The irregular variation of geomagnetic activity, caused by the solar wind interaction with the magnetosphere/ionosphere, occurs in a wide time interval (from seconds to annual and even larger periods) and amplitude ranges (from few to hundreds of nT). Major variation events (geomagnetic storms) can cause damaging effects and important perturbations to different human activities such as satellite communications, long distance radio broadcasting, navigation, surveying, etc. Furthermore, the induction of electric currents (geomagnetically induced currents – GICs) may affect all grounded conducting networks, in particular electrical power transmission systems. Useful proxies of GIC effects are still under debate; here, we identify the pros and cons of some different candidates. The intensity of geomagnetic activity is usually characterized by geomagnetic indices, among which the 3-hourly indices Kp and local K. In this study we compare 3-hour K and 1-hour K-derived local range indices, using geomagnetic time series from the mid-latitude Coimbra observatory (COI), in Portugal. We also compute smaller time-resolution geomagnetic and GIC indices such as the geomagnetic horizontal field components and their time derivatives, horizontal field magnitude and its time derivative and the LDi/LCi indices [Cid19]. We compare the computed indices with GIC simulations in the Portuguese transmission power grid, to evaluate which of them may be used to nowcast the induced currents. We suggest as a better GIC proxy, an index obtained from geomagnetic field components filtered by convolution with the uniform conductivity Earth filter (new EGIC index). Previous studies have considered the local ground conductivity to be an important factor to determine GIC amplitudes (e.g. [Tri07] and [Rib21]). We then obtain GIC estimations for power grid substations lying at different geological regions. Acknowledgements: This study is funded by national funds through FCT (Portuguese Foundation for Science and Technology, I.P.), under the project MAG-GIC (PTDC/CTA-GEO/31744/2017). FCT is also acknowledged for support through projects UIDB/50019/2020-IDL, PTDC/CTA-GEF/1666/2020 (MN) and PTDC/CTA-GEO/031885/2017 (MN). CITEUC is funded by FCT (UIDB/00611/2020 and UIDP/00611/2020). We acknowledge the collaboration with REN (Redes Energéticas Nacionais).