We propose the use of SNR for the assessment of AEC performance. SNR was used to assess AEC kV reproducibility and AEC attenuator reproducibility on six DR systems from three different manufacturers. AEC devices are used with DR systems to maintain a suitably adequate level of image quality, consistent with the level of exposure to the detector. Therefore, it is appropriate to use a measure of image quality to assess the performance of AEC devices used in DR. This study highlights the weakness of the current IPEM report 32 recommended metrics of detector air kerma (Doyle and Martin, 2006) [1] and detector dose index (Rowlands, 2002) [2]. This study also investigates the inconsistency of patient equivalent attenuators used across different hospitals in the UK (Rose, 2015) [3]. SNR is again used to compare four different attenuator combinations. Our results obtained indicate that AEC performance can be accurately assessed by measuring SNR when using different thicknesses of aluminium.