A power bank is a battery in a box, and the box has a number on it. That number is honest but easy to misread. This guide explains what you really get, how to pick the right size and output, and how to keep it safe.
Rated mAh vs real capacity
The capacity printed on the label, say 10,000 mAh, is measured at the battery cell's own voltage of about 3.7 V. Your phone is charged at 5 V or higher through USB. Converting between those voltages, plus heat in the circuitry and the cable, loses energy. In practice you can expect roughly 60 to 70 percent of the rated figure to reach your device, so a 10,000 mAh power bank delivers about 6,000 to 7,000 mAh of useful charge.
Some labels also print the energy in watt-hours (Wh), which is the clearer unit: 10,000 mAh at 3.7 V is 37 Wh. Comparing Wh between products is fairer than comparing mAh.
How many charges will you really get?
Divide the useful capacity by your phone's battery. A phone with a 5,000 mAh battery charged from a 10,000 mAh power bank gets a bit more than one full charge, not two. A 20,000 mAh unit gives that phone roughly two and a half charges. Earbuds and watches have tiny batteries, so even a small power bank refills them many times.
Pick the size by how long you are away from a wall socket:
- 5,000 mAh: a top-up for one phone, fits in a pocket.
- 10,000 mAh: a day out or a long commute, one full phone charge.
- 20,000 mAh: a weekend trip, a tablet, or two people sharing.
- 25,000 mAh and above: laptops, camping and anyone who charges several devices; heavy to carry.
Output watts
Capacity tells you how much energy is stored; watts tell you how fast it comes out. A power bank with a 10 W output will charge a modern phone slowly even if it is huge. Look for a USB-C port with USB Power Delivery and an output of at least 18 to 20 W for phones, 30 W for tablets, and 45 to 65 W or more for laptops. As with wall chargers, a bank with more watts than your device needs is fine; the device takes only what it accepts. If you also charge Android phones that need PPS for full speed, check for PPS support.
Input: how fast the bank itself recharges
A large power bank with a slow input takes all night, or longer, to refill. Check the input watts and prefer USB-C input with PD. A 20,000 mAh bank with an 18 W input takes around five to six hours; one with a 45 W input can finish in under two.
Pass-through charging
Pass-through means the bank can charge a device while it is itself plugged into the wall. It is convenient on a nightstand with one socket. Not every power bank supports it, and on those that do it often runs slower and warmer. If you plan to use pass-through daily, look for a model that states it clearly.
Airline limits
Power banks are lithium batteries and must travel in your carry-on bag, never in checked luggage. A very common rule is that banks up to 100 Wh (about 27,000 mAh at 3.7 V) are allowed without approval, those between 100 and 160 Wh need airline approval, and anything above 160 Wh is not allowed. Many airlines also limit the number of banks per passenger and require the Wh figure to be printed on the unit; a bank with no readable rating can be refused at the gate. Rules change and vary by airline and country, so check your airline's own page before you fly.
Safety and care in the heat
- Do not leave a power bank in a parked car or in direct sun. Egyptian summer heat is enough to damage the cells and, in rare cases, cause swelling or fire.
- Charge it on a hard surface, not under a pillow or on a bed.
- If the casing bulges, cracks or smells, or the bank gets unusually hot, stop using it and dispose of it at an electronics collection point, not in household rubbish.
- Use a decent cable and charger; a damaged cable can overheat the port.
- Avoid draining it to zero and leaving it empty for months. Storing it at around half charge is kindest to the cells.
- Expect the capacity to fall gradually after a few hundred cycles; that is normal ageing, not a fault.
Before you buy
- Confirm the rated capacity and, if printed, the Wh figure.
- Check the output watts and USB-C PD support for your main device.
- Check the input watts so the bank itself recharges in reasonable time.
- Count the ports you actually need.
- Weigh it: 20,000 mAh units are often 400 g or more.
- Look for recognisable safety certification marks and keep the receipt for the warranty.
If you are unsure, ask us through the contact page and we will point you to a matching option.