What's the difference between battery capacity and battery life?

The higher the battery capacity of a device, the more energy it can store, which theoretically increases its run duration per charge (i.e. battery life). So a phone with a 5,000mAh battery should, on paper, last longer than one with a 4,200mAh battery. Similarly, in theory, the capacity of a power bank should indicate how many times it will charge your phone. 

For example, an iPhone 18 Pro (US model) has a battery capacity of 4,288mAh, meaning that a 10,000mAh power bank could theoretically charge it a little more than two full times. However, charging isn't perfectly efficient; you'll usually see a 20 to 30 percent loss with normal charging. Fast charging and wireless charging generate more heat and thus have even higher loss percentages.

When it comes to real-world battery life, many factors like this are at play. You need to consider how quickly the device consumes energy and how efficient the battery is. If you're using a laptop with a large battery capacity to play modern games at high graphic settings, its battery will likely deplete faster than one with a lesser battery capacity used only for word processing. 

Elements like screen size matter; an iPad has a bigger battery than an iPhone, along with a much larger screen that uses more energy. Temperature also impacts battery life; you've likely noticed your phone battery draining more quickly when taking photos outside on a hot summer day. OS and hardware efficiency plays a part too, which is why Apple silicon MacBooks last so much longer than older Intel models. 

Battery life can never be a perfect measurement, since you don't use your devices exactly the same way each day. Because of this, companies often provide battery life numbers based on hours of playing video files, with Apple adding a "typical battery life" number for some of its newer devices. 

And as batteries age, their capacity decreases. This is why knowing your battery health is important; after 1,000 charge cycles, that "100 percent" battery life icon doesn't mean what it did when you got your device.