Backend instance hours count

Question

I'm using the AppEngine's Backend instances and the daily free quota is 9 instance hours. However, I've been using a Backend with 10 instances for around 16-17 minutes and my usage has already crossed 66%.

The calculation I had in my mind was 17 mins * 10 instances = 170 mins ~ 2.8hrs which is definitely less than 66% of 9 hours.

Can someone explain me the billing scheme here?

Answer 1

From https://developers.google.com/appengine/docs/quotas#Requests:

Instance Hours (billable) In general, instance usage is billed on an hourly basis based on the instance's uptime. Billing begins when the instance starts and ends fifteen minutes after the instance shuts down. You will be billed only for idle instances up to the number of maximum idle instances set in the Performance Settings tab of the Admin Console. Runtime overhead is counted against the instance memory.

In your case, you'd have 17min of activity + 15min after activity = 32 minutes. So 320 minutes (32 * 10) is pretty close to 2/3 of 9 hours.

You should be able to see the details in the Usage History of your application.