IEC 60211: Maximum Demand Indicators — The Core Metering Tool Behind Your Industrial Power Bill

Why “Demand Charges” Account for 30–50% of Your Industrial Power Bill

IEC 60211 specifies maximum demand indicators (Class 1.0 accuracy). Most industrial users do not realize: demand charges may represent 30–50% of their electricity bill — billed on maximum average power (kW or kVA) over 15- or 30-minute intervals, not total energy consumption (kWh).

Operating Principle: Thermal-Lag vs. Digital Integration

Traditional thermal-lag demand indicators use a bimetallic strip thermal inertia to simulate the utility billing window (15/30 min). Current passing through a heating resistor bends the bimetallic strip; the pointer deflection is proportional to the integrated I²R heating effect over the interval — essentially the integral of current squared over time. Modern digital indicators sample voltage and current in real time, computing sliding-window average power — eliminating environmental temperature sensitivity of the thermal type.

Why Utilities Charge by Demand

Because generation and transmission infrastructure must be built for maximum possible load, not average load. A factory consuming 100,000 kWh/month with all equipment running simultaneously at a 500 kW peak demands 500 kW of transformer and line capacity from the grid. Another factory with identical 100,000 kWh but flat load profile (100 kW peak) requires only 100 kW. The demand charge is essentially “rent” for grid capacity.

Optimization strategy: Install a demand controller — when the predicted 15-minute window power approaches the historical peak, automatically shed non-critical loads (HVAC compressors, chargers) to “shave” the peak. Payback period for a 20–30% demand charge reduction is typically only 6–12 months.

TN Lab — The demand indicator is the “fair scale” between utility and customer. Understanding its principle lets you optimize your bill.