Monolithic Power Systems, Inc. (NASDAQ:MPWR) shares have had a really impressive month, gaining 28% after a shaky period beforehand. The last 30 days bring the annual gain to a very sharp 86%.

Since its price has surged higher, Monolithic Power Systems' price-to-earnings (or "P/E") ratio of 29.9x might make it look like a strong sell right now compared to the market in the United States, where around half of the companies have P/E ratios below 19x and even P/E's below 11x are quite common. Nonetheless, we'd need to dig a little deeper to determine if there is a rational basis for the highly elevated P/E.

Recent times have been advantageous for Monolithic Power Systems as its earnings have been rising faster than most other companies. It seems that many are expecting the strong earnings performance to persist, which has raised the P/E. You'd really hope so, otherwise you're paying a pretty hefty price for no particular reason.

Want the full picture on analyst estimates for the company? Then our free report on Monolithic Power Systems will help you uncover what's on the horizon.

What Are Growth Metrics Telling Us About The High P/E?

Monolithic Power Systems' P/E ratio would be typical for a company that's expected to deliver very strong growth, and importantly, perform much better than the market.

If we review the last year of earnings growth, the company posted a terrific increase of 341%. The strong recent performance means it was also able to grow EPS by 370% in total over the last three years. Accordingly, shareholders would have probably welcomed those medium-term rates of earnings growth.

Shifting to the future, estimates from the analysts covering the company suggest earnings growth is heading into negative territory, declining 18% each year over the next three years. Meanwhile, the broader market is forecast to expand by 12% each year, which paints a poor picture.

With this information, we find it concerning that Monolithic Power Systems is trading at a P/E higher than the market. Apparently many investors in the company reject the analyst cohort's pessimism and aren't willing to let go of their stock at any price. Only the boldest would assume these prices are sustainable as these declining earnings are likely to weigh heavily on the share price eventually.

The Final Word

The strong share price surge has got Monolithic Power Systems' P/E rushing to great heights as well. Using the price-to-earnings ratio alone to determine if you should sell your stock isn't sensible, however it can be a practical guide to the company's future prospects.

We've established that Monolithic Power Systems currently trades on a much higher than expected P/E for a company whose earnings are forecast to decline. When we see a poor outlook with earnings heading backwards, we suspect the share price is at risk of declining, sending the high P/E lower. Unless these conditions improve markedly, it's very challenging to accept these prices as being reasonable.

It's important to make sure you look for a great company, not just the first idea you come across. So take a peek at this free list of interesting companies with strong recent earnings growth (and a low P/E).

This article by Simply Wall St is general in nature. We provide commentary based on historical data and analyst forecasts only using an unbiased methodology and our articles are not intended to be financial advice. It does not constitute a recommendation to buy or sell any stock, and does not take account of your objectives, or your financial situation. We aim to bring you long-term focused analysis driven by fundamental data. Note that our analysis may not factor in the latest price-sensitive company announcements or qualitative material. Simply Wall St has no position in any stocks mentioned.