Which of the following is an appropriate power supply rating for a typical 50-watt output mobile FM transceiver?
Why Mobile radios are designed for the vehicle's nominal 13.8-volt charging system, so the supply voltage must be 13.8 V, not 24 V. Current matters too: a 50-watt output FM transmitter is only about 40-50 percent efficient, so it draws roughly 100-150 watts of DC input, and 13.8 V times 12 A is about 165 watts, which covers it with margin. A supply must be able to deliver both the correct voltage and at least the peak current the radio draws on transmit.
Watch out The 13.8-volt, 4-ampere choice has the right voltage but only supplies about 55 watts of DC input, far short of what a 50-watt transmitter needs; the 24-volt choices are the wrong voltage for automotive gear entirely.
Cars run 13.8 V. For 50 W out, figure roughly double the DC input, so about 12 A.