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Why 20000mAh is not really 20000mAh
The big number on a power bank is the capacity of the cells inside, measured at about 3.7 volts. But USB power leaves at 5 volts or more. The power bank has to raise the voltage, and some energy turns into heat on the way. Your phone loses a little more as it charges.
As a rule of thumb, plan on about 65% of the printed mAh reaching your phone. Many power banks also print a smaller "rated capacity" figure on the back. Read that one, because it is closer to what you get.
How many charges: the simple sum
Printed mAh × 0.65 ÷ your phone's battery mAh = number of charges.
These are rough estimates from that sum, not test results.
| Phone battery | 10000mAh power bank | 20000mAh power bank |
|---|---|---|
| 4000mAh | About 1.6 | About 3.2 |
| 5000mAh | About 1.3 | About 2.6 |
| 6000mAh | About 1.1 | About 2.2 |
| 7000mAh | About 0.9 | About 1.9 |
Many budget phones in India now carry large batteries, so check yours in the phone's specifications. A bigger phone battery means fewer charges.
For a long train journey or a day of work calls, a 10000mAh pack is lighter and may be enough. A 20000mAh pack makes more sense for several days away, or for two people sharing, but it is heavy in a pocket. Check the weight on the listing.
Watts: buy what your phone can use
Watts decide speed, not capacity. A 67W power bank will not charge a phone faster than that phone allows. Three things must all agree: the power bank, the cable and the phone.
Check the fast charging type too. Some phones reach their top speed only with the brand's own charging system, and fall back to a slower speed on other chargers. USB Power Delivery, often written as PD, is the common standard for iPhones, many Android phones and laptops.
| Your use | Watts to look for | Check this |
|---|---|---|
| Phone top-ups, no hurry | 15W to 22.5W | Any basic cable works |
| Fast charging budget or mid-range phone | 22.5W to 33W | Your phone's listed charging speed |
| Phone with very fast charging | 45W or more | That the phone accepts the power bank's charging type |
| Thin laptop or tablet | 45W to 65W | USB-C port and your laptop charger's watts |
| Phone with magnets, no cable wanted | 15W wireless | Slower, and less efficient than a cable |
Cable types
- USB-C to USB-C. The usual choice for fast charging. Cables differ, so use one rated for the watts you want.
- USB-A to USB-C. Common and handy, but often slower than a USB-C port.
- Lightning. Older iPhones need this cable. Newer iPhones use USB-C.
- In-built cable. You cannot forget it at home. But if it breaks, it is harder to replace than a loose cable.
- Magnetic wireless. Handy on phones with magnets. Other phones need a magnetic case or ring. Wireless charging loses more energy as heat, so you get fewer charges from the same power bank.
Top 5 power banks to look at
We have not tested these. The details come from each Amazon listing, and the pros and cons are our reading of those details and of this type of product. This is a shortlist, not a ranking. We picked popular listings on Amazon.in, so the order does not mean one is better. Check the current price, warranty and reviews on Amazon.
| Product | Type | Listed detail |
|---|---|---|
| Xiaomi 4i | Power bank | 20000mAh, 33W |
| Xiaomi 5i | In-built cable | 20000mAh, 67W |
| Portronics Ignis 65 | Power bank | 20000mAh, 65W |
| Portronics Power Shutter | Magnetic wireless | 20000mAh, 15W |
| Ambrane 20000mAh | In-built Type-C | 20000mAh, 22.5W |
All five list the same capacity, so the choice comes down to watts and cable type.
1. Xiaomi 4i
The Xiaomi 4i is a plain 20000mAh pack with three ports, and the only one of these five without an in-built cable. It is aimed at someone with a fast charging phone who is happy to carry a cable.
Key details
- Capacity: 20000mAh
- Output: listed as up to 33W, with PD 3.0 support
- Ports: 3 in total, with Type-C for both input and output
- Recharge time: listed as about 4.8 hours
- Size and weight: 15 x 7.2 x 2.6 cm, 440 grams
- Warranty: 1 year manufacturer, with a cable in the box
- Three ports give you a choice of cables for different devices.
- Two-way fast charging runs through the Type-C port.
- A loose cable is easy to replace if it breaks.
- At 440 grams it is the heaviest of these five, so it belongs in a bag more than a pocket.
- Laptops are not among the listed devices, and 33W is below the 45W our table suggests for one.
Good for: A fast charging phone or tablet, if the weight does not bother you.
2. Xiaomi 5i
The Xiaomi 5i has the highest listed output of these five, plus an in-built USB-C cable that also works as a lanyard. It is aimed at people who charge a laptop or tablet as well as a phone.
Key details
- Capacity: 20000mAh
- Output: listed as up to 67W, with PD 3.0 and PPS
- Input: listed as 65W, filling it in about 2.5 hours with a 65W or higher charger
- Ports: 3 listed, with USB-A and USB-C connectors, and a digital display
- Size and weight: 14 x 7.2 x 3.2 cm, 415 grams
- Warranty: 1 year manufacturer
- The cable is always attached, so you cannot leave it at home.
- The fast listed recharge is useful when power returns for a short time after a cut.
- Laptops are named among the listed devices, including MacBooks.
- If the in-built cable breaks, it is harder to replace than a loose one.
- The fast recharge needs a 65W or higher charger, and none is listed in the box.
Good for: One pack for a laptop and a phone, if you own a strong wall charger.
3. Portronics Ignis 65
The Portronics Ignis 65 is a 65W pack with a flat braided Type-C cable built in. It is lighter than the Xiaomi 5i, but lists only one port beside that cable. It is aimed at people who charge a laptop or tablet from one pack.
Key details
- Capacity: 20000mAh
- Output: listed as up to 65W from both the Type-C PD port and the in-built cable
- Ports: 1 Type-C PD port for input and output
- Size and weight: 13.2 x 10.7 x 3.6 cm, 380 grams
- Warranty: 1 year, with a user manual in the box
- The in-built cable is listed as carrying the full 65W, so you need no separately rated cable.
- Laptops and MacBooks are named in the listing.
- The display shows the exact battery level, so you are not guessing.
- No USB-A port is listed, so USB-A cables will not plug in.
- The input is called fast, but no recharge time is listed, so confirm it.
Good for: A USB-C laptop and phone user who wants a lighter 65W pack and needs only USB-C.
4. Portronics Power Shutter
The Portronics Power Shutter adds 15W magnetic wireless charging, which sets it apart from the other four, and has in-built Type-C and 8 pin cables. It is aimed at someone who wants to top up a phone with no loose cable.
Key details
- Capacity: 20000mAh
- Wireless: listed as up to 15W for Qi phones, with a magnetic ring
- Wired: listed as 22.5W in total across USB and Type-C outputs
- Cables and port: in-built Type-C and 8 pin cables, plus a two-way Type-C port
- Size and weight: 14 x 10.8 x 3.8 cm, 370 grams
- Warranty: 1 year
- Magnetic wireless charging means no cable at all on a suitable phone.
- Two in-built cables cover Type-C phones and older 8 pin iPhones.
- The LED display shows the battery percentage and charging status.
- Wireless is slower and loses more energy as heat, so you get fewer charges.
- The specs show 1 port while the features mention USB and Type-C outputs, so confirm what you get.
Good for: An iPhone or magnet-ready phone user who values cable-free top-ups over speed.
5. Ambrane 20000mAh
The Ambrane MiniCharge 20 is the lightest and slimmest of these five, with an in-built Type-C cable and output listed as 22.5W. It is aimed at people who carry a pack every day and mainly top up a phone.
Key details
- Capacity: 20000mAh
- Output: listed as 22.5W through PD and USB
- Ports: 2, for charging two devices at once
- Cable: in-built Type-C with a loop design
- Size and weight: 12 x 9.3 x 2 cm, 320 grams
- Warranty: 6 months from the manufacturer
- It is the easiest of the five to keep on you.
- The in-built cable means one less thing to carry each day.
- Two devices can charge at the same time.
- The 6 month warranty is shorter than the 1 year listed on the other four.
- Input watts and recharge time are not listed, so confirm them, because a pack this size takes hours.
- One feature line mentions wireless charging but the title and bullets do not, so do not count on it.
Good for: Daily phone top-ups, for someone who cares more about weight than speed.
Power cuts: two more sums
A 20000mAh power bank holds about 74 watt-hours (20000 × 3.7 ÷ 1000). Using the same 65%, about 48 watt-hours is usable.
- Running a 5 watt USB fan or light: 48 ÷ 5 is about 9 hours, as a rough guide.
- Recharging it: 74 ÷ 20 is about 3.7 hours on a 20W input. In practice it takes longer, because charging slows near the end.
If your area has long cuts, look at the input watts as well as the output watts. Those are two different figures, and the big number on the box is usually the output. Recharge as soon as the power returns, so the pack is ready for the next cut.
Flying with a power bank
Rules change, so check your airline's page before each trip. In general:
- Power banks go in cabin baggage, not checked baggage.
- Airlines commonly allow packs up to 100 watt-hours. A 20000mAh pack at 3.7 volts is about 74 watt-hours.
- The watt-hour figure should be readable on the power bank itself.
- Some airlines limit how many you carry, and may restrict using one during the flight.
BIS mark and safety
Power banks sold in India need BIS registration. Look for the mark with a number starting with R, then check it in the BIS Care app. Skip any power bank that shows no such number.
Keep it away from direct sun, including a parked car in summer. Do not charge it under a pillow. If it swells, gets very hot or smells odd, stop using it, and do not put it in household dustbins.
Read next
We read the Amazon listings, compared the details and noted what they leave out. We have not tested these products, and brands do not pay to be listed.
Common questions
Why does my 20000mAh power bank not give four full charges?
The printed number is measured at the low cell voltage. Raising that to USB voltage loses energy as heat, and your phone loses a little more.
Can I take a power bank on a flight?
Generally it must go in cabin baggage, not checked baggage. Airlines also set their own limits on size, number and use on board, and these change, so check with your airline before you fly.
Will a power bank charge my laptop?
Only if the laptop charges through USB-C and the power bank can supply enough watts at that port. Check the watts on your laptop charger first.
How do I check the BIS mark?
Find the registration number starting with R on the product or box, then look it up in the BIS Care app.