A recent publication (Wang et al 2013 Laser Phys. Lett. 10 115804) described an actively Q-switched and diode-pumped Er:YAG (yttrium aluminum garnet) laser source. The novelty of that letter lies in the use of a very bright and spectrally narrowed fiber-coupled laser diode as a pump, thus leading to very good performance in continuous wave mode. In Q-switch operation, the output energy reported is surprisingly high (6.6 mJ) for this kind of setup with a low pump power (only 10 W) and with small beam diameter (250 µm). In this letter, we demonstrate that the output energy exceeds the maximum extractable energy of the gain medium.