Choosing a USB-C power bank, especially one meant to charge more than just your phone, can quickly become confusing. You see big numbers like 10,000mAh, 20,000mAh, or even higher, alongside wattage ratings like 30W, 65W, or 100W PD. It’s tempting to just grab the power bank with the highest mAh (milliamp-hour) rating, assuming it’s the “best.” However, when dealing with USB-C Power Delivery (PD) for fast charging phones and powering laptops, the output wattage is just as crucial, if not more so, than the capacity alone.
Understanding the difference between power bank capacity and output wattage (PD) is key to selecting a portable charger that actually meets your needs. This MTI Software guide breaks down these two critical specifications.
Defining the Specs: Capacity vs. Output Wattage
These two numbers tell you different things about a power bank’s capabilities:
Capacity: How Much Energy is Stored?
- mAh (milliamp-hours): This is the most commonly advertised capacity metric. It represents the charge the internal battery cells can hold (specifically, how many milliamps of current can be supplied for one hour). While useful, comparing mAh ratings directly can be misleading because it doesn’t account for battery voltage, which differs between power bank internal cells (often ~3.7V) and the standard USB output voltage (5V or higher with PD).
- Wh (Watt-hours): This is the true measure of stored energy. It takes voltage into account (Watt-hours = (mAh * Nominal Voltage) / 1000). A 20,000mAh power bank with 3.7V internal cells holds about 74Wh of energy (20,000 * 3.7 / 1000). Watt-hours provide a much better way to compare the total energy storage between different power banks and estimate how many device charges you might get.
- Analogy: Think of capacity (especially Wh) as the size of the power bank’s fuel tank.
- Flight Limit: Remember that airlines typically restrict carry-on batteries to 100Wh (around 27,000mAh @ 3.7V nominal voltage) without special approval.
Output Wattage (PD): How Fast Can Energy Flow?
- W (Watts): This measures the maximum rate at which the power bank can deliver energy through its USB-C port(s), using the USB Power Delivery (PD) standard. Common PD output levels are 30W, 45W, 65W, 100W, and even 140W or higher on some advanced models.
- PD Negotiation: The power bank communicates with your device to determine the fastest safe charging speed they both support, up to the power bank’s maximum output wattage. [See: Understanding USB PD 3.0 vs 3.1 and PPS].
- Analogy: Think of output wattage as the diameter of the fuel hose delivering energy to your device.
Why BOTH Capacity (Wh) and Wattage (PD) Are Crucial
You need the right combination of both specs for optimal performance:
- High Capacity (Wh), Low Wattage (W): This power bank is like a massive fuel tank with a tiny hose. It stores a lot of energy, but it delivers it too slowly to effectively charge demanding devices like laptops. Your laptop might charge extremely slowly or not at all.
- Low Capacity (Wh), High Wattage (W): This is a small fuel tank with a wide hose. It can deliver power quickly (e.g., fast-charge a phone or briefly power a laptop), but it will deplete its stored energy very fast.
- The Goal: Find a power bank with sufficient output wattage (W) to meet your device’s charging requirements and enough capacity (Wh) to provide the amount of runtime or number of recharges you need.
Prioritizing: What Matters More for Your Needs?
How you weigh capacity versus wattage depends on what you’re charging:
Charging Laptops
- Output Wattage (PD) is CRITICAL: The power bank’s USB-C PD output must meet or exceed your laptop’s minimum charging wattage (check your laptop’s original charger or specs – often 45W, 65W, or 100W+). A 20,000mAh bank with only 20W output cannot effectively charge a 65W laptop, regardless of its capacity. [See: Choosing the Right USB-C Charger Wattage].
- Capacity (Wh) Determines Runtime: Once the wattage requirement is met, choose a capacity based on how much charge you need. A ~74Wh (20,000mAh) bank might give a modern ultrabook about one full charge (considering efficiency losses). A ~100Wh (27,000mAh) bank provides more runtime, pushing the airline limit.
Fast Charging Phones/Tablets
- Output Wattage (PD) Enables Speed: Ensure the power bank’s PD output matches your device’s fast-charging capability (e.g., 20W for recent iPhones, 25W/45W PPS for Samsung, 30W+ for some Pixels/tablets). [See: Understanding USB PD 3.0 vs 3.1 and PPS].
- Capacity (mAh/Wh) Determines Number of Charges: A 10,000mAh (~37Wh) bank typically provides 2-3 phone charges. A 20,000mAh (~74Wh) provides significantly more. Choose based on how many recharges you anticipate needing.
Charging Multiple Devices
- Consider the total combined output wattage and how the power bank distributes it across ports when multiple are used.
- Ensure the total capacity (Wh) is sufficient for the combined energy needs of the devices you plan to charge simultaneously.
Reading the Specs: Finding Wh and PD Wattage
- Always check the detailed specifications on the manufacturer’s website or product listing.
- Look specifically for USB-C PD Output wattage (e.g., “65W PD Output”).
- Look for capacity listed in Watt-hours (Wh) if available. If only mAh is listed, check if the nominal voltage is provided to calculate Wh (Wh = mAh * V / 1000). Assume ~3.7V internal voltage if not specified, but a direct Wh rating is more reliable.
- Remember real-world output capacity will be lower (~60-70%) than the internal battery’s stored energy due to conversion losses.
Conclusion
When choosing a USB-C PD power bank, looking solely at the mAh capacity isn’t enough, especially if you need to charge a laptop or utilize fast charging. Output Wattage (PD) determines how fast the power bank can deliver energy and whether it’s compatible with demanding devices like laptops. Capacity (Wh) determines how much total energy is stored, dictating runtime or the number of recharges.
Prioritize finding a power bank that meets the minimum PD Wattage required by your most power-hungry device. Then, select a Capacity (Wh) that provides the energy reserves you need, balancing it with portability and airline regulations. By understanding both specs, you can choose a portable charger that truly meets your needs.
Ready to choose? See our recommendations: Best USB-C PD Power Banks. For more on wattage: Choosing the Right USB-C Charger Wattage. Explore the full ecosystem: The Ultimate Guide to USB-C.