Check the labels on your equipment
A thermostat’s fan setting, a window unit’s auto fan speed and an energy-saver operating mode are not necessarily the same control. Read the manual to learn whether “auto” changes speed, follows compressor cycles or does something else.
The linked LG operating instructions illustrate a model where energy-saver operation stops the fan when cooling stops. Use that as a reminder to verify behavior, not as a rule for every window, portable or central system.
Count only additional fan hours
Suppose the fan would run four hours during cooling and another eight hours if left continuously on. Only those additional eight hours belong in this direct comparison. A hypothetical 40 W fan would add 0.04 × 8 = 0.32 kWh per day.
At $0.18/kWh for 25 days, that is $1.44. The example excludes any change in compressor operation or room conditions. It is a fan-only accounting exercise, not a complete prediction of the AC bill.
Avoid an incorrect wattage shortcut
If the AC label says 800 W, that does not establish an 800 W fan. Applying the whole-unit rating to a fan-only period can substantially distort the estimate. Use credible mode-specific data or a permitted measurement of the relevant operating state.
For a fixed system, do not attempt household plug-meter measurement. Existing monitoring data or an appropriate professional measurement may be needed if a precise answer is worth obtaining.
Observe comfort as well as cost
Record noise, air movement and humidity concerns while using each permitted setting. A continuously moving fan may suit one routine but be unnecessary in another; it does not supply outdoor ventilation merely because air is moving. Keep required building ventilation separate from optional comfort airflow. Choose a setting based on the manual, the room’s needs and a realistic cost difference instead of assuming that either button is universally best.
Choose your next step
Continue with the question that matches your next decision:
Sources & further reading
Numerical examples are illustrative unless explicitly described as measured. This guide does not claim hands-on product testing. Read the methodology and editorial disclosure.