So then I could look at monthly statistical data. I guess the only part I didn't totally understand was the different levels and what they mean (1,2,3,4). Or how the number I pulled from the PowerCLI relates to the number in vCenter.
It basically works like this:
You have a whole bunch of different performance metrics on every vCenter object, in the VM-context you have for example "datastore.numberWriteAveraged.average" for Write IOPS or "cpu.usagemhz.average" for Mhz CPU usage and many, many more.
All of these performance metrics are tied to a certain statistics collection level controlled by vCenter. In your case "datastore.numberWriteAveraged.average" belongs to level 2 and "cpu.usagemhz.average" (will most certainly) belongs to level 1.
Your vCenter is configured to store metrics of statistics level 1 only (as you have confirmed in the vCenter Server Settings window), meaning it won't rollup and store any historical data of metrics belonging to higher statistics levels such as "datastore.numberWriteAveraged.average" which is at level 2.
Now you can either globally increase the statistics collection level to 2, which will result in a lot more historical performance metrics being collected (including the desired "datastore.numberWriteAveraged.average") and thus increasing the size of the vCenter DB by probably a lot more than what would be necessary, or you can configure just selected metrics to belong to statistics level 1 instead in order to avoid collecting too much useless data.
The first option is a bit easier as it's just a couple of clicks, the latter option is described here: