If you have a quality unit - heating down to -20 seems to indicate that - the fan speed is gonna do much effeciency wise. The fans watts usage will be little difference.
Auto is just the units best guess to keep you comfortable with the amount of heat it has determined is needed.
For measurment, make sure you are measuring actual billable usage that is PF corrected. The compressor will be all over the place in PF, but residential users don't have to pay for that.
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