Next year, Apple (AAPL 0.01%) is expected to launch a successor to this year's iPhone X, as well as a larger version with a huge 6.46-inch display. 

To keep things simple, I'm going to refer to this year's iPhone X as iPhone X (2017), the successor to that as the iPhone X (2018), and the large-screen version of the iPhone X (2018) as the iPhone X Plus (2018).

Not too many specifics are known about the iPhone X Plus (2018) aside from that it'll have a larger, sharper display than the one found on the iPhone X (2017). However, there is one prediction about the phone that I'm extremely confident about that I'd like to share.

Apple's iPhone X.

Image source: Apple.

Bigger phone, more battery life

Since next year's iPhone X Plus (2018) is going to have a much larger screen than either the iPhone X (2017) or the iPhone X (2018), it will necessarily have a larger casing. That larger casing means that the iPhone X Plus (2018) will be able to house a significantly larger battery than the iPhone X (2017) does or the iPhone X (2018) will.

To put things into perspective, the iPhone 8, which has a 4.7-inch display, has a battery with a rated capacity of 1,821 milliampere hours (the larger this value, the more power the battery can store).

Apple's iPhone 8 Plus (left) and its iPhone 8 (right).

Image source: Apple.

The iPhone 8 Plus, which has a larger screen and correspondingly larger casing, contains a 2,691 milliampere hour battery, which is 47.8% more capacity than the battery inside of the iPhone 8.

I think that the iPhone X Plus (2018) will have a similarly larger battery capacity than the ones in either the iPhone X (2017) or the iPhone X (2018).

That increased battery size should mostly translate into better battery life, which is something that a lot of smartphone users care about.

Why "mostly"?

The reason that the much larger battery won't translate into much longer battery life is simple: Power requirements scale with device size.

The iPhone X Plus (2018) will have a larger screen than either the iPhone X (2017) or the iPhone X (2018). This means that if the iPhone X Plus (2018) had a display that had the same pixel density as the display on the iPhone X (2017) or the iPhone X (2018), then it would still have more overall pixels.

More pixels mean greater power consumption, which ultimately translates into lower battery life.

To further compound the issue, the iPhone X Plus (2018) is expected to have a slightly sharper display than the iPhone X (2017) and iPhone X (2018). This means even more pixels on the screen that draw power and chew up even more battery power. 

That's one of the reasons, for example, that the iPhone 8 Plus doesn't feature nearly 50% better battery life than the iPhone 8 -- the battery capacity gain relative to the iPhone 8 is being split between improved battery life, a larger display, and a sharper display.

Now, to be clear, this is by no means meant to be a jab at Apple. I think that customers care about display image quality and if the company can deliver a sharper display while retaining similar battery life, then that's a clear win for iPhone buyers.

Ultimately, everything in engineering is about managing a complex set of trade-offs to deliver the best user experience possible in a reasonable cost structure. I believe Kuo's claim that the iPhone X Plus (2018) will have a larger and sharper display than the iPhone X (2017) and the iPhone X (2018), but at the same time I don't think all the battery capacity gains will go toward the better display -- I think the iPhone X Plus (2018) will have a clear battery life advantage over the iPhone X (2018).

The improved battery life of the iPhone X Plus (2018) compared to the iPhone X (2018) should make the former quite compelling to Apple's most enthusiastic iPhone customers, ultimately driving said customers to buy the larger, more expensive iPhone over the smaller, cheaper iPhone -- something that'll help fuel Apple's iPhone average selling price growth story.