Seasonal buy-back (purchase) rates may be selected in lieu of annual rates at the cooperative/utility's option if there are seasonal rates for standard retail service. If selected, the seasonal rates will be calculated as follows: the sum of the months in effect for each period times the rate for each period must equal twelve months times the annual rate, and the difference between the summer and winter rates for purchase must be the same as the difference between rates for service at the standard residential service rates. The following formula will calculate the summer purchase rate (SPR): (SMO)(SPR)+(12–SMO)(SPR-(SSR-WSR)=12(APR)Where SPR = Summer purchase rateSMO = Months summer service rate in effectSSR = Summer service rateWSR = Winter service rateAPR = Annual purchase rate