The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the G15's measured sensitivities are about 1/3 stop lower than indicated across the ISO range (i.e. images are fractionally darker than expected for any given set of exposure values). A discrepancy this small has little practical impact in real world use.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
In terms of measured JPEG noise the Canon Powershot G15 maintains relatively low levels of both luminance and chroma noise up to ISO 1600 before increasing significantly at ISO 3200 and beyond. However, when looking at the sample shots it becomes obvious that some of this 'image cleanliness' is being paid for by a loss of detail at higher sensitivities. That said, Canon's approach to noise reduction if fairly well balanced. The images show comparatively little chroma noise up to higher sensitivities while some luminance noise is tolerated. This gives the G15 high-ISO output a grainy appearance that is not too dissimilar to film grain, and much preferable to the 'blotched' noise than can be seen on some other small-sensor cameras.
Overall the Canon performs very well for a camera with a 1/1.7" sensor. If you want visibly better high ISO results in a small package you'll have to invest in a compact camera with a larger sensor, such as the G1 X or Sony RX100. That said, both these cameras haven't got lenses as bright as the G15's, which means you can keep the ISO down for longer on the latter.
Raw noise (ACR 7.3 RC noise reduction set to zero)
As usual, looking at the Adobe Camera Raw results gives a pretty clear explanation of the JPEG results. With noise reduction in ACR set to zero the noise level curve is much steeper than for the out-of-camera JPEGs. Some noise is visible even at base ISO but both chroma and luminance noise become quite intrusive at ISO 400. Of course much cleaner results could be achieved by applying a customized noise reduction mix in your raw converter.