HOUSTON (Reuters) - Troubled Texas power grid operator ERCOT on Friday said $2 billion in charges for power and services went unpaid, according to an official notice, signaling the financial fallout from high electricity rates is spreading to utilities. It will cover $800 million of the shortfall by borrowing from internal accounts, and will draw an undisclosed amount from grid users with credit balances. A spokeswoman was unavailable for immediate comment.