variable costs
- Definition
- Costs that rise and fall with actual consumption — the pay-as-you-go model.
- Also called operational expenditure (OpEx).
- How AWS bills variably
- Per second or per hour for compute, per GB-month for storage, per request for AWS Lambda and API calls, per GB for data transfer out.
- No charge when a resource is not running (a stopped EC2 instance still incurs EBS storage cost, but no compute cost).
- Why it matters
- No idle capacity to pay for; costs track business activity.
- Failed experiments cost hours of compute rather than a hardware purchase.
- The catch
- Spend is unbounded by default — which is why AWS Budgets, AWS Cost Explorer, and cost allocation tags exist.
- Contrast with
- fixed costs — bought up front, sized for peak, paid regardless of use.