The maximum power or high power, which can be legally transmitted by a marine VHF radio is 25 watts. When reliable VHF communication is available, that is for ranges less than 10 NM, it is perfectly satisfactory to use less transmitter power. The advantage for the user is that there is less drain on the radio’s power source, that is the battery supply. However, the main advantage of using less power is that by reducing the range over which one’s vessel transmissions can be heard, it allows others a few nautical miles away to utilize the same channel. Another important advantage is the possibility of avoiding 'capture effect', as explained below.

For these reasons, all marine VHF radios must be able to transmit on a reduced or low power setting of 1 watt. With one’s VHF antenna sited as high as possible, good communication ranges in excess of 5 to 10 NM on even the smallest boat are possible.

When selecting VHF CH 15 or VHF CH 17 for onboard communications only, the power setting should automatically switch to low power.

When using US channel scheme outside US waters, the lower power setting should be used.