Speaker Question

Hi I am new to this forum and this question might not be in the right place. I am mounting an amplifier and extension speaker on a metal detector which normally uses headphones. The speaker is small diameter about 2inches wide. It is only 0.5 watt by 4 ohm. would a 5watt by 8 ohm speaker use more power. I am only getting 3hrs usage before the battery is flat in its present set up and dont want to reduce its time any further. Thanks Nello

The power is dependent on the impedance of the speaker, the 8 ohm speaker will take much less power than the 4 ohm one, but will only provide half the power output.

If the amplifier is fairly efficient then the life of the amplifier's battery depends entirely on how loud you turn up the volume and the duration it produces the sound.

You forgot to show which amplifier circuit.
You forgot to say its supply voltage.
You forgot to say which battery size.

Does the metal detector produce a tone only when it detects?