We use device hours. On an average one device will consume about .75 amps at 12vdc (9 watts). If you have a 1200Ah (14,400 watt hours) battery pack you have 1600 device hours. The total devices need to include everything from cameras, communications, power regulators, POE power supplies, inverters, transformers, and the status monitoring system. So a system with 1 PTZ Camera (no heater) , 1 camera power supply, 1 communications device, 1 Status monitor = 4 device hours x 24 hours = 96 device hours / day. Then 1600 device hours ÷ 96 device hours/day = 16.6 days.