Cloud Economics: Fixed vs. Variable Costs

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Lesson 3: Cloud economics. In lesson one, we met the six advantages of cloud computing. Now we'll look at the money behind them, because the Cloud Concepts domain asks about cloud economics directly. We'll cover two ideas. First, the difference between fixed and variable costs, and between capital and operating expenses. Second, what running your own data center really costs, and why the cloud can cost less.

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Fixed costs vs. variable costs. Start with two kinds of cost. A fixed cost stays the same no matter how much you use. When the store buys a server, it has paid for it whether the server is busy or idle. A variable cost rises and falls with usage. In the cloud, the store's monthly bill follows its traffic: a quiet month costs less, and a busy month costs more.

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CapEx vs. OpEx. Accountants describe the same idea with two terms. Capital expenses, or CapEx, are up-front investments in physical assets, such as servers, buildings and perpetual software licenses, used over several years. Operating expenses, or OpEx, are the day-to-day costs of running the business. Moving to the cloud shifts spending from CapEx to OpEx, so you don't have to forecast and pay for years of capacity in advance.

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The costs you can see. So what does running your own data center actually cost? The visible part is the equipment: servers, storage, network gear such as routers, switches and firewalls, and software licenses. When people compare on premises with the cloud, they often stop here. But this is only the tip of the iceberg.

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…and the costs below the waterline. Below the waterline are the costs that are easy to forget: data center space, power and cooling, physical security, the staff who install, patch and repair everything, maintenance contracts, and replacing hardware every few years. And there's idle capacity: everything bought for the busiest day sits unused most of the time, but it was paid for anyway.

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Total cost of ownership (TCO). That's why fair comparisons use total cost of ownership, or TCO: every cost, over the same period, often three years. Count everything on premises, from hardware to staff time. Then count everything in the cloud too: usage, data transferred out of AWS, the migration itself, and training. The cloud isn't free, but once the hidden costs are included, it often costs much less.

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Tools for the comparison. AWS offers tools for this. The free AWS Pricing Calculator lets you model a solution and estimate its AWS costs before you build anything. Migration Evaluator helps you build a data-driven business case: it analyzes your existing servers, how much they're really used, and the licenses you already own, then compares what running them on AWS would cost.

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Economies of scale lower the price. Finally, remember economies of scale from lesson one. Because AWS combines the usage of hundreds of thousands of customers, it buys and runs infrastructure at a scale no single company can match, so its cost per unit is lower. AWS passes those savings on as lower pay-as-you-go prices. On the exam, that's a cost saving of moving to the cloud, not something you configure.

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How the exam asks about this. Here's how the exam asks about this. Spending that doesn't change with usage is a fixed cost; spending that follows usage is variable. Buying hardware up front is a capital expense; paying monthly for cloud services is an operating expense. Forgotten costs like power, cooling and staff point to total cost of ownership. Estimating AWS costs before building means the AWS Pricing Calculator, and a business case from your current servers means Migration Evaluator.

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Recap: cloud economics. To recap: fixed costs stay the same while variable costs follow usage, and the cloud turns large capital expenses into operating expenses. A fair comparison counts the total cost of ownership, including the hidden costs below the waterline. And economies of scale let AWS charge lower prices. Next, we'll see how to lower your own cloud bill by rightsizing, automating and choosing the right licenses.

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