Question: how much power does a fan use per hour?
Answer: At the midpoint, 70 watts, if a kilowatt costs 10 cents per hour, the fan would cost 0.7 cents per hour (0.07 kwh x 10 cents). Extend that out to a month and it works out to $5.04 per month if it runs around the clock (0.7 cents per hour x 24 hours x 30 days).