Power Banks Explained: mAh, Wattage and What You Actually Need
Why a 10,000mAh power bank never charges your phone three times, what fast charging really requires, and which capacity suits you.
Power banks are sold on one number, and that number is the most misunderstood specification in consumer electronics.
Why 10,000mAh does not mean three phone charges
Your phone battery might be 5,000mAh. A 10,000mAh power bank therefore looks like two full charges with some to spare. In practice you will get roughly one and a half.
Two things eat the difference.
Voltage conversion. The cells inside a power bank run at about 3.7 volts. USB output is 5 volts or higher. Converting between them loses energy as heat. That alone costs 10 to 20 percent.
Charging losses. Your phone loses more energy again while charging, particularly at high speed, which is why phones get warm.
Realistic rule: expect about 60 to 70 percent of the printed capacity to reach your phone. A 10,000mAh bank delivers roughly 6,000 to 7,000mAh of usable charge.
Wattage decides speed, and both ends have to agree
mAh is the size of the tank. Wattage is the width of the hose.
Fast charging only happens when three things all support it: the power bank, the cable, and the phone. A 22.5W power bank charging through a cheap cable rated for 2.4A will deliver 2.4A, not 22.5W.
Common standards in India:
- PD, or Power Delivery: the USB-C standard. Works across iPhone, most Android and many laptops.
- QC, or Quick Charge: Qualcomm's standard, common on Android phones with Snapdragon processors.
A bank supporting both PD and QC is the safe buy, because it will fast-charge almost anything.
Which capacity you actually need
- 5,000mAh: pocketable, one emergency top-up. Good as a second bank, not a main one.
- 10,000mAh: the sensible default. Roughly one and a half phone charges, still light, and permitted in cabin baggage without question.
- 20,000mAh: two to three charges, or a phone plus earbuds plus a watch over a weekend. Noticeably heavier.
- Above 20,000mAh: only if you are charging a laptop.
Flying with a power bank
Indian carriers follow the international rule. Power banks must go in cabin baggage, never checked. Capacity is regulated by watt-hours, not mAh. Under 100Wh is generally fine, which covers everything up to about 27,000mAh at 3.7 volts. Between 100Wh and 160Wh usually needs airline approval.
A 20,000mAh bank is about 74Wh, so it is comfortably within limits.
What to check on the box
- Both PD and QC support if you own a mix of devices
- Two-way fast charging so the bank itself recharges quickly. A 20,000mAh bank charging at 10W takes most of a day.
- Pass-through charging if you want to charge the bank and a device at once
- Number of output ports if you carry several devices
- BIS certification which is mandatory in India for safety
What we stock
Xiaomi Power Bank 4i 10000mAh, 1,299 rupees
22.5W with PD and QC 3.0, USB-C two-way input and output, and three output ports. The sensible default for most people.
Xiaomi Power Bank 4i 20000mAh, 2,199 rupees
33W super fast charging with PD and QC 3.0. Choose this if you carry a phone, earbuds and a watch, or if two people share one bank.
boAt Energyshroom PB300 10000mAh, 949 rupees
22.5W with 12-layer protection and an aluminium alloy casing. The cheapest here that is genuinely worth carrying.
boAt Energyshroom PB400 Pro 20000mAh, 1,699 rupees
Two-way 22.5W, three output ports and pass-through charging.
Frequently asked questions
How many times will a 10,000mAh power bank charge my phone?
Roughly one and a half times for a phone with a 5,000mAh battery. Voltage conversion and charging losses mean only about 60 to 70 percent of the printed capacity actually reaches your device.
What is the difference between mAh and wattage?
mAh is capacity, or how much charge the bank holds. Wattage is speed, or how fast it can deliver that charge. A high mAh bank with low wattage holds a lot but charges slowly.
Why is my phone not fast charging from my power bank?
Fast charging requires the power bank, the cable and the phone to all support the same standard. A cheap cable is the most common cause. Check the cable is rated for the current you expect and that your phone supports PD or QC as appropriate.
Can I take a power bank on a flight in India?
Yes, in cabin baggage only, never checked. Limits are set in watt-hours. Under 100Wh, which covers everything up to about 27,000mAh, is generally permitted without approval. A 20,000mAh bank is around 74Wh.
What capacity should I buy?
10,000mAh suits most people, giving about one and a half phone charges while staying light. Choose 20,000mAh if you charge several devices or share the bank, and 5,000mAh only as a pocketable secondary.