What voltage is a 9V battery dead?

I'm trying to find information about when a 9 volt battery is sub par to use. I have a voltage meter, but the problem is that I do not know the cutoff in a battery loses its effectiveness. For example, on a regular, non-rechargeable, alkaline batteries, a reading of 8.9 volts means it still has more than 75% of its original capacity? What is the point at which the battery needs replacing - as the voltage reading? I feel that I am replacing the batteries installed in the backup devices must leave office, when they might still do the job at a lower reading but have no idea what the readings actually mean.
Thanks, Terri

Sorry Terri, there is no good answer to this question. First, the voltage is measured in large part due to the stress of chemistry. The battery may have very little power and still be read high voltage when it is "open circuit". You have to measure the battery to get an idea of its state of charge. The load can also be used to develop the same power that the gadget in power. Then, depending on the device's battery is happening. These transistor radio batteries are generally used to power devices that do not have much power. In addition, some devices are designed to operate in a wide range of voltages, so even if the 9 volt battery is only 5 volts to keep running. Finally, 9-volt batteries consist of several cells in series. I have been using six cells in series and five cells in series. So a 9 volt battery could be nominally 7.5 volts. The use of five cells that gives less voltage, but the cells are larger and therefore more time to run if the device is operating can use the lower voltage.

The answer to your question can only be "When the device can not work, then the battery needs to be replaced." I know this does not help much, if necessary, when the batteries are used for emergencies, sorry again.