1. A computer-readable storage medium storing computer-executable program instructions which, when executed by a processor, perform a method for computing a chargeable time, the method comprising: storing virtual currency units; exchanging data with an external entity; storing the data in a memory; updating the virtual currency units held in the memory in accordance with the received data; transmitting the data from the memory to the external entity; and computing a chargeable time by using a varying load factor that is determined by dividing a processing time, corresponding to an operation period excluding a sleep time, by a used time, which includes the sleep time, and multiplying the result by a coefficient of variation.