How to Choose a Power Bank: What the Numbers Actually Mean

Power banks are one of the most-bought gadgets in Bangladesh, and one of the easiest to buy badly. Load-shedding, long commutes and all-day phone use make a spare battery genuinely useful — but the numbers printed on the box are widely misunderstood, and some sellers rely on that.

Here is what actually matters.

1. Capacity is not the number you think it is

A 20,000mAh power bank will not put 20,000mAh into your phone.

The cells inside a power bank run at about 3.7 volts. Your phone charges at 5 volts or more. Converting between them loses energy as heat, and the electronics take a small share too. In practice you get roughly 60 to 70 percent of the printed capacity delivered to your device.

So a 20,000mAh power bank realistically delivers around 12,000 to 14,000mAh. If your phone has a 5,000mAh battery, that is about two and a half to three full charges — not four.

This is true of every brand, including the expensive ones. It is physics, not deception. Be suspicious of any seller who claims otherwise, and more suspicious of a 50,000mAh power bank that weighs 300 grams — real cells of that capacity would weigh close to a kilogram.

A useful check: genuine capacity has weight. Roughly 20 grams per 1,000mAh is normal. A light power bank with a huge number on the case is almost always overstated.

2. Watts decide the speed, not mAh

Capacity tells you how much. Wattage tells you how fast.

• 10W (5V/2A) — basic. An overnight charge.
• 18W to 22.5W — the sweet spot for most phones. Roughly half a charge in 30 minutes.
• 30W to 65W — needed only if you charge a tablet or a laptop.

Look for USB-C Power Delivery (PD), which is the standard nearly all modern phones and laptops use. Some Android phones also support Quick Charge (QC). A power bank listing both will work quickly with almost anything.

One catch worth knowing: the cable matters. A cheap USB-C cable rated for 3A will not carry 60W no matter what the power bank supports.

3. Match the capacity to the actual problem

Bigger is not better — it is heavier, slower to refill, and more expensive.

Your situation	Sensible capacity
Topping up during a workday	10,000mAh
Full day out, or load-shedding at home	20,000mAh
Travel with several devices	20,000mAh with two outputs
Charging a laptop	20,000mAh or more, 45W+ PD

A 10,000mAh unit fits a pocket. A 20,000mAh unit does not, really — it lives in a bag.

4. Check how the power bank itself recharges

This gets overlooked constantly. A 20,000mAh power bank with a 10W input takes eight hours or more to refill. The same capacity with an 18W input takes about five. If you recharge it overnight, this does not matter. If you refill it between outings, it matters a lot.

5. Safety features are worth paying for

Look for stated protection against overcharging, over-discharge, over-current, short circuit and overheating. A protection circuit is a few taka of components that prevents a lithium cell from becoming a fire in your bag.

Avoid any power bank that gets hot enough to be uncomfortable to hold, swells at the seams, or has a loose port. Stop using it immediately if it does.

A quick checklist before you order

• Capacity stated in mAh and Wh — Wh is harder to fake
• Output wattage listed per port, not just "fast charging"
• USB-C PD support
• Input wattage stated
• Weight that is plausible for the capacity
• Named protection features

The honest summary

For most people in Bangladesh, a 20,000mAh power bank with USB-C PD at 18–22.5W is the right answer. It covers a full day, refills overnight, and works with almost every phone sold here.

If you are choosing between two similar options, pick the one that publishes real specifications — wattage per port, input rating, weight. A seller willing to state the boring numbers is usually the one with nothing to hide.