nope your math is right on!
But some of these big inline fans and squirrel cage fans can use 100 watts or more, but then again they can also move like 600+ cubic feet per minute of air. But for big grows making a lot of heat, these fans aren't that uncommon. Thanks for checkin in !
If you're using a box fan, that's part of why it is so low in power use. Inline fans have to compress the air to push it through the ducting which is why they don't push as many cubic feet of air per minute... they have to use more power. What setup are you using for your fan?