How to calculate the BTUs a device generates?

I’m trying to decide whether a new data closet will require some type of air conditioner, or whether simple vents in the door and ceiling will provide enough airflow to keep things cool. Is there any way to calculate how many BTUs a device generates by looking at its power requirements? Or does a device’s efficiency rating play too big a part?

Someone gave me a formula. You multiply the current drawn times the wall voltage, and then multiply by 3.17. Supposedly this gives how many BTUs the device will throw off in heat, but I’m nervous about using this.

Anybody know how to figure this out?

Thanks -maera