. The first crewed free flight by an electrically powered aeroplane, the MB-E1, was made in 1973, and most crewed electric aircraft today are still only experimental prototypes. Between 2015 and 2016, Solar Impulse 2 completed a circumnavigation of the Earth using solar power. Electric VTOL aircraft or personal air vehicles are being considered for Urban Air Mobility. Electric commercial airliners could lower operating costs. Environmental effects of aviation The environmental effects of aviation on climate change have become a major driving force for the development of electric aircraft, with a zero-emissions electric powertrain being the goal for some development teams. Aviation accounts for 2.4% of all fossil fuel derived emissions, and its emissions of air transportation altogether increased by 32% between 2013 and 2018. While estimating aviation's non- effects on climate change is complex, NOx and contrails could increase this responsibility to 3.5%. Other benefits are the potential for noise reduction, in an industry with a severe noise pollution and abatement problem. Offboard power supply Mechanisms for supplying the necessary electricity without storing all of it onboard include: Solar cells convert sunlight directly into electricity using photovoltaic materials. Microwave energy that is beamed from a remote transmitter. Power cables connected to a ground-based electrical supply. Solar cells A solar cell converts sunlight directly into electricity, either for direct power or temporary storage. The power output of solar cells is low and requires that many be connected together, which limits their use. Typical solar panels running at 15–20% conversion efficiency (sunlight energy to electrical power) produce about in direct sunlight. Usable areas are further limited as output from a poorly performing panel impacts the output of all the panels on its circuit, meaning they all require similar conditions, including