In addition to the voltage specification, batteries and power supplies are rated in ampere - or amp - hours, which indicate the product's capacity. One amp-hour is the amount of power needed to provide 1 amp of power for an hour - although the president of one power supply manufacturer says that formula is a bit "idealized." In theory, dividing a power supply's capacity in amp-hours by its load, measured in amps, should give your system run time, notes Dan DeMerchant, president of Highpower Security Products LLC of Meridien, Conn. But that number will be just an approximation, he says. "Leave a 20 percent cushion for longer power supply life," DeMerchant advises.
Stephen Custer, vice president of Oberlander Alarm Systems, a Peoria, Ill.-based security installing company, leaves an even wider margin. "We won't load a power supply to more than 70 percent of its value," he says. "If the power supply isn't stressed, you get good longevity. If we get to 60 percent to 70 percent of the load on an alarm system, we add auxiliary power supplies."