Reserved capacity vs savings plans vs committed use
These products discount compute or a spend commitment in exchange for a term. They are not interchangeable. Current rates belong on the official pricing pages, not in this table.
Last verified: · Written and technically verified by Ankit Mehta
How the products actually differ
AWS documents Reserved Instances (capacity and billing constructs by instance family and region) and Savings Plans (compute or EC2 instance spend commitments). Azure documents reservations for specific VM sizes and related resources. Google Cloud documents committed use discounts (resource-based or spend-based) against a project or billing account. All three reduce on-demand rates only if usage matches the commitment.
| Topic | AWS RI / Savings Plans | Azure reservations | Google Cloud CUD |
|---|---|---|---|
| What you commit | Hours or $/hour of compute (product-specific) | Quantity of a SKU for a term | vCPU/memory or spend for a term |
| Mismatch risk | Family or region change leaves unused commitment | Wrong VM size reservation sits unused | Machine-type change misses CUD |
| Where to verify rate | AWS Pricing Calculator / Savings Plans pages | Azure pricing calculator and reservation docs | Google Cloud CUD documentation |
Decision table
Factual differences are in the table above. The rows below are recommendations for a named situation.
| When | Choose |
|---|---|
| Steady EC2 or VM usage in one family and region for a year or more | A reservation or resource-based commitment that matches that SKU. Confirm the current rate on the official calculator. |
| Compute mix will change (family, region, or container vs VM) inside the term | A spend-based instrument (Savings Plans or spend CUD) if the provider offers one that covers the new mix. Otherwise stay on-demand until the mix settles. |
When neither option is appropriate
Do not buy a commitment to “save money” without a usage forecast that survives a migration or shrink. If the workload is seasonal, experimental, or about to change instance family, neither a reservation nor a CUD is appropriate until that is written down.
Limitations
- Discount percentages change with term, payment option and SKU. Read the current official calculator for the month you are planning.
- A commitment can raise cost if you migrate or shrink. That is documented behavior of prepaid discounts, not a CloudArch Pro claim.