Here are some additional details about how banks generate income:

• Loan interest is by far the largest source of revenue for banks. Banks make money by charging borrowers a higher interest rate than they pay depositors. The difference is the bank's profit, known as the interest spread or margin.

• Fees and charges have been increasing as a source of revenue for banks in recent years. Banks charge fees for everything from checking accounts to wire transfers to overdrafts. The larger the bank, the more fee-based income they tend to have.

• Mortgage lending has become highly profitable for many banks. Mortgage interest rates are usually higher than other loan rates, and mortgages are paid back slowly over 30 years.

• Credit card income also consists of both interest charges on balances and annual fees. Late fees and penalty fees also add to card-related revenue. However, credit card loans tend to be riskier for banks.

• Revenue from investment activities is dependent on interest rates and financial market conditions. When rates rise, banks' investment returns typically increase.

• Interchange fees - the small percentage banks get from every credit/debit card transaction - can add up substantially, though regulation of these fees is increasing.

• ATM fees have become a notable source of revenue for large banks that have extensive ATM networks. But customers generally dislike these fees.

• The "float" is the least transparent source of income for banks, but it can generate millions due to the large deposit volumes that banks handle.

So in short, loan interest remains the most important revenue source for sustaining bank operations and profitability. However, fees, premium services, and other sources are important additions to a bank's overall income statement.