EC2 reserved instance calculation

AWS automatically optimizes which instances are charged at the discounted rate to ensure you always pay the lowest amount. For information about billing, and how it applies to RIs, see Billing Benefits and Payment Options.

So if you purchase 2 RI of type xyz. And your autoscaling group runs 3 instances of type xyz, then 2 of those instances will be charge at the RI rate and the other one will be charge at the On-demand rate. If during night you only have 1 instance type xyz running then you will pay the RI rate for that running instance.

Reserved Instances are billed for every clock-hour during the term that you select, regardless of whether an instance is running. Each clock-hour starts on the hour (zero minutes and zero seconds past the hour) of a standard 24-hour clock. For example, 1:00:00 to 1:59:59 is one clock-hour.

If you purchase one xyz Reserved Instance and run four xyz instances concurrently for one hour, one instance is charged at one hour of Reserved Instance usage and the other three instances are charged at three hours of On-Demand usage.

However, if you purchase one xyz Reserved Instance and run four xyz instances for 15 minutes (900 seconds) each within the same hour, the total running time for the instances is one hour, which results in one hour of Reserved Instance usage and 0 hours of On-Demand usage.

You can check the How you are billed section to better understand the billing process for RI.