<p>Agricultural waste, such as coconut fibre, is used to prepare activated carbon, which is then mixed with graphene to form a composite for electromagnetic shielding applications. The preparation of activated carbon involves pretreatment, chemical activation, and carbonisation, followed by the addition of graphene, yielding a lightweight, dielectric composite, i.e., graphene/activated carbon. The characterisation of various GaC composites prepared with graphene concentrations ranging from 0.05 to 0.5 wt% has been conducted, and a waveguide measurement setup was used to measure electromagnetic properties. Further, a numerical model is proposed to predict the performance of the GaC composite in terms of shielding effectiveness for battery-housing applications in electric vehicles. The TGA analysis shows that graphene concentrations ranging from 0.05 to 0.4 wt% exhibit enhanced thermal stability, whereas 0.5 wt% shows significant weight loss. It is observed that the total shielding effectiveness (<i>SE</i>_<i>t</i>) increases by 2.37dB with a weight concentration increment of 0.1wt%. It is observed that graphene concentrations of 0.1–0.3 wt% exhibit enhanced, stable shielding effectiveness in X-band; moreover, a maximum shielding effectiveness of 21.5dB is attained with GaC0.5 wt%. Also, it is observed that conducting numerical simulations using GaC for battery housing results in effective EMI reduction, with <i>SE</i>_<i>R</i> values ranging from − 14.5 to -16.5 dB.</p>