The Per-Unit System — Why Power Engineers Don't Use Ohms

One transformer, two wildly different ohm values depending which side you measure from — and exactly one per-unit value that works on both.

Power system studies almost never quote impedance in ohms. Instead everything — voltage, current, impedance, power — gets expressed as a fraction of a chosen base value: the per-unit (pu) system. It looks like an extra layer of abstraction bolted on top of Ohm's law, but it exists to solve a specific, concrete headache: comparing and combining equipment of wildly different physical sizes and voltage levels without a mess of turns-ratio conversions at every step.