Make an estimate of the duration of the batteries you use in a device.
From the electrical charge of the battery in mAh (or sum of the batteries in series) and the consumption indicated in the device (mA or W and V) you can obtain an estimate of the duration until its exhaustion.
You will get the nominal duration (theoretical duration, if everything was perfect, if that will never happen) and the approximate duration (empirical duration, an approximation to the real world).
Keep in mind that the consumption indicated on a device is the maximum consumption it is capable of producing. It does not mean that you are always consuming that amount, but it will always be according to the type of device.
Keep in mind also that the batteries are not perfect and they will not always have the indicated electric charge (usually they will have something less, but they can also have something more).
If you have several batteries in series, the total electric charge will be the sum of the individual charges.
If you indicate the electrical current of the device (mA), this will be correct if the total voltage of the batteries (sum of V) is that indicated in the device.