If I have an external crystal fitted - but I set "Internal" with the bootloader, will this also reduce power without having to remove crystal from the board?
The crystal circuit should not draw any current.
If the internal oscillator runs at a slower clock speed, yes. As @in0 points out, the crystal itself doesn't consume power.
What's the application and how come you're concerned with the difference in power consumption between the internal and the external oscillator? You're making me curious.
Application is a simple battery powered sensor that transmits via Sigfox.
I have found that running an internal oscillator, I do not have reliable comms with the Sigfox module.
Yeah, I see, so you intend to run the internal RC oscillator for most of the time and switch to external crystal whenever you want to communicate with Sigfox?
I've never tried this; I've mostly played with ATMega and ATtiny uC's that don't support switching oscillators on the fly.
Have you considered using the external crystal, but downclocking during periods when speed isn't critical? Or, perhaps even more obviously - using the various power saving/sleep states of your microcontroller of choice? This would be the most obvious approach IMO.
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