When you shop online for e-cigarettes, you’ll frequently see the term “mAh” mentioned in battery specifications. You might correctly assume that a higher mAh number must be better, but what does the term actually mean?

mAh is short for milliampere-hour, and it’s a term that describes the total charge capacity of a battery. In other words, a battery rated at 1000 mAh can deliver one amp of power for one hour before it dies.

To understand how the mAh rating of an e-cigarette battery translates to actual usage, you need to know the number of amps that your battery and atomizer or cartomizer draw. A 2.5 ohm atomizer and 3.7 volt battery — most e-cigarettes fit this description — would draw 1.48 amps. If you’re using a battery with a different voltage or an attachment with a different resistance, use the Ohm’s Law Calculator to find the current that the combination would draw in amps. Multiply the number by 1000 to convert amps to milliamps.

Because atomizer and cartomizer resistance can vary by around .2 ohms in either direction and the voltage of an e-cigarette battery drops as it’s used, this isn’t an exact science. However, it’s possible to use the information above to figure out how many puffs you’ll get from an e-cigarette battery based on its mAh rating.

Next, take a look at the Battery Life Calculator. Enter the mAh rating of the e-cigarette battery you’re curious about, and then enter “1480” in the “Consumption of Device” field or use the mAh value that you found using the Ohm’s Law calculator. Click “Calculate” to get the expected life of the battery in hours.

Once you find the life of the e-cigarette battery in hours — you’ll get a small number like .132 — multiply it by 3600, the number of seconds in an hour. That’s the total number of seconds you can spend puffing with the battery. If the battery in question has a capacity of 280 mAh, for example, the result will be 475 seconds.

Finally, divide that total by the length of your average puff in seconds. For example, if the battery in question has an estimated life of 475 seconds per charge and your average puff is three seconds long, you can expect to get around 158 puffs out of the battery before it does, although you’ll probably get more puffs in practice because, as I mentioned above, the battery’s voltage will drop a bit as you use it.