In AC electrical circuits, two key power metrics come into play:
- Apparent power (KVA): The product of voltage and the root-mean-square (RMS) value of current, often called “capacity” in transformer terminology. It accounts for both the power that performs actual work and the power required to sustain magnetic fields in inductive equipment.
- Active power (KW): The power that directly powers devices and performs useful work (what most people refer to as “power” in daily conversations).
The critical difference lies in how these metrics apply to real-world loads. Most electrical systems rely heavily on inductive loads—think motors, compressors, or industrial machinery. For these loads, apparent power (KVA) is the vectorial sum of active power (KW) and reactive power (the non-working power needed for magnetic field operation). This means the apparent power of inductive loads is always greater than their active power.
If you sized a distribution transformer based solely on active power (KW) instead of apparent power (KVA), you’d run into serious issues with inductive loads. Since the inductive load’s apparent power exceeds its active power, the transformer’s capacity would be insufficient to handle the actual current draw. This leads to overcurrent conditions, which can damage the transformer, trigger safety protections, or cause premature equipment failure.
The rule changes only for purely resistive loads—such as electric heaters, incandescent lights, or resistive heating elements. For these loads, reactive power is negligible, so the total power consumption (in KW) can be roughly equivalent to the transformer’s KVA rating. However, purely resistive load setups are rare in most commercial, industrial, or residential applications.
KVA is specifically tailored to AC circuit dynamics, directly reflecting the voltage-current relationship that defines transformer performance. When sizing a transformer, the goal is to accommodate the total electrical demand—including both working power and reactive power—rather than just the power doing tangible work. Using KVA ensures the transformer can handle the full current load, preventing overloads and optimizing operational safety.
In contrast, KW focuses solely on active power and ignores reactive power’s impact on current draw. This oversight can lead to undersized transformers that struggle to support real-world loads, even if the KW rating seems sufficient on paper.
For distribution transformer sizing, KVA remains the gold standard—it aligns with how electrical systems operate, accounts for all types of loads, and ensures reliable, efficient performance for years to come.
