Best power bank for emergencies: how many mAh do you need?
For a 72-hour power cut, a 20,000 mAh power bank per household (or per two people) is the sweet spot: enough to keep a smartphone running for several days, small enough to carry. Choose USB-C Power Delivery, keep it charged to around three-quarters, and top it up every few months.
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mAh, Wh and phone charges explained
Power banks are sold in milliamp-hours (mAh), but that number is measured at the battery’s internal voltage of about 3.7 V. Watt-hours (Wh) are the honest unit: Wh = mAh × 3.7 ÷ 1,000. Some energy is lost converting to 5–20 V USB output, so expect around 60–70% of the rated capacity to reach your phone.
| Power bank | Energy | Typical phone charges* | Weight | Good for |
|---|---|---|---|---|
| 10,000 mAh | ≈ 37 Wh | ≈ 1.5–2.5 | ≈ 200 g (7 oz) | Grab bag, daily carry |
| 20,000 mAh | ≈ 74 Wh | ≈ 3–5 | ≈ 350–500 g (0.8–1.1 lb) | 1–2 people for 72 hours |
| 24,000–27,000 mAh | ≈ 89–100 Wh | ≈ 4–6 | ≈ 550–650 g (1.2–1.4 lb) | Families, laptops |
| Portable power station | 250–2,000+ Wh | Dozens | 3–20 kg (7–44 lb) | Router, fridge, medical devices |
*Depends on the phone’s battery (most are 3,000–5,000 mAh) and how efficient the power bank is.
How much do you really need for 72 hours?
With the phone in low-power mode, screen dim and data used only for checking messages, a smartphone can often last a day or more on one charge. For three days, plan roughly one full charge per phone per day, plus a top-up for a radio or head torch if they charge by USB. For two people that is about six charges: a 20,000 mAh bank gets you most of the way, two get you there comfortably.
If you rely on a medical device, a router for working from home, or a fridge for insulin, go straight to a portable power station.
What to look for
- USB-C Power Delivery (PD) in and out, ideally 20 W or more, so it recharges quickly when power returns.
- Two or more ports to charge a phone and a radio at the same time.
- Pass-through charging if you want to charge it from a solar panel while using it.
- A charge indicator with percentage or at least four LEDs.
- A known brand with safety certifications. Very cheap no-name banks often overstate capacity.
Solar power banks: the honest view
The small panel on a solar power bank produces very little: even in strong summer sun it can take days to refill a 20,000 mAh battery, and far longer in a northern winter. They are fine as a “never completely flat” backup. If you want real solar charging, pair an ordinary power bank with a foldable 20–30 W USB solar panel, or a power station with solar input.
Our picks
| Product | Best for | Key spec | Buy |
|---|---|---|---|
| 20,000 mAh power bank | Most households | USB-C PD, several charges | Check price on Amazon |
| Anker Nano 10,000 mAh with built-in cable | Grab bag, daily carry | No cable to forget | Check price on Amazon |
| Anker 737 Power Bank (24,000 mAh) | Families, laptops | High-wattage USB-C | Check price on Amazon |
| Solar power bank (20,000 mAh) | Backup that never fully dies | Slow trickle charge | Check price on Amazon |
| USB-C car charger | Recharging from the car | Engine running, outdoors | Check price on Amazon |
| EcoFlow River 3 | Small step up to a power station | 245 Wh with AC socket | Check price on Amazon |
We don’t list prices because they change often. Use the button to see the current price on Amazon.
How to store a power bank
- Keep it charged to about 50–80% for long storage, and top it up every three months.
- Store it at room temperature: cold reduces usable capacity, heat shortens its life.
- Keep the right cables with it (USB-C, Lightning or micro-USB as needed).
- Replace it if it swells, gets hot when idle or loses charge quickly.
The US FAA and most airlines allow lithium power banks up to 100 Wh in cabin baggage without approval, 101–160 Wh with airline approval, and never in checked bags. Rules vary, so check before you fly.
Save battery in a blackout
- Turn on low-power or battery-saver mode and dim the screen.
- Send text messages instead of calls; they need less network and less power.
- Turn off Wi-Fi and Bluetooth if you don’t need them, and close apps that refresh in the background.
- Get news from a battery radio, not the phone.
A power bank is on every kit list on this site, from the basic kit to the communication kit.
Questions people ask
How many mAh power bank do I need for an emergency?
A 20,000 mAh bank (about 74 Wh) gives roughly three to five phone charges, enough for one or two people for 72 hours if you use the phone sparingly. A 10,000 mAh bank suits a grab bag; 24,000 mAh or more suits families.
Are solar power banks worth it?
As a backup, sometimes. The built-in panel is tiny and can take days of strong sun to refill a large battery. A separate foldable solar panel is much faster.
How long does a power bank hold its charge in storage?
Good lithium power banks lose only a little charge per month, but they do lose some. Check and top up every three months, and store them at room temperature.
Can I take a 20,000 mAh power bank on a plane?
Usually yes: 20,000 mAh at 3.7 V is about 74 Wh, below the common 100 Wh limit for carry-on. Power banks must not go in checked baggage. Check your airline’s rules.
Power bank or portable power station?
A power bank charges phones, radios and lights. If you need to run a router, a fridge or a medical device, you need a portable power station.
Keep reading
- /best-portable-power-station/
- Best emergency radio: crank, solar and battery
- The 2025 Iberian blackout: 10 lessons for your kit
- /kits/communication/