Need Layman's terms clarification on free tier

Lightsail's pricing page has the exact terms of its Free Tier offering https://aws.amazon.com/lightsail/pricing/

since then the Ubuntu host has been powered off

One major distinction from EC2 is that Lightsail offers pricing bundles that include data transfer and disk usage and as such billing does not stop when the instance is stopped.

You will have to delete your Lightsail instance in order to stop being billed. You may choose to create a snapshot, at a lower cost than an instance, so you can preserve your work and create a new instance in the future.

Let me explain how the AWS Free Tier works and why you might be seeing charges even though you haven't actively used the service since August.

The AWS Free Tier offers certain services for free within specific usage limits for a set period, typically 12 months from when you first sign up. However, it's important to understand that simply not using a service doesn't automatically stop it from running or incurring charges.

In your case with the Lightsail VPS (Virtual Private Server), even if you're not actively using it, the instance still exists in your account and is consuming resources. The 690 hours you mentioned roughly equates to about 29 days of continuous operation (24 hours * 29 days = 696 hours). This suggests that your VPS has existed continuously throughout September, regardless of whether you were actively using it or not. Stopped Lightsail instance continues to accrue charges.

The 0.10 GB of data transfer could indeed be from small amounts of network traffic, such as ACKs or other system-level communications, even if you weren't actively using the VPS.

To avoid these charges, you need to take active steps to shut down or delete resources you're not using. For Lightsail, this means deleting the VPS entirely when you're not using it.

To prevent future unexpected charges:

1. Regularly check your AWS account for running resources.
2. Stop (for lower costs) or delete (for final cleanup) resources you're not actively using.
3. Set up billing alerts to notify you when your usage approaches the Free Tier limits.
4. Use the AWS Free Tier usage tracking tools to monitor your usage.

Remember, it's your responsibility to manage your resources and usage, even under the Free Tier. Always shut down or delete resources you're not using to avoid unexpected charges.

Sources
Avoiding unexpected charges after Free Tier - AWS Billing
Getting unexpected bill while using a free tier ressources | AWS re:Post
Trying services using AWS Free Tier - AWS Billing
Confirming eligibility to use AWS Free Tier - AWS Billing