TGEN vs PESI vs CLNE vs ERII

For growth investors, Tecogen Inc.

(TGEN) is the stronger pick with 19. 7% revenue growth year-over-year, versus -7. 1% for Energy Recovery, Inc. (ERII). Energy Recovery, Inc. (ERII) offers the better valuation at 22. 5x trailing P/E (22. 9x forward), making it the more compelling value choice. Analysts rate Tecogen Inc. (TGEN) a "Buy" — based on 4 analyst ratings — the highest consensus in this comparison. The "better buy" depends entirely on your goals: growth investors should weight revenue trajectory, value investors should weight P/E and PEG, and income investors should weight dividend yield and streak.

Over the past 5 years, Tecogen Inc.

(TGEN) delivered a total return of +180. 2%, compared to -73. 8% for Clean Energy Fuels Corp. (CLNE). Over 10 years, the gap is even starker: PESI returned +178. 6% versus CLNE's -26. 9%. Past returns do not guarantee future results, and the stock with the higher historical return may already have its best growth priced in.

By beta (market sensitivity over 5 years), Clean Energy Fuels Corp.

(CLNE) is the lower-risk stock at 1. 19β versus Tecogen Inc. 's 3. 43β — meaning TGEN is approximately 188% more volatile than CLNE relative to the S&P 500. On balance sheet safety, Energy Recovery, Inc. (ERII) carries a lower debt/equity ratio of 5% versus 18% for Clean Energy Fuels Corp. — giving it more financial flexibility in a downturn.

By revenue growth (latest reported year), Tecogen Inc.

(TGEN) is pulling ahead at 19. 7% versus -7. 1% for Energy Recovery, Inc. (ERII). On earnings-per-share growth, the picture is similar: Perma-Fix Environmental Services, Inc. grew EPS 43. 6% year-over-year, compared to -173. 0% for Clean Energy Fuels Corp.. Over a 3-year CAGR, TGEN leads at 2. 7% annualised revenue growth. Higher growth typically commands a higher valuation multiple — check whether the premium P/E or P/S is justified by the growth rate using the PEG ratio.

Energy Recovery, Inc.

(ERII) is the more profitable company, earning 17. 0% net margin versus -52. 3% for Clean Energy Fuels Corp. — meaning it keeps 17. 0% of every revenue dollar as bottom-line profit. Operating margin tells a similar story: ERII leads at 18. 2% versus -26. 3% for TGEN. At the gross margin level — before operating expenses — ERII leads at 65. 2%, reflecting greater pricing power or product mix advantage. Stronger margins indicate durable pricing power, lower cost of revenue, or higher mix of software/services. They are one of the clearest signs of business quality.

Analyst consensus price targets imply the most upside for TGEN: 211.

2% to $15. 00.

None of the stocks in this comparison currently pay a material dividend.

All are effectively zero-yield and should be held for capital appreciation rather than income.

For long-horizon retirement investors, Clean Energy Fuels Corp.

(CLNE) is the stronger choice — it scores higher on the combination of lower volatility, dividend reliability, and long-term compounding (low volatility (β 1. 19)). Tecogen Inc. (TGEN) carries a higher beta of 3. 43 — meaning larger drawdowns in market downturns, which matters significantly when you cannot wait years for a recovery. Both have compounded well over 10 years (CLNE: -26. 9%, TGEN: -3. 2%), confirming both are viable long-term holds — but the lower-volatility option typically results in less emotional selling during corrections. Retirement portfolios generally favour predictability over maximum returns. Consult a financial advisor before making allocation decisions.

These companies operate in different sectors (TGEN (Industrials) and PESI (Industrials) and CLNE (Energy) and ERII (Industrials)), which means they face different economic cycles, regulatory environments, and macro sensitivities — making direct comparison nuanced.

In terms of investment character: TGEN is a small-cap high-growth stock; PESI is a small-cap quality compounder stock; CLNE is a small-cap quality compounder stock; ERII is a small-cap quality compounder stock. These fundamental differences mean investors should not choose between them on a single metric — the "better stock" depends entirely on which of these characteristics aligns with your investment strategy.