Measuring Specific Power Consumption Per Application
What We Measure, And For How Long
There are applications where the GPU load remains pretty much the same during the whole period of use (for instance, simple 2D and 3D applications in windowed mode), and those where the load fluctuates a lot more (games). Because of this, it is of no use to sample power consumption just once, but it has to be measured over a long period of time and then turned into an average value.
For games, we selected different levels and tested for at least 15 minutes. Of course, we measured the consumption at idle as well. Each measurement was done with our low-end card first and then repeated with each of the high-end cards.
What We Measure With
We used a standard power consumption data logger, so that our results are easier to reproduce for anyone.
Despite the relatively low purchase price, it provides a usage log that is quite sufficient for analysis. The Voltcraft Energy Logger 4000 from Conrad Electronic is a product available in Germany, but we found that there are similar products available in other markets, including the US. Our device also outputs the power consumption in volt-amperes, as well as in watts. Of course, we chose the latter.
We would also like to point out that many cheap devices are not capable of measuring the consumption of hardware with switching power supplies with sufficient accuracy. Our device should be fine, as long as the measured values do not fall below 1-2 W.
The device stores all data on a regular SD card. Charts are created easily by importing the data files into Microsoft Excel or something similar. In the end, the important value for us is the average power consumption of the overall system, per specific application, in watts.
Measurement Accuracy and Value Determination
Since the measurement interval (sampling rate) of one minute can be a bit too long to catch the varying values when playing games, we instead set the logger to record the total consumption for our 15 minute gaming period. This way, we get a value with much higher resolution through a simple calculation: a measured power consumption of 57 Wh over a 15 minutes period of time equals an average consumption of 228 W.