To test how effective a photovoltaic device is, NREL uses solar simulators that can produce from 1-sun—which equals 1,000 W/m²—to about 2,000 suns. Solar Cell Efficiency Tables list records set by 23 different technologies. NREL validated 6 records, including one for mechanically stacked solar cell made of gallium indium phosphide and crystalline silicon that was developed jointly by NREL and Swiss Center for Electronics and Microtechnology. Efficiency of dual-junction cell was measured at 29.8%.