Capacity: This is the amount of charge that the portable battery pack can deliver and it's measured in milliamp-hours (mAh). The higher the capacity, the more likely it's able to fully charge your iPhone. Generally, a portable battery with a 5,000mAh capacity can almost fully recharge an iPhone 15.
Size: This refers to the size or bulk of the portable battery and not its capacity (although the two are very connected). This is important because all these portable batteries add bulk to your iPhone 15, 14, 13 or 12 — but some more than others. Also, larger portable batteries will take up more real estate on the back of your iPhone and could block its camera, especially if you have a smaller iPhone mini.
Recharging ability: All of the below portable batteries can wirelessly charge your iPhone over MagSafe, however how you can recharge them once they are out of juice differs. Some support wireless charging, so you can place them on a wireless charging pad. Some support reverse wireless charging, meaning you can connect it to your iPhone and then recharge your iPhone (via Lightning) and it'll charge them both at the same time. But most of the cheaper options require a separate USB-C wired connection, which is admittedly a little inconvenient.
Special features: Not all these MagSafe portable batteries have the same abilities. For example, Apple's option works particularly well with your iPhone (and its Control Center) and can turn into a high-speed MagSafe Charger when connected to power. Others have integrated stands and extra ports to simultaneously charge other devices along with your iPhone. The more features, generally the higher the price.
