When looking to keep Google Cloud Platform (GCP) costs in control, the first place users turn are the discount options offered by the cloud service provider itself, such as Google’s Sustained Use discounts. The question is: do Google Sustained Use discounts actually save you money, when you could just turn the instance off?

How Google Sustained Use discounts work

The idea of the Sustained Use discount is that the longer you run a VM instance in any given month, the bigger discount you will get from the list price. The following shows the incremental discount, and its cumulative impact on a hypothetical $100/month VM instance, where the percentages are against the baseline 730-hour month.

I have to say here that the GCP prices listed can be somewhat misleading unless you read the fine print where it says “Note: Listed monthly pricing includes applicable, automatic sustained use discounts, assuming the instance runs for a 730 hour month.” What this means to us is that the list prices of the instances are actually much higher, but their progressive discount means that no one ever actually pays list price. That said – the list price is what you need to know in order to estimate the actual cost you will pay if you do not plan to leave the instance up for 730 hours/month.

For example, the price shown on the GCP pricing link for an n1-standard-8 instance in the Iowa region is (as of this writing) $194.1800. The list price for this instance would be $194.1800/0.7 = $277.40. This is the figure that must be used as the entry point for the table above to calculate the actual cost, given a certain level of utilization.

What if you parked the VM instance instead?

Here at ParkMyCloud, we’re all about scheduling resources to turn off when you’re not using them, i.e., “parking” them. With this mindset, I wondered about the impact of the sustained use discounts on the schedule-based savings. The following chart plots the cost of that n1-standard-8 VM instance, showing Google sustained use discounts combined with a parking schedule.

We can definitely see progressively more sustained use savings added to progressively less schedule-based savings. I am sure this would end up getting described as the typical hype of “the more you spend, the more you save!” But, the reality of the matter must intrude here and show the more you spend…the more you spend!

Looking at what this means for ParkMyCloud users, here is the monthly uptime for a few common parking schedules, and the associated cost:

These are a far cry from the $277.40 list price, and even the $194.18 max discounted price. From this, it can be seen that even with the most wide-open “work day” schedule of 12 hours per weekday, the schedule is barely nudging over the 182.5 hours needed to hit the first price break of 20%. And even then, the 20% discount is only applied to those hours above 182.5 hours. A welcome discount to be sure, but not very enormously impactful to the bottom line.

Another way our users keep these utilization hours low is by keeping their VM instances “always parked” and temporarily overriding the schedule for a set number of hours (such as for an 8-hour workday) when their non-production resources are needed. When the duration of the override expires, the instance is automatically shut down. Giving the best possible savings, and usually never even hitting the first GCP discount tier.

Do Google Sustained Use discounts save you money?

In short: definitely! At least, they do save you money over the price listed by Google. Do they save you the maximum amount of money possible? No, not if it’s a non-production VM instance that is only needed during a regular workday (although it’s close).

To get the optimal savings on your resources, keep them running only when you’re actually using them, and park them when you’re not. If you meet the threshold of 25% usage for the month, Google’s Sustained Use discounts will kick in, and further lower your cost from the list price. These two savings options combined will optimize your costs and provide the maximum savings.