In my previous post, I looked at the European Union (EU) Energy Star specifications for computers. This week, I decided to apply the specifications to a computer that is currently available to purchase; I walk through the calculations used to determine whether the computer meets the Energy Star requirements as set out in Commission Decision 2009/489/EC.
The computer I have used in this example is the Apple iMac, 21.5 inch, 3.06GHz (model MB950). I have used the values for power consumption from the Energy Star Product Database.
Apple also publishes power consumption information for the iMac in their Environmental Report. I was interested to note that values for idle power consumption in the Energy Star database and the Apple report are different. The values for power consumption in off mode and sleep mode are the same. I suspect the difference in idle power consumption values is due to differences in the definition of idle.
I decided to use the power consumption values from the Energy Star database, as they are more likely to have been measured in accordance with the specification.
Here are the details of the iMac I used from the Energy Star database:
- Category: B
- Idle mode (W): 28.1
- Off mode (W): 0.8
- Sleep mode(W): 1.9
- ETEC (kWh): 103.0
- CPU: Intel Core2 Duo
- Core speed (GHz): 3.1
- System memory (GB): 4
- Discrete GPU: no
- Hard drives: 1
- Operating system: Mac OS X
This iMac falls within category B in the Energy Star specifications. The specifications provide four different categories for desktop computers based on their capabilities. To be classed as category B, the computer must have equal to two physical cores and two gigabytes of system memory.
To qualify for Energy Star status, a category B desktop computer must have a typical energy consumption equal to or less than 175kWh per year. The typical energy consumption limit can be increased by adjustments made for additional system memory, a discrete GPU, and additional internal storage. The iMac we are looking at does not meet the requirements for any adjustments based on additional system memory, a discrete GPU, or additional internal storage.
The formula for calculating the typical energy consumption of a desktop computer is set out in the specifications. The formula calculates the amount of power the computer would use over the year, based on the power it uses in different states and the average amount of time spent in those states:
ETEC = (8760/1000) x (Poff x Toff + Psleep x Tsleep + Pidle x Tidle)
The P-values represent power consumption measured in watts.
The T-values represent time measured as a percentage of the year.
The amount of time the computer spends in each state (the T-values) is set out in the specification. For a desktop computer the percentage of the year spent in each state is below:
- Off: 55% (Toff = 0.55)
- Sleep: 5% (Tsleep = 0.05)
- Idle: 40% (Tidle = 0.40)
The power consumption for each state (the P-values) are determined by measuring the particular computer in accordance with the specification. I have used the values from the Energy Star database for the iMac:
- Off mode: 0.8W (Poff = 0.8)
- Sleep mode: 1.9W (Psleep = 1.9)
- Idle mode: 28.1W (Pidle = 28.1)
When we put the P-values for this iMac and T-values for desktop computers into the formula, it looks like this:
ETEC= (8760/1000) x (0.8x0.55 + 1.9x0.05 + 28.1x0.40)
To meet the Energy Star requirements, this iMac needs typical energy consumption of of 175kWh or less. At 103.1kWh, the iMac meets the Energy Star requirements.
I was interested to note that the Energy Star database gives an ETEC value for this iMac of 103.0, whereas my calculations came out as 103.1. I'm not sure where this discrepancy comes from, but in either case, the iMac's typical energy consumption is well within the requirements.