Known originally as "PP3" batteries, rectangular 9-volt batteries continue to be very popular with designers of radio-controlled (RC) toys, digital alarm clocks and smoke detectors. Like 6-volt "lantern" models, 9-volt batteries actually consist of a plastic outer shell that encases several small, cylindrical cells wired in a series. However, 9-volt batteries use different types of a cells (e.g. alkaline, lithium, nickel-cadmium), which have different output capacities To calculate the approximate lifetime of a battery powering a specific appliance, you simply need to know the appliance's power rating and the battery's capacity.
Determine the power rating (in watts) for the appliance using the battery. In general, this information is printed on a label on the bottom or rear of the device. If you're not sure, visit the manufacturer's website, pull up the device's model number and search under "Technical Specifications."
Divide the power rating by 9 volts. The result will be the number of amperes or "amps" that the appliance draws from the battery.
Find the "capacity" for the 9-volt battery by checking the Technical Specifications section of the battery's packaging. Note: the battery's capacity will most likely be measured in milliampere-hours or "mAh."
Divide the battery's capacity by 1000 to convert its units to ampere-hours or "AH."
Divide the battery's AH capacity (from Step 4 ) by the amps drawn (from Step 2). The result is the amount of time (in hours) that the battery will be able to power the appliance.