For years, automated savings programs such as Bank of America's Keep the Change and Way2Save from Wells Fargo have been helping consumers trying to save a little cash.

Swipe your debit card at the grocery store or, say, Starbucks and if the charge comes to $6.50, for example, the bank would round up the amount to $7 and put the extra 50 cents in spare change into your savings account.

Great idea, but with yields on savings accounts paying close to zero percent interest, such a service that moves funds from one account to another seems a bit pointless.

But now there may be a better way. Acorns, a startup with an app for iPhone and Android smartphones, is taking the automatic micro-transfer a step further, by investing those spare change roundups in low-cost index funds.

A gold piggy bank. New firms allow you to invest in small amounts.

From Swipes to ETFs

To begin investing your spare change, you typically link at least one account, often a debit or credit card, to Acorns, which will then monitor your expenses for possible roundups, and a bank account, from which those roundups will ultimately be deducted. Acorns will round up every transaction made to the nearest dollar. If you use your debit card to spend $8.49 on lunch, then that 51 cents in spare change will be set aside in your checking account. Once the total amount reaches $5, Acorns deducts it from your checking account and funds your investment account.

Those funds are then used to purchase six exchange-traded funds, in a portfolio chosen to match your tolerance for risk (Acorns administers a short risk tolerance quiz to prospective investors). The accounts are SIPC insured, just as they are at an online discount brokerage.

How much money could you save? It depends on how much you spend, but Acorns projects that in 15 years a $10 monthly investment into its moderately aggressive portfolio will grow to $3,200. If that doesn't seem worthwhile, an alternative could be Betterment's Smart Deposit. Instead of basing periodic investments on transactions, it sweeps a preset amount (it can be hundreds of dollars per week or month) into its similar ETF-based portfolios.

Then there are the fees. Acorns fees are undeniably clear, but depending on how much you save, expensive. For an investor who has accumulated $500, for example, the $1 monthly fee amounts to a huge annual expense of 2.4 percent. Only after you've invested $3,000 or more with Acorns do the fees become more commensurate with other robo-adviser portfolios like Betterment and Wealthfront, although they're still somewhat higher (Acorns does facilitate larger lump sum investments).

On the other hand, Acorns charges just $12 a year, which isn't much of a hardship if it helps educate consumers otherwise intimidated by investing. And Acorns waives the fees for students and anyone between the ages 18 and 24, a market that might be suited for a basic service like this. Best of all, saving five dollars at a time using Acorns will likely offer a better return than less sophisticated methods.