What a Power Bank Actually Runs in a Blackout (Real Numbers)
The number on the box is not the number you get.
This is not a scandal and nobody is lying to you. It is physics, and it is consistent enough that you can plan around it once you know the correction. But it does mean that the answer to how long would my power bank last is roughly two thirds of what most people assume, and that gap is worth closing before you find out empirically.
The correction
Power banks are sold in milliamp hours. A common size is 20,000mAh.
Two things eat into that. The cells inside run at about 3.7 volts while your phone charges at 5, so the conversion alone costs you a slice. Then the electronics that do the converting lose more as heat.
The practical result: assume you get about 60 to 70 percent of the printed capacity. A 20,000mAh bank delivers something like 12,000 to 14,000mAh of actual charge. Cold weather takes a little more.
So when you see “charges your phone six times,” expect four.
What that means in hours
Rough figures for a 20,000mAh bank, fully charged, at ordinary room temperature:
Phone, from flat to full: three to four charges for a typical modern phone with a 4,000 to 5,000mAh battery. If you are using the phone lightly and keeping it topped up rather than running it flat, that stretches to roughly three or four days of a phone that stays alive.
Phone kept alive on standby, screen mostly off: close to a week. Airplane mode with the radio switched on only to check messages is the single biggest thing you can do to extend this.
LED lamp or torch, USB powered, a few watts: thirty to fifty hours. A small USB light is the most efficient thing you can attach to a battery, and it is the reason a power bank is a genuinely useful lighting solution and not just a phone accessory.
Small DAB or internet radio, USB powered: ten to twenty hours. A battery-powered analogue radio will beat this comfortably and does not touch your bank at all.
Laptop: most 20,000mAh banks will not charge a laptop, and the ones that do will manage roughly half a charge. If a laptop matters to you, that is a different and much larger purchase.
A fridge, a kettle, heating, or anything with a plug: no. Not close. These are not in the same category of device and no reasonable power bank will run them. Anything claiming otherwise is a portable power station, which is a different product at ten times the price and weight.
What this is actually for
The temptation is to think of backup power as keeping the household running. It will not. Even a proper power station runs a fridge for a day at best.
The realistic goal is smaller and more useful: light, a phone, and news. That trio is what turns a blackout from frightening into merely inconvenient, and it is comfortably within reach of a single mid-sized power bank plus a cheap light. Everything above that trio is a much bigger conversation about batteries and generators, and most households never need to have it.
So the honest specification for a flat is:
One power bank in the 20,000mAh range, kept charged. One small USB light or a torch. Ideally a battery or wind-up radio so news does not compete with your phone for the same battery.
That is perhaps forty or fifty euros of equipment, it fits in a shoebox, and it moves the power lifeline from four hours to something like two or three days.
The failure that actually happens
Power banks self-discharge. Left in a drawer, a bank loses charge slowly, and after a year it may be at half or less. Lithium cells also age faster when stored at full charge or at empty.
The result is that the most common way this preparation fails is not that the bank was too small. It is that the bank was flat when it was needed.
Charge it every three to six months. The easiest way is to make it your ordinary daily power bank, so it cycles naturally, and simply top it up whenever you notice it low. A bank in use is a bank that works. A bank preserved carefully in a drawer for the emergency is the one that will disappoint you.
If you would rather keep it stored, aim to leave it around three quarters full rather than completely full, and put the recharge on the same annual rhythm as rotating your water.
Buying, briefly
Capacity between 10,000 and 20,000mAh covers almost every household. Larger banks are heavier, slower to recharge, and often restricted in hand luggage.
Look for USB-C with power delivery, because it charges the bank itself much faster and will handle newer devices. Pass-through charging, where the bank can charge a device while itself being charged, is genuinely useful and not universal.
Ignore capacity claims that seem impossibly cheap. Overstated capacity is the most common form of dishonesty in this category, and a bank that claims 30,000mAh for fifteen euros is claiming something that costs more than fifteen euros to make.
What to do this week
Find the power bank you already own. There is usually one in a drawer. Charge it fully and note the date.
If there is no bank in the house, buy one in the 10,000 to 20,000mAh range and a small USB light with it.
Then, optionally, the test that tells you more than any specification: unplug for one evening. Lights off, phone on the bank, no laptop. Two hours will teach you more about your household’s real power position than any amount of reading, including this.
Not sure which lifeline is your limit? The 72-hour check takes ninety seconds and names the weakest one.
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