<Li> power to run the sensors, active heating, cooling and telemetry . </Li> <Li> power for spacecraft propulsion--electric propulsion, sometimes called solar - electric propulsion . </Li> <P> For both uses, a key figure of merit of the solar panels is the specific power (watts generated divided by solar array mass), which indicates on a relative basis how much power one array will generate for a given launch mass relative to another . Another key metric is stowed packing efficiency (deployed watts produced divided by stowed volume), which indicates how easily the array will fit into a launch vehicle . Yet another key metric is cost (dollars per watt). </P> <P> To increase the specific power, typical solar panels on spacecraft use close - packed solar cell rectangles that cover nearly 100% of the sun - visible area of the solar panels, rather than the solar wafer circles which, even though close - packed, cover about 90% of the sun - visible area of typical solar panels on earth . However, some solar panels on spacecraft have solar cells that cover only 30% of the sun - visible area . </P>

What advantages do solar cells have in space
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