The EC cap for 'Silver' accounts is 10 Million per character. The way it works is...

• If your character is above the 10 Million cap when you transition to 'Silver', you don't lose any ECs, but you can't gain any until your balance is below 10 Million.
• If your balance is below 10 Million, and you earn something that will take you over the 10 Million cap, you only gain up to the 10 Million Cap, and anything above that is lost.

Purchasing the EC cap extension moves everything up to a 1 Billion EC cap limit.