there's a nice article that puts ideas out there at worldchanging:
"According to the United Nations, the entire debt for the entire continent of Africa was about $320 billion in 2003. Adjusting for inflation and further accumulated debt, let's call it an even $350 billion.
You could install solar panels on 20 million American homes for $300 billion. (I'm ballparking a rather conservative $15k for full installation of enough solar infrastructure to fully power an average American house; the price would surely come down drastically at that scale.) By the way, 20 million houses is more than one-quarter of the entire stock of occupied detached houses in the U.S."