Cha-ching! That's the sound of money. It's also the top reason to use an online only bank.

Online only banks can offer better interest rates because they don't have to pay for brick-and-mortar operations. They pass the savings on to you. The best online banks sometimes offer as much as four times the interest rates of traditional banks, at least in this protracted low-interest environment spawned by the Great Recession. Rates are subject to change daily, but at times can remain stable for six months or longer.

Interest rates in large part are determined by the U.S. Federal Reserve, which sets the Fed Funds Rate, or the rate at which banks lend money to each other on a short-term basis, usually overnight. When the Fed Funds Rate is low, the cost of money is low. There's an abundance of it, and banks don't need to pay you for it. Subsequently, interest rates paid on your deposits are low. When the Fed Funds Rate is high, the cost of money is high. It's in short supply, and thus in high demand, which means banks pay you higher interest to hold your money, which they loan to others.

Another factor in determining your interest rate is the length of time you agree to keep your money in a bank. That's how certificates of deposit, or CDs, function. If you put $5,000 into a 60-month CD, you agree to leave it there for five years for the bank to use. Withdraw it before the term is up, and you will incur a monetary penalty.

You often can secure a CD for as little as three months. But remember, the longer the term, the higher the rate. CDs pay higher interest rates than traditional savings accounts. The same goes for money market accounts. The best online banks offer both CDs and money market accounts. They also offer other savings vehicles such as traditional and Roth IRAs, as well as educational savings plans.

Let's not forget loans. The best online banks offer a full complement of loans, including personal, auto, home equity and mortgages, as well as business loans.

The second most important reason to bank online is convenience. You can do everything through online only banks that you can do through a bank teller. The difference is that traditional banks close every night and on weekends. Online only banks are accessible 24-hours a day from any location you choose. All you need is an internet connection and a computer, tablet or smartphone.

When you're looking to bank online, remember that the most important criteria are the interest rates paid on your deposits and whether the online banking services work smoothly and easily on your smartphone, tablet or computer. If full services such as loans, long-term savings options and checking accounts are important to you, be sure to compare the offerings of a variety of online only banks. Also be sure to check for fees and charges that may be attached to some services. The best online only banks have no hidden fees.

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