Which Reserved Instance Mix Cuts EC2 Costs for Sunday-Only Load?

SAA-C03 By islever Editorial Last updated Oct 13, 2026

Design cost-optimized compute solutions.

Short answer

A Use Scheduled Reserved Instances for the 12-hour Sunday run and Standard Reserved Instances for the Monday–Saturday constant workload.

Question

A company's production environment consists of Amazon EC2 On-Demand Instances that run constantly between Monday and Saturday. The instances must run for only 12 hours on Sunday and cannot tolerate interruptions. The company wants to cost-optimize the production environment. Which solution will meet these requirements MOST cost-effectively?

Purchase Scheduled Reserved Instances for the EC2 instances that run for only 12 hours on Sunday. Purchase Standard Reserved Instances for the EC2 instances that run constantly between Monday and Saturday. Correct Answer
Purchase Convertible Reserved Instances for the EC2 instances that run for only 12 hours on Sunday. Purchase Standard Reserved Instances for the EC2 instances that run constantly between Monday and Saturday.
Use Spot Instances for the EC2 instances that run for only 12 hours on Sunday. Purchase Standard Reserved Instances for the EC2 instances that run constantly between Monday and Saturday.
Use Spot Instances for the EC2 instances that run for only 12 hours on Sunday. Purchase Convertible Reserved Instances for the EC2 instances that run constantly between Monday and Saturday.

Key Takeaways

The exam tests matching Reserved Instance purchase models to usage patterns—predictable recurring schedules need Scheduled RIs while constant workloads get the deepest discount from Standard RIs—and the trap is choosing Spot for the cheaper Sunday hours despite the no-interruption requirement.

This SAA-C03 question tests cost-optimizing EC2 with Reserved Instances for a steady Monday–Saturday workload plus a 12-hour Sunday schedule that cannot be interrupted. The correct mix is Scheduled Reserved Instances for the recurring Sunday window and Standard Reserved Instances for the constant production run.

Common mistake: Many learners pick Spot Instances for the Sunday-only window because they are the cheapest option, but Spot capacity can be reclaimed at any time, which violates the explicit requirement that the instances cannot tolerate interruptions.

Expert Analysis

Why the Answer Is Correct

Option A uses Scheduled Reserved Instances for the 12-hour Sunday run, which AWS designed for predictable, recurring schedules; they reserve capacity for the exact time window ahead of time and are not interruptible. For the Monday–Saturday constant production load, Standard Reserved Instances give the largest discount among RI types because no instance-family or operating-system flexibility is needed. Together these two purchases satisfy both the no-interruption requirement and the “MOST cost-effectively” requirement. The community votes and the source key agree on A, and the reasoning is grounded in the AWS EC2 Reserved Instances purchasing options.

Why the Other Options Are Wrong

Option B uses Convertible Reserved Instances for the Sunday-only window; Convertible RIs are designed for steady commitments where you may need to exchange instance attributes, and they offer a smaller discount than Scheduled RIs for a recurring 12-hour schedule. Options C and D both use Spot Instances for Sunday, but Spot Instances can be terminated with a two-minute warning, directly violating the requirement that the instances cannot tolerate interruptions. Option D also replaces the Standard RI with a Convertible RI for the always-on Monday–Saturday workload, which is more expensive than the Standard RI discount for a constant workload with no flexibility needs.

Community Comment Notes

Commenters largely converged on A. LeonSauveterre explained that Scheduled Reserved Instances fit "predictable and recurring schedules (like 12 hours on Sunday)" and ensure uninterrupted capacity, while Convertible RIs are not suited to short predictable schedules. FlyingHawk reasoned that Spot instances are out because "the instance cannot tolerate interryptions", leaving only A and B, then preferred Scheduled RIs for the recurring Sunday window. The commenter 8621a7 added that Standard RIs are cheaper than Convertible RIs for the constant Monday–Saturday run. GOTJ noted a real-world caveat: the EC2 console may recommend Savings Plans over Reserved Instances and Scheduled RIs could eventually be replaced, but for this exam scenario A remains the intended cost-optimized solution. bujuman and EllenLiu pointed to the AWS Reserved Instances documentation and purchasing page as the source of truth.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

When a question says instances cannot tolerate interruptions, immediately eliminate any option that includes Spot Instances. Then match the remaining Reserved Instance types to usage duration: Scheduled RIs for recurring short windows and Standard RIs for constant workloads.

Frequently Asked Questions

Why are Spot Instances wrong for the 12-hour Sunday workload?

Spot Instances can be reclaimed by AWS at any time, and the question explicitly states the instances cannot tolerate interruptions, so options C and D are eliminated.

Why use Standard RIs instead of Convertible RIs for Monday–Saturday?

The Monday–Saturday workload runs constantly and needs no instance-family or OS flexibility, so Standard RIs provide a deeper discount than Convertible RIs.

More SAA-C03 FAQ →

Community Votes

100%

100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 6 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 6

8621a7c 👍 4 Answered A

standard reserved instance is cheaper than convertible reserved instance
LeonSauveterre 👍 3 Answered A

Scheduled Reserved Instance: Perfect for workloads with predictable and recurring schedules (like 12 hours on Sunday). This is cost-efficient and ensures uninterrupted instances. Convertible Reserved Instance: Allows flexibility to change instance types, sizes, or OS during the term. However, they are not ideal for short, predictable schedules like 12 hours on Sunday. Scheduled Reserved Instances are a better fit. Spot Instances can be interrupted at any time. They do not guarantee uninterrupted operations.
bujuman 👍 3 Answered A

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-reserved-instances.html
GOTJ 👍 1 Answered A

"A" is the most straightforward option. However, I'm afraid this feature could be eventually removed and substituted by On-Demand Capacity Reservations and Savings Plans. If you try to use scheduled reservation in the EC2 console, you'll get the following banner: "We recommend Savings Plans over Reserved Instances. Savings Plans are the easiest and most flexible way to save money on your AWS compute costs and offer lower prices (up to 72% off) just like Reserved Instances. To learn more and get started with Savings Plans click here"
FlyingHawk 👍 1 Answered A

as the instance cannot tolerate interryptions, so spot instances are out, C and D are incorrect, leave only A and B, Scheduled RI or Convertible RI, based on the AWS doc, Purchasing Reserved Instances on a recurring schedule lets you pay for compute capacity by the hour and obtain a capacity reservation ahead of time for only the time periods you will need it. as it only 12 hours on Sunday, so A will be cheaper than B https://aws.amazon.com/ec2/pricing/reserved-instances/pricing/.

Show 1 more comment

EllenLiu 👍 1 Answered A

Scheduled reserved instances are purchase of reserved Instances on a recurring schedule https://aws.amazon.com/ec2/pricing/reserved-instances/buyer/

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