Billing commences when Amazon EC2 initiates the boot sequence of an AMI instance. Billing endswhen the instance terminates, which could occur through a web services command, by running"shutdown -h", or through instance failure. When you stop an instance, Amazon shuts it down butdoesn’t charge hourly usage for a stopped instance, or data transfer fees, but charges for the storagefor any Amazon EBS volumes. Reference: http://aws.amazon.com/ec2/faqs/