The stock market continues to zig and zag. After a strong July, share prices are sinking again, with the Nasdaq Composite (^IXIC 1.59%) now down 24% year to date. If this month pans out like most Septembers do, the index could soon find itself even deeper in the red.

Not every Nasdaq-listed name is caught up in the sweeping bearishness, though. A couple of them are defying the sell-off so firmly, in fact, that investors are taking notice. This strength may well be a sign that it's time to add one or both to your portfolio.

Enphase Energy

The past few weeks have been great ones for solar stocks, for a couple of reasons. One of them is June's executive order from President Joe Biden, which facilitates an expansion of the United States' solar panel manufacturing industry. And August's passage of the Inflation Reduction Act provides tax credits to homeowners who install new solar power systems at their homes; the credit is even greater for those homeowners who purchase American-made solar panels and related equipment. Manufacturers, however, will also receive tax credits of their own simply for continuing to make what they're already making.

Enter Enphase Energy (ENPH 2.69%). It doesn't make solar panels. Rather, it makes a piece of technology that may be even more important at this point in the solar power era. Namely, Enphase's IQ8 microinverters don't just convert the Sun's rays into electricity. They're part of a smart system that optimizes the collection of this energy, and when applicable, stores it for use when the Sun isn't shining. The company's IQ systems also integrate with electric vehicle charging stations, and all of its functions can be managed with a smartphone app.

This is arguably the big leap solar power was waiting for -- a means of making it work as reliably as conventional, utility-delivered electricity, but doing so more efficiently. That's what the company's revenue and earnings outlook suggests, anyway. Analysts are calling for top-line growth of 62% this year, to be followed by 34% sales growth next year. The stage is set for even better earnings growth -- from last year's $2.41 per share, to $4.09 this year, to $4.98 per share in 2023.

This is increasingly looking like a pivotal, breakout year for Enphase Energy.

DexCom

The other Nasdaq ticker that's clearly caught the eyes of investors is DexCom (DXCM 1.89%). Although it's down by a third of its value year to date, that pullback arguably has more to do with 2021's overheated rally. Regardless, the stock's up nearly 33% from June's low, making it one of that timeframe's biggest winners. It's also back within sight of new multi-month highs, thanks to this week's surge most likely prompted by the company's presentation at an industry conference.

It's not exactly a household name. It's a name, however, that many people depend on every day. DexCom is the maker of continuous glucose monitoring systems (or CGMs) needed by diabetics to manage their condition. As of the latest public report, the company says 1.25 million people are users of one of its CGMs. That makes it one of the bigger players in the business.

This still only scratches the surface of the opportunity, however. DexCom estimates the market for this sort of CGM technology is between 7 million and 8 million people worldwide, with nearly half of that figure being readily accessible diabetics within the United States.

The market is growing too -- unfortunately. Due to a combination of poor diets and lack of exercise, the International Diabetes Federation estimates the percentage of the world's population diagnosed with some form of diabetes will swell from 2019's 9.3% to 10.9% by 2045. And bear in mind that the global population will be nearly 20% bigger than it is now by that time, according to projections from Worldometer.

Given all this, the company's expected revenue growth of 18% this year, followed by next year's 20%, makes a lot of sense. So does the market's renewed interest following the stock's pullback from earlier this year.