The Rayleigh-Modal method is used to calculate the electromagnetic field within the grooves of a perfectly conducting, rectangular-shaped one-dimensional diffraction grating. An enhancement coefficient (η) is introduced in order to quantify such an energy concentration. Accordingly, $\eta >1$ means that the amount of electromagnetic energy present within the grooves is larger than that one will have, over the same volume, if the diffraction grating is replaced by a perfectly reflecting mirror. The results in this paper show that η can be as large as several decades at certain, often narrow, ranges of wavelengths. However, it reduces to approximately 20% under sunlight illumination. In this latter case, such values are achieved when the optical spacing between the grooves $dn$ is greater than $500\mathrm{nm}$ , where d is the groove spacing and n is the refractive index of the substance within the grooves. For $dn$ smaller than $500\mathrm{nm}$ the enhancement coefficient turns negligibly small.