  E.G. Bessonov suggested the time integrated strength of electric field
$\int_{-\infty}^{\infty}E(r,t)dt=S_E(r)$ as a parameter to classify
electromagnetic (EM) waves. Since then, this parameter has been studied and
used in many works on microwave and laser physics, especially when it comes to
unipolar, bipolar and few cycle EM pulses. In this paper, it is shown that
$S_E(r)=0$ is an identity for a wide class of free space pulses of finite total
energy. This property can be useful in various applications of few cycle
radiation and as a benchmark in EM and QED computations.
