Numbers can lie -- but they're the best first step in determining whether a stock is a buy. In this series, we use some carefully chosen metrics to size up a stock's true value based on the following clues:

• The current price multiples.
• The consistency of past earnings and cash flow.
• How much growth we can expect.

Let's see what those numbers can tell us about how expensive or cheap AES (NYSE: AES) might be.

The current price multiples
First, we'll look at most investors' favorite metric: the P/E ratio. It divides the company's share price by its earnings per share (EPS) -- the lower, the better.

Then, we'll take things up a notch with a more advanced metric: enterprise value to unlevered free cash flow. This divides the company's enterprise value (basically, its market cap plus its debt, minus its cash) by its unlevered free cash flow (its free cash flow, adding back the interest payments on its debt). Like the P/E, the lower this number is, the better.

Analysts argue about which is more important -- earnings or cash flow. Who cares? A good buy ideally has low multiples on both.

AES has a P/E ratio of 219.8 and an EV/FCF ratio of 15.3 over the trailing 12 months. If we stretch and compare current valuations to the five-year averages for earnings and free cash flow, AES has a P/E ratio of 26.1 and a five-year EV/FCF ratio of 25.6.

A positive one-year ratio under 10 for both metrics is ideal. For a five-year metric, under 20 is ideal.

AES is zero for four on hitting the ideal targets, but let's see how it compares against some competitors and industry mates.

Company

1-Year P/E

1-Year EV/FCF

5-Year P/E

5-Year EV/FCF

AES 219.8 15.3 26.1 25.6
Duke Energy (NYSE: DUK) 18.3 169.1 17.5 526.5
Dominion Resources (NYSE: D) 8.9 NM 14.2 NM
FirstEnergy (NYSE: FE) 27.6 23 17 47.5

Source: Capital IQ, a division of Standard & Poor's; NM = not meaningful.

Numerically, we've seen how AES' valuation rates on both an absolute and relative basis. Next, let's examine …

The consistency of past earnings and cash flow
An ideal company will be consistently strong in its earnings and cash flow generation.

In the past five years, AES' net income margin has ranged from -4.7% to 8.4%. In that same time frame, unlevered free cash flow margin has ranged from 2.4% to 15.7%.

How do those figures compare with those of the company's peers? See for yourself:

Source: Capital IQ, a division of Standard & Poor's; margin ranges are combined.

Additionally, over the last five years, AES has tallied up four years of positive earnings and five years of positive free cash flow.

Next, let's figure out …

How much growth we can expect
Analysts tend to comically overstate their five-year growth estimates. If you accept them at face value, you will overpay for stocks.  But while you should definitely take the analysts' prognostications with a grain of salt, they can still provide a useful starting point when compared to similar numbers from a company's closest rivals.

Let's start by seeing what this company's done over the past five years. Due to some losses, AES' past EPS growth rate isn't meaningful. However, Wall Street's analysts expect future growth rates of 8%.

Here's how AES compares to its peers for trailing five-year growth:

Source: Capital IQ, a division of Standard & Poor's; EPS growth shown.

And here's how it measures up with regard to the growth analysts expect over the next five years:

Source: Capital IQ, a division of Standard & Poor's; estimates for EPS growth.

The bottom line
The pile of numbers we've plowed through has shown us the price multiples shares of AES are trading at, the volatility of its operational performance, and what kind of growth profile it has -- both on an absolute and a relative basis.

The more consistent a company's performance has been and the more growth we can expect, the more we should be willing to pay. We've gone well beyond looking at a 219.8 P/E ratio, but this is just a start. Digging a bit further on that high P/E ratio, it's that high because of a \$1.2 billion asset writedown. We should also note that free cash flow numbers for utilities aren't as useful as in other industries because of regulations that affect the value proposition of capital projects (yet, AES' are still pretty good). And in terms of dividend yield (which is a big reason investors like utilities), AES doesn't pay one. If you find AES' numbers or story compelling, don't stop. Continue your due diligence process until you're confident one way or the other. As a start, add it to My Watchlist to find all of our Foolish analysis.

If you want some more stock ideas, check out my recent article: The Greatest Companies of 2020.