It's on every Apple (NASDAQ:AAPL) investor's mind: How well will the new Apple Watch sell when the device goes on sale next year? Sure, any estimate of the number of smartwatches the company will sell in 2015 is purely speculation at this point, with very little substance to back up any guess. But this doesn't mean investors shouldn't think about the Apple Watch's implications on the tech giant's business. After all, Apple only sells a few different categories of products. So just one product category can have a large impact on the company's bottom line.

In this context, let's examine potential bear and bull cases for how the device could impact Apple's business.

Apple Watch. Image source: Apple.

What we know
On average, analysts estimate Apple will deliver 22.6 million Apple Watches in 2015, according to estimates compiled by Fortune magazine's Philip Elmer-DeWitt. But with the highest estimate at 60 million and the lowest at 10 million, there is great disparity between the most optimistic and the most pessimistic projections, confirming that no one really has a clue.

Citing industry sources, DigiTimes reported that Apple has put in enough orders from suppliers for 30 to 40 million Apple Watches. If that is true, Apple seems to be more bullish on its new product than the consensus analyst estimate.

One version of the Apple Watch Edition. 18-karat rose gold. Image source: Apple.

As far as pricing, all we know for certain is that the entry-level Apple Watch Sport will start at $349. Daring Fireball's well-connected John Gruber predicts the stainless-steel Apple Watch will start around $999 and the gold Apple Watch Edition could be priced as high as $4,999.

Bear case
Worst case, we'll assume Apple sells only 15 million Apple Watch devices in calendar year 2015. This would peg Apple Watch sales at about 7.5% of Apple's estimated iPhone sales for fiscal 2015. Notably, however, this 15 million would be achieved in a shorter time frame than an entire fiscal year, since Apple isn't expected to launch the device until around March.

For our bear case, we'll assume the Apple Watch Sport dominates sales and brings the device's average selling price down to $550. Furthermore, we'll assume the product line's gross profit margin is in line with Apple's corporate gross profit margin of 39%.

In this bear case, Apple Watch sales could boost Apple's net income for calendar year 2015 by $1.8 billion, or 4.5%.

Bull case
In our bull case, let's assume Apple sells 40 million Apple Watches in the first year of production at a higher average selling price of $650. Gross profit margin for the product line will again be in line with Apple's corporate average.

The impact on the bottom line? Apple's net income for 2015 would increase by $5.5 billion, or 13.9%.

While either the bear or bull case could be respectively not pessimistic or optimistic enough of the actual outcomes, these stabs in the dark still paint a clear picture: For a company priced at just 18 times earnings, the Apple Watch's potential impact on Apple's bottom line is too great to ignore.

Even more, given Apple's history of convincing its loyal base of consumers to pay up for new devices, we are likely safe in predicting the product will be a moderate success and live up to our bear scenario. And even a 4.5% increase to Apple's net income is meaningful given Apple stock's conservative valuation.