iMac 27'' Power Consumption

I got my iMac 27'' about 4 months ago and these days I became really curious on how much does it cost me to run this machine 18 hours/day.

So I unpacked my small Watt meter and started to measure things.

Here it is:

• During start of the OS: It was between 70 - 140 Watts. I hate to say here I have 2 HDDs in. One 1TB SATA drive and the second 256GB SSD drive.
• Normal usage:  power consumption varies between 70 - 90 Watts for me
• Idle state (monitor disabled) : 48 Watts
• And the big surprise... When I used sleep mode my Watt meter shown Zero Watts. I think there must be something wrong because I don't think it doesn't eat anything. Probably power consumption in this state is very small and my device simply cannot measure it.