Power Bank Capacity Is Not Always Bigger Is Better! How to Calculate the “Just Right” Capacity Based on Your Phone Model

In the world of portable chargers, the capacity of a power bank is commonly measured in milliampere-hours (mAh). Many consumers assume that the higher the capacity, the better—meaning more charges and longer device uptime. However, this is a misconception. Picking a power bank with the right capacity tailored to your smartphone model and usage habits is key to portability, cost-effectiveness, and practical daily use.

Understanding Power Bank Capacity and Phone Battery Size

The capacity rating on a power bank, such as 10,000 mAh or 20,000 mAh, represents the total amount of electrical charge the internal battery cells can store. However, the real usable capacity that reaches your device is lower due to voltage conversion losses and circuit inefficiencies. For example, a 10,000mAh power bank typically delivers around 6,000-7,000mAh of usable charge when stepping from 3.7V cell voltage to the standard 5V output voltage expected by USB devices (and factoring in roughly 82% efficiency).1

Smartphone batteries also have their own capacity ratings, commonly ranging between 2,500mAh and 5,000mAh depending on the model. To estimate how many full charges a power bank can provide for a phone, a practical formula is:

Number of Charges=(Power Bank Capacity in mAh)×0.85/(Phone Battery Capacity in mAh)

The 0.85 factor accounts for the typical energy losses during charging, such as voltage conversion and heat.

Why Bigger Capacity Power Banks Aren’t Always Better

It’s tempting to buy a huge capacity power bank to ensure multiple charges, but bigger means heavier and bulkier—reducing portability. For many users, a 10,000mAh power bank is enough to fully charge most smartphones once or twice, striking a good balance between convenience and capacity.23

Besides, charging needs depend on usage: light users who use their phones moderately should consider 5,000-10,000mAh models, while medium to heavy users who travel or rely on multiple devices may opt for 15,000-20,000mAh or even higher. But oversizing by too much can be wasteful if you rarely need multiple charges or only carry your phone with you.

How to Calculate the “Just Right” Capacity for Your Phone

1. Find your phone’s battery capacity. This is usually in specs or product manuals. For instance, the iPhone 14 has around 3,279mAh, Samsung Galaxy S23 Ultra about 5,000mAh.

2. Decide how many full charges you want on the go. For example, two full charges.

3. Calculate the baseline capacity need:

   Needed Capacity=(Phone Battery Capacity)×(Number of Charges) 

4. Add about 20% to cover power loss and inefficiencies:

   Adjusted Capacity=(Needed Capacity)×1.2

   For example, for a Samsung phone with a 5,000mAh battery and two desired charges:

5,000×2=10,000mAh⇒10,000×1.2=12,000mAh

You would look for a power bank with at least 12,000mAh capacity. Considering real-world factors, a 15,000mAh model would be ideal to maintain portability.

Additional Tips When Choosing a Power Bank

• Consider your typical usage. Are you mostly out for a few hours or traveling for days? Your power needs vary accordingly.
• Multiple devices. If you charge your phone, earbuds, smartwatch all from one bank, sum their battery capacities and consider total usage.
• Portability preferences. Bigger capacity power banks tend to be heavier and bulkier. Slim models with moderate capacity might fit better in pockets or bags without much weight.
• Brand and quality. Choose reputable brands with accurate capacity ratings and good efficiency to ensure you get what you pay for.45

By understanding how to calculate your ideal power bank capacity based on your specific phone model and usage patterns, you can avoid overspending on unnecessarily large batteries and enjoy the best balance between power, size, and convenience.