In a report, the company has calculated that the institutions would need this amount if a sharp increase in yields and a severe economic contraction were witnessed.
The study considered scenarios that the continent could face between 2011 and 2015 - including considerable downturns in the economies of Ireland, Portugal, Greece and Spain, an interest rate shock and limited market access for weaker sovereigns.
S&P noted in the report: "The overall effect on the creditworthiness of western European countries, if it were to happen, would be severe."
It explained such events might also result in higher debt levels throughout the region.
Bloomberg recently reported that the government in Portugal has predicted that the country's gross domestic product is likely to shrink this year due to cuts in spending to reduce the budget deficit and a fall in investment.