The Best Powerbank for a Cruise: Why Size Actually Matters

April 28, 2026

Here’s a question I didn’t think to ask before my first cruise: how many times can you charge your phone if there’s exactly one outlet in your cabin and two people sharing it?

The answer, if you’re not prepared, is “not enough.”

I live in the Bay Area. I work in a research lab. I’m used to thinking about problems in terms of variables and constraints. So before my second cruise I sat down and actually worked out what I needed from a powerbank – not just “something with a big battery” but the specific numbers that matter on a ship.

Here’s what I found.

Quick Answer

For a 7-day cruise, an Anker 20000mAh powerbank covers most people comfortably – it’s under the 100Wh airline and cruise terminal limit, charges two devices simultaneously, and gets you through 2-3 days without touching an outlet. If you travel light with one device, 10000mAh is enough. Anything under 10000mAh will leave you rationing.

The Outlet Problem on a Cruise Ship

Standard Carnival cabins have one North American outlet. Sometimes two, depending on the ship and cabin category. That’s it for 7 days.

There’s usually a USB port near the mirror or nightstand on newer ships – but it’s often a single port, and slow. Not something you want to rely on for a full charge.

If you’re traveling solo this is manageable. If you’re with someone else, you’re sharing. If you both have a phone, a smartwatch, wireless earbuds, and maybe a tablet – you’re doing math every night about what gets charged first.

A powerbank removes that math. You charge the powerbank when an outlet is free, and charge your devices from the powerbank whenever you want.

Simple system. But the numbers matter.

The 100Wh Rule: What You Need to Know Before You Buy

This is the constraint that most people don’t know about until it causes a problem.

Airlines limit lithium battery powerbanks to 100 watt-hours (Wh) in carry-on luggage. Cruise terminals follow the same standard. Anything over 100Wh can be confiscated at security.

The catch: powerbanks are marketed in milliamp-hours (mAh), not watt-hours. The conversion depends on voltage, which is almost always 3.7V for lithium cells.

The formula: mAh x 3.7 / 1000 = Wh

So:

10000mAh = 37Wh – well under the limit
20000mAh = 74Wh – under the limit, fine to bring
26800mAh = 99.16Wh – technically under, but cutting it very close
30000mAh = 111Wh – over the limit, not allowed

That 20000mAh is the sweet spot. Enough capacity to actually matter, comfortably under the limit.

How Much Capacity Do You Actually Need

Let me put some real numbers on this.

A typical iPhone 15 has a battery capacity of about 3274mAh. A Samsung Galaxy S24 is around 4000mAh. Most modern smartphones fall in the 3000-4500mAh range.

A 20000mAh powerbank – accounting for the roughly 85-90% efficiency typical of quality powerbanks – delivers about 17000-18000mAh of usable charge. That’s:

About 4-5 full charges for a standard iPhone
About 4 full charges for a standard Samsung
Enough to cover 2-3 days of heavy use without touching an outlet

A 10000mAh powerbank gives you about 2-2.5 full phone charges. That’s enough if you charge it every night and use it as a top-up during the day.

Anything under 10000mAh – I’d leave at home. It’s not enough to make a real difference on a 7-day cruise.

Capacity	Wh	Phone charges (approx)	Brings on cruise?
5000mAh	18.5Wh	1-1.5	Yes, but barely useful
10000mAh	37Wh	2-2.5	Yes – solo traveler minimum
20000mAh	74Wh	4-5	Yes – best all-around choice
26800mAh	99Wh	6-7	Yes, but risky at security
30000mAh	111Wh	7-8	No – over the limit

What to Look for Beyond Capacity

Capacity is the most important number but not the only one.

Charging speed – input and output.

Input is how fast the powerbank charges itself from an outlet. Output is how fast it charges your devices.

Look for USB-C Power Delivery (PD) on both input and output. A powerbank with 18W or higher PD input recharges itself overnight from the cabin outlet. One with 20W+ PD output fast-charges modern phones.

Older powerbanks with only 5W output will charge your phone, but slowly. Fine if you leave it charging while you sleep. Frustrating if you’re trying to top up in 20 minutes before heading to port.

Number of ports.

Two output ports minimum if you’re traveling with someone else. One USB-C and one USB-A covers most device combinations. Some models have two USB-C ports – useful if you and your travel partner both have modern phones.

Pass-through charging.

Some powerbanks can charge themselves and your device simultaneously from one outlet. On a ship with limited outlets this is genuinely useful. Not all powerbanks support this – check before you buy.

Size and weight.

A 20000mAh powerbank typically weighs around 12-14 oz (340-400g) and is roughly the size of a large smartphone. It fits in a daypack without issue. Nothing you’ll notice on a port day.

What I Actually Use

I’ve been using the Anker 737 (24000mAh, but 88.8Wh – under the limit) for about a year. Before that, an Anker PowerCore 20000. Both have been solid.

Anker is the brand I keep coming back to – not because I have any arrangement with them, but because in my experience the stated capacity is close to actual capacity, the build quality holds up, and the customer service is straightforward if something goes wrong.

That said – the powerbank market is competitive and there are good options from Baseus, Belkin, and RAVPower as well. Whatever you get, check the Wh rating before you pack it.

One habit I’ve developed: I keep the powerbank in the same side pocket of my bag every trip. I don’t think about whether it’s packed – it just lives there. I charge it the night before we leave Sacramento, and by the time we get to the port it’s at 100%.

The Port Day Situation

This is where a powerbank earns its spot.

Port days are long. Off the ship at 8 or 9 AM, back by 4 or 5 PM. You’re navigating with Google Maps, taking photos, maybe running a hotspot for someone else in the group. Your phone is working hard in the heat.

In Ensenada or Puerto Vallarta there’s no obvious place to find an outlet and sit down for 30 minutes while your phone charges. You’re moving.

A full 20000mAh powerbank in your bag means you leave the ship at 100% phone battery and come back with the powerbank at maybe 40%. That’s a comfortable margin.

If I forget the powerbank – and I have, once – I spend the afternoon rationing screen time and turning off features. It changes how the day feels. Not dramatically, but enough that I noticed.

A Note on Cheap Powerbanks

I’ve tested a few no-name powerbanks that showed up as “20000mAh” but delivered maybe 60-70% of that in practice. Capacity ratings from unknown brands are sometimes inflated.

That doesn’t mean you have to spend $60. But it does mean that a $12 powerbank from an unknown brand with 20000mAh printed on it probably isn’t actually 20000mAh.

Mid-range from a recognizable brand – $25-45 range – is where the value is. You get honest capacity numbers and a product that will still work on your third cruise.

When a Powerbank Is Not Enough

If you’re working remotely from the cruise – running video calls, uploading files, keeping a laptop charged – a phone powerbank is not the right tool. You need a separate laptop battery or a higher-capacity solution, and you need to plan your work around outlet access more carefully.

I wrote a separate piece on working remotely from a cruise – [link] – that gets into the specifics of that setup.

Also: if your phone battery is already degraded and holds maybe 60% of its original capacity, factor that in. An old phone that drains fast needs more powerbank capacity than the numbers above suggest.

What I’d Tell Someone Buying Their First Cruise Powerbank

Get the 20000mAh. Not the 10000mAh because it’s cheaper, not the 26800mAh because it sounds like more. The 20000mAh hits the right balance of capacity, weight, price, and staying well under the 100Wh limit without cutting it close.

Spend $30-45 on a recognizable brand. Check that the Wh rating is under 100. Make sure it has at least two output ports and USB-C PD if your phone supports it.

Then put it in your bag and stop thinking about it. That’s the point.

If you’ve found a powerbank that’s worked well across multiple cruises – drop it in the comments. I’m always curious what other people are actually using versus what gets recommended in listicles.

Related articles:

What to pack for a cruise from LA: the full checklist – [link]
Telcel instead of roaming in Mexico: staying connected in port – [link]
How to work remotely from a cruise – [link]

Last updated: April 2026

Disclaimer: Some links in this article are affiliate links (Amazon Associates). The price to you doesn’t change. I only recommend things I’ve personally used or researched in detail.