Which is more powerful 12.5hz or 50hz?

Neither... or both depending on how you view the issue.

12.5 hertz or 50 hertz are not measurements of power or energy. They are measurements of frequency. 12.5 hertz reverses the flow of electrons in the circuit 12.5 times each second. 50 hertz does so 50 times each second.

Power is measured in the voltage (how far above and below 0 volts this wave goes) and by how many electrons are moving (the current of the unit). We take and multiply the two units of measurement to derive the wattage of the signal.

Watts tell us how much instantaneous power each of these signals would have.