Charge a DC 5V – 2.15 A power bank with an Apple Macbook charger

The power bank capacity is just that, capacity. So the bigger the capacity, the more often you can recharge your iPhone before you need to recharge your power bank. Give or take a few mAh getting lost on the way, the number of recharges possible is basically Capacity of power bank divided by Capacity of iPhone.

This also means that there is no recommended capacity per se. Questions you may want to ask yourself before buying one include

• do I need to recharge several time a day or is one recharge usually enough?
• do I plan to take multi-day cross-country hikes without power outlets on the way
• is low weight important for me (more capacity usually means more weight)
• do I just want to recharge one device or is the goal to charge e.g. an iPad as well

Most MacBook use MagSafe charging which doesn't operate on the same voltage as USB - so the vast majority of battery banks that charge anything USB won't work. There are some inverters that would power the adapter using 120 or 240 VAC but those are quite large.

Only the new MacBook with USB-C connector is low power enough to charge over the new power delivery specification for USB 3.1 that generally is supported along with the change to USB-C connector. Any charging bank would need increased power than what is used to charge most any mobile devices made in 2015 and earlier.