Investing in stocks continues to prove to be the great American equalizer. While wage growth for the middle class continues to remain stagnant -- as it has for some three decades -- the U.S. stock market remains the best way for the average person to grow wealth. America continues to produce some of the best global growth opportunities.

It remains a business-friendly place to start and run a company, and it gives entrepreneurs access to vast resources, including global markets. Even for foreign businesses, the U.S. continues to prove valuable as a source of capital, as many will list on U.S. stock exchanges. For individual investors, that means a wealth of opportunities to invest in both domestic and global trends that will drive markets for years to come. Some of the best opportunities of all are small-cap stocks that don't get the publicity of bigger, better-known companies, or the attention from large investors because they're too small to buy enough of. 

Stopwatch with time to buy on the face instead of numbers.

Image source: Getty Images.

Three small-cap stocks that look particularly compelling now include an offshore bank for the uber-wealthy, Bank of N.T. Butterfield & Son (NYSE:NTB), an e-commerce company with its sights set of Africa, along with Jumia Technologies (NYSE:JMIA) and alternative fuels upstart Clean Energy Fuels (NASDAQ:CLNE). Keep reading to learn why they're worth buying now. 

Profit from the growth of global elites

The world is getting wealthier, and investors can profit from the growth of the world's ultra-rich by investing in Bank of N.T. Butterfield & Son. Butterfield is an offshore bank with operations in the Cayman Islands, the Channel Islands, and Bermuda. Butterfield has $12.6 billion in deposits and $14.2 billion in assets, making it quite small in the banking world, but it's been in business for more than a century. It went public just over four years ago and has a market cap of $1.8 billion. 

Why invest in Butterfield now? In short, because I think the market doesn't appreciate its long-term value over short-term worries. Here's a look at both its trailing and forward P/E ratios -- price to earnings -- and its price to tangible book value

NTB PE Ratio Chart

NTB PE Ratio data by YCharts

As the chart shows, the market has valued Butterfield less and less over the past couple of years, primarily over global economics, including the ongoing trade war between the U.S. and China, falling interest rates -- which isn't great for bank profits -- and a litany of other things that could temporarily weigh on Butterfield.

But when you look beyond any short-term speed bumps -- which Butterfield is built to ride out -- you find a bank that's incredibly profitable and ready to deliver market-beating returns for long-term investors. Butterfield touts a return on assets of 1.6% and a return on equity of 20.3% -- numbers that not only stand far above the benchmark targets of 1% and 10%, respectively, but are also much better than most other banks.

I think that makes it worth far more than 9.8 times earnings and 2 times tangible book value. That's before adding in a generous dividend that yields 5.2% at recent prices. 

We will find out more about the market's appreciation, or lack of it, when Butterfield reports earnings on Feb. 12. 

A bet that Jumia can right the ship and strike riches in Africa

It's been a pretty ugly ride for Jumia since going public. After almost tripling from its IPO on investor hopes that it would be the Amazon.com of Africa, the e-commerce company's shares have fallen sharply, even as the company has grown sales substantially. At this writing, the stock is down almost 80% from the IPO price. 

Why the big sell-off? A number of reasons, including a bit of a reality-check for investors that Jumia was a long way from matching the success of Amazon or MercadoLibre, as well as a short attack accusing the company of being an outright fraud. More recently, the company has had to retrench, selling its travel business to a partner and shuttering operations in Rwanda. 

I've never bought the "fraud" thesis. This is a legitimate business, and it's growing very quickly. The problem, plain and simple, is that Jumia was growing quickly, but it was burning cash even more quickly.

JMIA Chart

JMIA data by YCharts

But the moves to scale back and refocus on profitable growth are smart and should give the company an easier, more attainable and more sustainable path forward than it was on. 

To be clear, this is still a high-risk investment. Jumia will remain cash-consuming business for some time to come, and with operations in over a dozen countries where e-commerce is still a mystery to millions of consumers, there are no promises it will succeed. But if you have a high threshold for risk, and you're truly committing capital you can live without, Jumia looks worth a small, speculative bet right now. 

The under-the-radar alternative fuel

Clean Energy Fuels has fought its way out of an ugly situation. An aggressive move by management to take on a load of debt and build a big public natural gas refueling network, simply put, backfired. Without belaboring the details, the timing was awful, and the company took on over $600 million in debt to build a bunch of stations, only to see oil prices crash and take a big bite out of the momentum natural gas had been seeing as a fuel for heavy-duty and medium-duty transportation. 

The weight of that debt, along with a top-heavy operations structure, combined to wipe out millions in shareholder value and had the company on the ropes, but over the past four years and change, management has taken steps to cut costs and clean up the balance sheet. All that effort has paid off, too, as evidenced by the company's positive cash flows over the past year. 

Even better, its core refueling business continues to grow. In 2012, the company delivered about 200 million diesel-gallon-equivalents of natural gas, but when it reports its 2019 results later this month, it's likely to have delivered more than 400 gallons last year. Moreover, the 2019 result should be close to 10% higher than its 2018 deliveries. 

A big part of its fuel sales growth is the increased demand for RNG, or renewable natural gas, which comes from human-generated waste like landfills and agricultural production. Not only is using RNG better for the environment than diesel with fewer particulates and carbon emissions, but it's also a net positive, reducing the environmental impact of other human activities. 

And before you dismiss Clean Energy as an eventual loser to electric vehicles, the reality is, there aren't really any heavy-duty electric trucks anywhere close to commercial production today, and the ones in the pipeline will cost double -- or even triple -- what a comparable natural gas truck costs, and come with significant limitations to range and weight. 

Put it all together, and Clean Energy is lean, efficient, and growing quickly. Today's share price represents an excellent opportunity to profit on the most overlooked -- and most viable -- alternative fuel for heavy-duty transportation.