The measurements (45W) was the average power consumption at the wall by my DS918+ equipped with 4 x 8TB WD Red drives. WD Red drives do not support spin down, so at most they're idle.

Synology does publish power consumption for each model, i.e. for the DS920+ (https://www.synology.com/en-us/products/DS920+#specs) model they state :

- 32.17W access - 9.69W HD Hibernation

But, they also specify that the drives used for their calculation is

    Power consumption is measured when it is fully loaded with Western Digital 1TB WD10EFRX hard drive(s).

And looking up power consumption of that particular model we get

- read/write 3.7W - idle 3.1W - standby 0.6W

so for the "access" vaule we can subtract 4x3.7W (14.8) and see that the device itself uses 17.37W. Then lookup the power consumption of your drives, i.e. WD80EFAX (https://documents.westerndigital.com/content/dam/doc-library...), and find

- read/write 8.8W - idle 5.5W - standby 0.8W

So you add the 4x8.8W (35.2W) to the 17.37W and arrive at 52.57W for "access".