This paper studies ways to estimate inertia in power grids, a crucial property for maintaining grid stability during power imbalances. The authors compare two methods for calculating inertia constants: a formula based on power imbalance and rate of change of frequency (RoCoF), and using the total kinetic energy stored in rotating parts of generators. Simulating a power outage in the Czech Republic, the paper analyzes resulting data on frequency and power flow. The findings are calculating inertia for individual areas based on simulated data and RoCoF is unreliable. Estimating inertia for the entire network using WAMS data and a formula is possible, but accurate RoCoF determination is crucial. Obtaining total system load data can be difficult. The paper concludes that estimating inertia for the whole network using WAMS data is promising, but further research is needed to improve RoCoF calculations and address data acquisition challenges.