Buying a power bank isn’t just about grabbing the biggest number on the box. Pick one too small, and you’ll run out of juice halfway through the day. Pick one too big, and you’ll carry extra weight you never needed. Here’s how to figure out exactly how much mAh is right for you.
How to work out the mAh you need
You need a power bank with at least 1.5 times your device’s battery size if you want a reliable full charge in real life. That extra space covers the energy lost during charging, so your phone actually reaches 100% instead of stopping short.
Start by finding your device’s battery size. Phone makers list this in milliamp-hours (mAh). Most modern iPhones are between 2,800 mAh and 4,400 mAh. Android phones can be smaller or much larger, depending on the model.
Multiply that number by 1.5 to get a safe capacity for one full charge. If your phone has a 3,200 mAh battery, aim for at least 4,800–5,000 mAh. If you need more than one charge a day, multiply again by how many full charges you want.
Understanding power bank capacity ranges
Small (around 5,000 mAh)
These are slim, light, and easy to carry in a pocket. They’ll top up most phones from low battery to nearly full once. Perfect if you just need an emergency backup.
Medium (around 10,000 mAh)
This is the sweet spot for most people. It’s enough for one and a half to two full charges for many smartphones. Still compact enough for a bag, but with a lot more freedom than a 5,000 mAh unit.
Large (15,000–20,000 mAh)
These are travel workhorses. They’ll charge your phone multiple times, and often have the power to top up tablets or headphones too. The trade-off is size and weight. If you fly, check that it’s under 100 Wh so it’s airline-safe.
How charging efficiency changes the math
No power bank transfers 100% of its stored energy to your phone. Energy loss happens during voltage conversion, cable resistance, and heat.
Wired charging usually runs at about 85–90% efficiency. That means a 10,000 mAh wired bank really delivers about 8,500–9,000 mAh to your phone.
Wireless charging is less efficient, usually around 70–80%. You lose more energy as heat and electromagnetic transfer. The lower the efficiency, the bigger the bank you need to get the same usable power.
Special note for MagSafe users
MagSafe battery packs are even less efficient than wired charging. Heat and wireless transfer can drop usable capacity to around 75%. If your iPhone has a 3,200 mAh battery, a 5,000 mAh MagSafe power bank may only give you about 3,750 mAh of usable charge — just over one full charge if your phone isn’t completely drained.
The safe calculation for MagSafe is:
Device battery size × 1.3–1.4 for one full charge.
Multiply again if you need multiple charges without plugging the bank in.
Matching capacity to your lifestyle
The right mAh depends on when and how you use your phone. Here’s a quick guide.
| Usage type | Suggested capacity |
|---|---|
| Quick top-ups only | ~5,000 mAh |
| Full day away from outlet | ~10,000 mAh |
| Multiple devices or long trips | 15,000–20,000 mAh |
Occasional top-ups
If you’re usually near a wall charger and just want peace of mind, 5,000 mAh is plenty. It’s small, light, and gives you one solid boost without adding bulk to your pocket or bag.
Heavy daily use
If you’re on your phone all day for work, navigation, or streaming, 10,000 mAh will give you more freedom. It lets you top up without worrying about running low before the day ends.
No-outlet situations
If you’re camping, flying long-haul, or in situations where outlets aren’t an option, go for 15,000–20,000 mAh. It will keep your phone alive for several days, and can power other devices like earbuds or cameras.
Extra factors that can change your choice
- Multiple devices: Add up the total mAh for all the devices you want to charge, then adjust for efficiency.
- Fast charging: Some high-capacity banks also output more watts, letting you charge faster. This doesn’t change mAh needs but can be a deciding factor.
- Port count: If you’re charging more than one device at the same time, larger capacity helps prevent running dry too quickly.
- Travel rules: Airlines limit batteries to 100 Wh in carry-on (about 27,000 mAh at 3.7V). Check before buying for trips.
Final advice
Pick a power bank with at least 1.5× your device’s battery size for wired charging, or 1.3–1.4× for MagSafe. If you’re on the go all day, 10,000 mAh is a safe bet. If you’re travelling or charging multiple devices, go bigger while keeping it portable.
Choosing the right size means you’ll have enough power without carrying more than you need. A little calculation now saves you from carrying a brick you’ll never fully use — or worse, running out of charge when you need it most.