Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you're looking for stocks with low volatility to help you sleep better at night as your portfolio grows, the PowerShares S&P 500 Low Volatility ETF (NYSE: SPLV) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The PowerShares ETF is made up of the 100 stocks in the S&P 500 with the lowest volatility over the past year. The majority of them can be classified as value stocks, as growth stocks tend to be more volatile.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The PowerShares ETF's expense ratio -- its annual fee -- is a very low 0.25%.

This ETF doesn't have much of a performance to assess yet, as it's less than a year old. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
Let's take a look at some strong-performing examples of these low-volatility stocks. Tobacco giant Altria (NYSE: MO), for example, gained about 25% over the past year. Its future may not be as smoking as its past, though, due to increased taxes and regulations in the U.S., along with a shrinking smoking population. Those eager to invest in tobacco might want to look at Philip Morris International, poised to benefit as emerging-market populations become wealthier and start smoking more.

Duke Energy (NYSE: DUK), up 29%, is also benefiting from its international operations, with rising prices and volume in various regions, such as Brazil. It's also building its capacity, via a multi-billion-dollar, coal-fueled power station and investments in solar farms, among other things. Once it merges with Progress Energy, it will be the nation's largest electric utility.

Other companies haven't done as well lately but should be strong long-term performers. Procter & Gamble (NYSE: PG), up just 7%, has been shrinking some of its packaging in order to boost profits and is a consumer products titan, with many billion-dollar brands, such as Crest, Tide, Pampers, and Tampax.

These are the kinds of companies that are likely to perform well in all kinds of economic environments.

The big picture
A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 Stocks for Your Retirement Portfolio.