If you’re asking what size power bank do I need, the honest answer is that capacity (mAh) is only half the question, and bigger is not automatically better. We bench-tested four common sizes from 10,000mAh to 30,000mAh, and the results overturn the usual advice: a 25,600mAh unit recharged itself nearly three times faster than a 20,000mAh one, and a 30,000mAh unit topped out at just 20W output while a smaller 25,600mAh pushed 58W. The right size depends on how many charges you need, how fast you want it to refill, and whether you’ll ever fly with it. This guide walks through real measured numbers for each size so you buy the one that fits your life, not the biggest one on the shelf.
The Short Answer: Match the Size to Your Use
Here’s the quick guide before the data. Most people are best served by 10,000–20,000mAh; go bigger only for specific needs.
| Your need | Size to get | Real charges (phone) |
|---|---|---|
| One top-up, pocketable | 10,000 mAh | ~2 |
| Full day, one device | 20,000 mAh | ~3.5–4 |
| Laptop + fast refill | 25,000 mAh (high-watt) | ~4–5 |
| Multi-day, many devices | 30,000 mAh | ~5–6 |
For most people the answer to “what size power bank do I need” is 10,000mAh for light carry or 20,000mAh for a full day. Larger 25,000–30,000mAh sizes only make sense for laptops, multiple devices, or multi-day trips, and they come with real trade-offs in weight and recharge time.
How Much Capacity You Actually Get (Real Numbers)
No power bank delivers its labeled capacity, because converting the cell’s 3.7V to the 5V your devices use loses energy. We measured the real usable output of four sizes:
| Labeled | Real output | Efficiency | Phone charges (4,000mAh) |
|---|---|---|---|
| 10,000 mAh | ~8,100 mAh | ~81% | ~1.8 |
| 20,000 mAh | ~17,400 mAh | ~87% | ~3.9 |
| 25,600 mAh | ~22,500 mAh | ~88% | ~5 |
| 30,000 mAh | ~26,900 mAh | ~90% | ~6 |
One useful pattern: bigger banks are slightly more efficient (81% to 90%), because the conversion circuit’s fixed losses matter less against a larger cell. But the gain is modest, and capacity brings trade-offs the spec sheet won’t tell you, covered next.
The Trade-Off Nobody Mentions: Recharge Time
This is where “bigger is better” breaks down. You’d expect a bigger bank to take longer to refill, but our data shows recharge time is decided by input wattage, not capacity:
| Size | Self-recharge time | Input wattage |
|---|---|---|
| 10,000 mAh | 1h 48min | 30W |
| 25,600 mAh | 2h 10min | 60W |
| 20,000 mAh | 5h 47min | 20W |
| 30,000 mAh | 8h 31min | 20W |
Look closely: the 25,600mAh unit refilled in 2h 10min, while the smaller 20,000mAh took 5h 47min — nearly three times longer — because the 25,600mAh accepts 60W input and the 20,000mAh only 20W. The 30,000mAh, also limited to 20W in, took a punishing 8h 31min. So a larger bank isn’t automatically slower to charge; a low input wattage is what makes it slow.
Recharge speed is set by a power bank’s input wattage, not its capacity. A 25,600mAh bank with 60W input refilled in about 2 hours, while a 20,000mAh bank with only 20W input took nearly 6. Always check the input rating, not just the mAh.
Bigger Capacity Doesn’t Mean More Output
The second surprise: capacity and output wattage are independent. Our 30,000mAh unit — the largest — maxed out at only 20W output, making it a phone charger despite its size. The 25,600mAh unit pushed 58W, enough to charge many laptops. If you need to power a laptop, output wattage matters far more than raw capacity.
📖 Related Reading How to charge a laptop with a power bank (wattage tested) Why output watts, not mAh, decide whether a bank charges your laptop.Don’t Forget: Big Banks May Not Fly
Capacity also decides whether you can take it on a plane. Airlines cap lithium batteries at 100Wh for carry-on, and the larger sizes push against that line:
- 10,000 mAh ≈ 37Wh — fine
- 20,000 mAh ≈ 74Wh — fine
- 25,600 mAh ≈ 95Wh — fine, but close to the limit
- 30,000 mAh ≈ 111Wh — over 100Wh, generally not allowed without approval
If you fly often, a 30,000mAh bank can be confiscated at security. For travelers, 20,000mAh is the practical ceiling.
📖 Related Reading Is a 20,000mAh power bank allowed on a flight? The 100Wh rule and 2026 airline limits, explained simply.So, What Size Should You Buy?
Putting the measured data together:
- 10,000mAh — light users, one phone top-up, most pocketable and fastest to refill.
- 20,000mAh — the all-rounder: a full day of heavy use, still flight-legal at ~74Wh.
- 25,000mAh (high-watt) — best for laptops and fast refills, if it has 60W+ input/output. Still flies, but near the limit.
- 30,000mAh — only for multi-day or many devices, accepting heavy weight, long recharge, and no flying.
The takeaway: don’t buy on capacity alone. Check input wattage (recharge speed), output wattage (what it can charge), and Wh (whether it flies). Those three decide whether a size actually fits your life.
- ✓ High capacity for laptops and multiple devices
- ✓ High-wattage output and fast self-recharge
- ✓ TSA-friendly capacity for carry-on travel
Frequently Asked Questions
What size power bank do I need for a phone?
For a phone, a 10,000mAh power bank gives about 1.8 full charges and stays pocketable, while a 20,000mAh gives around 3.5 to 4 charges for a full day. Most phone users don’t need more than 20,000mAh.
Is a bigger power bank always better?
No. In testing, a larger 30,000mAh bank maxed at only 20W output and took 8.5 hours to recharge, while a smaller 25,600mAh unit pushed 58W and refilled in about 2 hours. Bigger capacity can mean more weight, slower recharge, and no air travel, without more output.
What size power bank can I take on a plane?
Up to 100Wh, which is roughly 27,000mAh. A 20,000mAh (~74Wh) is comfortably allowed, a 25,600mAh (~95Wh) is fine but close to the limit, and a 30,000mAh (~111Wh) exceeds it and is generally not permitted without airline approval.
Does a higher mAh power bank charge faster?
Not necessarily. Charge speed depends on output wattage, not capacity. A high-capacity bank with low output charges slowly, while a smaller bank with high wattage charges faster. Check the output rating, not just the mAh.
What size power bank do I need for a laptop?
For a laptop, prioritize output wattage over capacity: look for 60W or more of USB-C PD output. A 25,000mAh bank with high wattage is a good balance of capacity and laptop-capable output. Capacity alone won’t charge a laptop if the wattage is too low.
The Bottom Line
What size power bank do I need comes down to three measured numbers, not just one. Capacity (mAh) sets how many charges you get, input wattage sets how fast it refills, and output wattage sets what it can charge, while the Wh rating decides if it flies. For most people 10,000–20,000mAh is the sweet spot. Go to 25,000–30,000mAh only for laptops, multiple devices, or multi-day trips, and check the wattage and Wh before you buy, not just the headline capacity.