On Monday, more investors eagerly piled into Icarus-like First Solar (NASDAQ:FSLR), which touched a 300% year-to-date gain by the closing bell. The market leader in thin-film photovoltaic production has been rocketing higher on blazing top-line growth, which was further fueled by a clutch of new contract signings worth roughly $1.3 billion.

Sector news wasn't limited to First Solar. Trina Solar (NYSE:TSL) announced some European contract wins of its own, and JA Solar (NASDAQ:JASO) added manufacturing lines ahead of schedule.

Evergreen Solar (NASDAQ:ESLR) shares rose because ... its competitors are doing well?

Still, Monday was First Solar's day to shine. Shares rose roughly 24% -- quite the feat considering the firm's robust market capitalization, but not beyond belief. What I don't believe is the valuation being afforded this hot stock.

I'm not hung up on the P/E ratio of 500 on principle, though it would likely cause even Akamai (NASDAQ:AKAM) or Amazon.com (NASDAQ:AMZN) investors to blush. I have no doubt that the company's future earnings will leave the trailing numbers in the dust. However, there is a lot of long-term certainty baked into today's share price.

You can't buy this stock, or any for that matter, just because you think it's going to go higher. You need, at a minimum, a back-of-the-napkin sketch. If you used the PEG ratio (price/earnings to growth) to determine that today's price represents fair value for First Solar shares, you'd need to assume a long-term growth rate of -- guess what? -- 500%.

The PEG is merely inexact shorthand, however, and for me it breaks down when you're dealing with astronomical growth rates. Suffice it to say that I'd call today's P/E fair if First Solar could be expected to achieve 138.5% average annual earnings growth over the next five years.

Let's see what 138.5% earnings growth looks like over five years, the bare minimum of what could be considered a long-term forecast period:

Year 0 (trailing-12-month earnings): $15 million
Year 1: $36 million
Year 2: $85 million
Year 3: $203 million
Year 4: $485 million
Year 5: $1.16 billion

In the early years, I won't argue with these numbers. After all, first-quarter sales rose fivefold over those of the prior year, perhaps owing to some cosmic alignment with the Aztec Five Suns creation myth. That, or there's a ton of demand for thin-film solar modules. Believe what you wish.

Simply triple trailing sales in year one and then double sales in year two, and you've got your $85 million in earnings. No sweat? Maybe. But looking further out, I see serious reasons to sweat if you're holding these shares to bask in the long-term riches they'll supposedly bestow.

Given the fierce competition in the space, the company has little shot at raising its profit margins meaningfully over this period. That leaves just one way for First Solar to make these numbers: more than doubling sales year after year after year. Year five's sales projection of $14.5 billion is then roughly three times the present annual revenues of Peabody Energy (NYSE:BTU), the largest public coal company. I just don't see that happening.

For those of you who think you can keep flying closer to the sun without things ending badly, it might be time to brush up on your mythology.

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Fool contributor Toby Shute doesn't own shades or shares in any company mentioned. Amazon.com is a Motley Fool Stock Advisor recommendation and Akamai is a Motley Fool Rule Breakers pick. The Motley Fool's disclosure policy has no sunset clause.