Azure Reservations, Hybrid Benefit, and Spot VMs: Cutting Compute Costs

Azure gives you three main ways to pay less than standard pay-as-you-go rates for compute: Reserved Instances discount a committed VM size and term, Azure Hybrid Benefit lets you reuse on-premises licenses you already own, and Spot Virtual Machines sell unused capacity cheaply in exchange for accepting eviction risk. Which one saves you the most money depends entirely on how predictable and interruption-tolerant your workload is — pick wrong and you either overpay for flexibility you don’t need or accept downtime risk you can’t afford. Working out those savings ahead of time, before you commit to any of the three, is exactly what the Azure Pricing Calculator and TCO Calculator are built for.

Reserved Instances: pay less for committing to steady usage

An Azure Reservation (Reserved VM Instance) discounts compute cost in exchange for committing to a specific VM size for a fixed one-year or three-year term. The trade is straightforward: Azure locks in a lower rate because it can count on that capacity being used continuously, and you lock in a lower rate because you’re giving up the flexibility to walk away at any time. This fits workloads with steady, predictable, long-running demand — a production order-processing system, a database tier, anything that runs 24/7 and isn’t going anywhere for the foreseeable future.

Reservations don’t automatically relocate workloads between regions, clean up idle VMs, or connect to your support plan — they purely discount the compute rate for the size and term you commit to. If usage patterns are unclear, Cost Management’s reservation recommendations feature analyzes your historical usage and suggests an appropriate size and term to maximize savings before you commit.

Azure Hybrid Benefit: stop paying twice for licenses you already own

If your organization already owns on-premises Windows Server or SQL Server licenses with active Software Assurance, Azure Hybrid Benefit lets you apply those existing licenses toward equivalent Azure virtual machine costs instead of paying for Windows or SQL licensing a second time bundled into the VM’s price. This solves a licensing problem, not a compute-commitment problem — it’s about reusing an asset you’ve already paid for, and it works independently of whether you’re also using pay-as-you-go pricing or a reservation. In fact, Hybrid Benefit and Reserved Instances stack: a company running Windows Server VMs continuously, with Software Assurance licenses already in hand, gets the deepest discount by combining a Reserved Instance for the compute commitment with Hybrid Benefit for the licensing, rather than choosing one or the other.

Spot Virtual Machines: the deepest discount, with eviction risk attached

Spot Virtual Machines draw on Azure’s unused spare capacity at a steep discount — often the cheapest per-hour compute rate available — but Azure can evict a Spot VM with little advance notice whenever it needs that capacity back for other customers. There’s no uptime guarantee here; accepting eviction risk is the entire basis of the discount. That makes Spot VMs a poor fit for anything that must stay running, but a strong fit for short, restartable, interruption-tolerant work: batch image resizing, video rendering jobs, or overnight data processing that can pick back up if it gets interrupted.

Option	Best for	Trade-off
Reserved Instance	Steady, predictable, long-running workloads (1- or 3-year term)	Locks in a term commitment; loses value if usage drops
Azure Hybrid Benefit	Organizations with existing Software Assurance-covered licenses	Only helps if you already own eligible licenses
Spot Virtual Machines	Short, interruptible, restartable batch or dev/test work	Azure can evict the VM with little notice; no SLA

Matching the option to the workload

The exam (and real budgeting) usually presents workloads with mixed characteristics and expects you to route each piece to the right purchasing option rather than picking one option for everything. A retail company with a production order-processing VM that must run continuously for three years, a nightly batch image-resizing job that tolerates interruption, and existing Windows Server licenses no longer needed on-premises would combine all three: a Reserved Instance for the production VM, Azure Hybrid Benefit for the licensing, and a Spot VM for the batch job. Putting the production VM on Spot would risk exactly the availability it needs to protect; reserving the interruptible batch job would waste a multi-year commitment on work that doesn’t need one.

Beyond purchasing options: finding waste with Azure Advisor

Purchasing options only save money on resources you actually need. Azure Advisor closes the other gap by analyzing real usage telemetry and recommending resizing or shutting down virtual machines that have gone underutilized — say, a VM that’s shown low CPU and memory usage for 30 days straight. Advisor’s cost recommendations are about identifying waste after the fact, distinct from choosing a purchasing model up front; the two work together, since a right-sized VM on a Reserved Instance saves more than an oversized one on the same reservation.

Subscription and agreement type also affects the rate

One more lever sits above all three purchasing options: the type of subscription or agreement you’re billed under. An organization on an Enterprise Agreement (EA) can receive negotiated pricing and different invoicing terms than an equivalent pay-as-you-go subscription, even for identical usage — the agreement itself carries an upfront usage commitment in exchange for better rates. An EA doesn’t remove the need to size VMs correctly, doesn’t make egress free, and doesn’t automatically apply Hybrid Benefit to your resources; it changes the baseline rate and terms you’re negotiating those other savings on top of.

How the AZ-900 exam tests this

• A scenario describes a steady, always-on workload expected to run for years and asks which purchasing option cuts cost — the answer is a Reserved Instance, not Spot (which risks eviction) and not simply “pay-as-you-go with no changes.”
• A scenario mentions a company already owns Windows Server or SQL Server licenses with Software Assurance and wants to lower Azure VM costs — the exam wants Azure Hybrid Benefit specifically, not a reservation or a support-plan change.
• A scenario describes work that “can be interrupted and restarted” or “tolerates interruption” at the lowest possible price — that phrasing is the tell for Spot Virtual Machines, and the exam is testing whether you’ll wrongly pick Spot for something that actually needs to stay running continuously.
• A multi-part scenario mixes a steady workload, an interruptible workload, and existing licenses in one question — the exam expects you to assign each piece to a different purchasing option rather than force one option to cover everything.

Quick reference

• Reserved Instances discount a committed VM size for a 1- or 3-year term — best for steady, predictable, long-running workloads.
• Azure Hybrid Benefit reuses existing Windows Server/SQL Server licenses (with Software Assurance) toward Azure compute — a licensing benefit, not a compute-commitment discount.
• Reservations and Hybrid Benefit can be combined for additional savings on the same VM.
• Spot Virtual Machines offer the steepest discount but carry eviction risk and no SLA — reserve them for interruption-tolerant, restartable work.
• Subscription/agreement type (pay-as-you-go vs. Enterprise Agreement) is a separate cost factor that changes the baseline rate before any purchasing option is applied.
• Azure Advisor’s cost recommendations find underutilized resources to resize or shut down — a complement to, not a replacement for, choosing the right purchasing option.

For the full picture of what drives an Azure bill before you optimize it, see the cost factors behind every Azure invoice, and for the tools that track savings once they’re in place, Azure Cost Management budgets and tags covers the reporting side. Then practice AZ-900 questions that test whether you can match the right purchasing option to a workload’s usage pattern.