The core difference
Neither product is a machine. Both are billing discounts applied to compute you already run. With a Savings Plan, you commit to an amount in dollars per hour. With a Reserved Instance, you commit to a configuration: instance type, Region, tenancy and platform. Both last one or three years.
AWS recommends Savings Plans over Reserved Instances in its EC2 documentation. It says they offer lower prices of up to 72% off On-Demand, just like Reserved Instances, with more freedom on configurations.
Savings Plans: a commitment per hour
The prices you pay stay the same through the term, and you can pay all upfront, partial upfront or no upfront. These prices do not change with the size of your hourly commitment.
Know the limits before you buy. The terms of a plan cannot change after purchase, and it cannot be cancelled during its term. A plan reserves no capacity. It does not apply to Spot usage or to usage covered by a Reserved Instance.
AWS offers four types. The figures are maximum discounts against On-Demand rates:
- Compute Savings Plans, up to 66%. They apply across instance family, size, Region, operating system and tenancy, and they also cover Fargate and Lambda.
- EC2 Instance Savings Plans, up to 72%. You pick one instance family in one Region. Size, operating system and tenancy stay free.
- Database Savings Plans, up to 35%, on services such as Aurora, RDS and DynamoDB.
- SageMaker AI Savings Plans, up to 64%, on SageMaker AI instances.
Reserved Instances: a commitment to a configuration
A Reserved Instance discounts the usage that matches its attributes, and you pay for the whole term whether you use it or not. You cannot cancel the purchase. It does not renew: at the end of the term, usage returns to On-Demand rates. Depending on its class, you might modify it, exchange it or sell it on the Reserved Instance Marketplace.
You choose a class, then a scope:
- Offering class. Standard goes up to 72% and can only be modified. Convertible goes up to 66% and can be exchanged for another Convertible with different attributes.
- Regional scope. The Reserved Instance applies in any Availability Zone of its Region and reserves no capacity. On Linux with default tenancy, it applies to any size of the family.
- Zonal scope. The Reserved Instance reserves capacity in one zone and applies to one instance size only. Scope does not affect the price.
How to choose
Azure follows a close logic. A savings plan commits you to an hourly spend for one or three years and applies across eligible services and regions. Reservations lock in a specific resource, size and region for deeper discounts. A savings plan cannot be cancelled, and an hour of unused commitment is lost.
On AWS, start from what might change during the term:
- Choose a Compute Savings Plan if instance types, Regions or services may change, or if an autoscaler such as Karpenter picks your instances.
- Consider an EC2 Instance Savings Plan if one family in one Region will stay stable for the whole term.
- Consider a zonal Reserved Instance if you need capacity held in a zone. A Savings Plan can also pair with an On-Demand Capacity Reservation.
Size the commitment for a Kubernetes cluster
On EKS, Compute and EC2 Instance plans apply to the EC2 instances under your cluster, and Compute plans also cover Fargate. The Amazon EKS charges themselves stay outside.
A commitment buys a floor. It pays off when usage stays above that floor, hour after hour, for the whole term. Any saving you compute is an estimate based on past usage, and none is guaranteed. If usage falls below the commitment, you pay for the gap.
Work in this order:
- Reduce first. Fix oversized requests, remove idle workloads and stop test environments at night. A commitment on waste only locks it in.
- Measure the floor. Look at hourly compute spend after those changes and find the level you rarely go below.
- Commit to part of that floor. Leave room for consolidation, migrations and Spot. You can add a plan later if usage grows.
- Review every month. Coverage shows what stays at On-Demand rates. Utilization shows what you pay for without using it.
What Onicost adds: see coverage before you commit
The Cloud workspace of Onicost shows commitment coverage by service: how much of your committable spend is covered. For the stable part of your On-Demand spend, it proposes a Savings Plan or a Reserved Instance, with the amount to commit before you subscribe. It also flags commitments that are underused or ending soon.
Onicost buys nothing for you. Your team decides and subscribes. Savings are estimates. On the Kubernetes side, cost estimates use public prices and pool savings ignore commitments (RIs, Savings Plans, CUDs), so compare them with your bill.
The Cloud workspace comes with Team, from €39 per month before tax, and with its 14-day trial with no card.
Frequently asked questions
Can I use both at the same time?
Yes. A Savings Plan does not apply to usage already covered by a Reserved Instance, so the two discounts do not stack on the same usage.
Do Savings Plans cover Spot instances?
No. AWS states that Savings Plans do not apply to Spot usage. Size your commitment on On-Demand usage only.
Can I cancel a commitment?
Treat both as final. AWS says a Savings Plan cannot be cancelled during its term and a Reserved Instance purchase cannot be cancelled. A Reserved Instance can sometimes be modified, exchanged or sold.