The Japanese government issued a power usage warning for the first time Sunday as sweltering temperatures in Tokyo and surrounding areas are expected to put pressure on the system as businesses reopen Monday. The Ministry of Economy, Trade and Industry is particularly urging the public in Tokyo Electric Power Co.’s service area to save electricity via measures including turning off unnecessary lights in the demand-intensive 3 p.m. to 6 p.m. period on Monday. The call comes as projections show the area’s reserve power supply capacity ratio will be short of 5 percent Monday. The lowest level nec…