Monitors with back-up for baby's room

Does anyone know of a monitor available that has a back-up battery in the unit that goes in the child's room? We have power outages-usually at night-and I will be dependant on the monitor to hear the baby at night. I have tried out a couple Fisher Price models and one by Safety First, but none have back-up batteries in the child's unit.
Help!! Finding this would put my mind at ease!!