Abstract: A calculator having six discs concentrically mounted for relative rotation having scales and indices which are graduated and are selectively registrable. The calculator is operable to determine the period, in years, to recover the higher price of a higher efficiency air conditioner over a standard air conditioner taking into consideration the differences in price and efficiency; the energy saved; the climate region in which the air conditioner is to be used; the electric rate in the region in which the air conditioner is to be used; the capacity of the air conditioner; and the available interest rate. The calculator will indicate the number of years over which the air conditioner must be used in order to recover the initial price differential of the higher efficiency unit.