The size of your phone’s battery is measured in mAh. It stands for milliamp-hour and is a great first indicator for how long a battery will last. For example, you could assume that theSony Xperia Z2has a longer battery life than the LG G2 because it has a 3,200mAh battery compared to a 3,000mAh one.
But remember, a higher mAh doesn’t always mean a longer battery life. Different screens, processors and other hardware can be more efficient than others and make better use of a lower capacity battery. Typically, newer phones will have more efficient technology in them so they’ll last longer off of a single charge.
Portable chargers and battery packs also measure their capacity in mAh. And for these it’s even more important to look at the mAh number. That’s because there’s no screen or processor to affect its life. So you know that a 4,000mAh battery pack, like the Mophie Juice Pack Powerstation, would fully charge the 1,560mAh battery in an iPhone 5s twice with some power left over in case of an emergency.