Energy efficiency of an air conditioner is calculated as the amount of Btu (British thermal units) per hour drawn for every watt of power that the air conditioner uses. In the case of room air conditioners, this is calculated as an energy efficiency ratio (EER). Efficiency in central air conditioners is measured as a seasonal energy efficiency ratio (SEER). These ratings are available on all new air conditioners and can be seen in the energy guide that is provided with them. The energy guide provides valuable information such as the annual operating cost of the air conditioner. Manufacturers that adhere to energy star standards will voluntarily label their products. Appliances with higher EER and SEER ratings will often cost more. However, the amount of savings incurred when using these conditioners makes the investment worthwhile.