Reactive Power Is Not Wasted Energy

Part 8 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Call an electrical engineer's reactive power "wasted energy" and watch them wince. It is the most common misconception on a TNB bill, and it gets the physics exactly backwards. The magnetizing current that utilities penalise is not energy being thrown away. It is energy being borrowed and returned, over and over, a hundred times a second, and without it not one induction motor in your plant would turn.

The waste is real, but it is somewhere else entirely. Understanding where is the difference between throwing money at the wrong fix and solving the problem at its source.

Is reactive power wasted energy?

No. Reactive power does no net work, but it is not consumed. It flows from the source into a motor's magnetic field, is stored there, and flows straight back to the source on the next half-cycle. Over a full cycle the two flows cancel, so the field draws zero net energy. Yet the field itself is indispensable: it is the mechanism that turns electricity into torque.

This is easiest to see in an induction motor. The motor spins because a rotating magnetic field drags the rotor around with it. That field has to be built and maintained, and building it takes current, the magnetizing current. This is the reactive component. It surges into the windings, sets up the flux, then collapses back toward the supply as the AC waveform reverses, only to surge back in again. The oscillation runs at twice the line frequency, so 100 Hz on Malaysia's 50 Hz grid. The energy sloshes back and forth. It never leaves as heat or light or motion. But remove it and the motor cannot produce a single newton-metre of torque.

Transformers do the same thing. Every transformer in the grid needs magnetizing current to sustain the flux in its core, and that flux is what couples the primary winding to the secondary. No flux, no voltage transformation, no power delivered. The reactive current that maintains it is no leak: it is the price of admission for AC to do anything useful at all.

Then why do utilities penalise it?

Because moving reactive power around costs real money, even though the reactive energy itself is returned. The back-and-forth magnetizing current still has to travel through physical copper: your cables, your switchboard, the utility's transformers and distribution lines. And copper does not care whether the current it carries is doing useful work or just sloshing. It heats up regardless.

That heating is the genuine waste. It is resistive loss, proportional to the square of the current (I²R), and it is dissipated as heat in every conductor between the power station and your motor. Consider a load drawing 4 A of working current and 3 A of reactive current. Those do not add to 7 A; they combine as a right triangle to 5 A total. The cable carries the full 5 A, and its heating loss is set by that 5 A, not by the 4 A doing the work. You pay to lose energy warming up wires that are hauling current which accomplishes nothing at the far end.

The second cost is capacity. Every cable, transformer and switchboard is rated in kVA, apparent power, because its limit is set by current and voltage, not by how much of that current happens to be useful. A 1000 kVA transformer running at a power factor of 0.75 (the fraction of its current that does real work) can deliver only 750 kW of real power instead of 1000 kW, a quarter less than its rating. The gap opens because the load also draws roughly 660 kVAR of reactive power (kilovolt-amperes reactive), and the transformer has to carry that reactive current alongside the working current. That is grid capacity built, paid for, and then spent on transport rather than work. The relationship between these quantities is the power triangle, and it explains why a low power factor quietly steals headroom you already own.

So the penalty targets transport, not the reactive energy itself: TNB is recovering the cost of the extra current their network has to carry, and nudging you to stop making them carry it. In Peninsular Malaysia the threshold is a power factor of 0.85 for supply below 132 kV, and 0.90 for supply at 132 kV and above. Fall below and a surcharge lands on your bill. We cover exactly how that charge is computed in the power factor surcharge explainer.

Why do lightly loaded motors have such bad power factor?

Because the magnetizing current stays roughly constant no matter how little work the motor does, while the working current shrinks with the load. Power factor is the ratio of real power, the working part, to total power, so as the useful share collapses, the ratio collapses with it.

A motor's field has to be fully established whether it is driving a full load or idling. The reactive current that builds it is typically 20 to 60 percent of the motor's full-load current, and it barely moves as the shaft load changes. At full load an induction motor might run at a power factor of 0.85, a healthy figure, because the large working current dominates. Take the load away and the working current drops toward zero while the magnetizing current holds firm. Power factor at no load can fall to 0.15 or 0.30.

This is why oversized motors are one of the most common causes of poor site power factor in Malaysian plants. A 30 kW motor specified for a 12 kW duty spends its whole life lightly loaded, drawing near-full magnetizing current to do a fraction of the work it could. Every one of those motors is dragging your site power factor down and, below 0.85, adding to your monthly surcharge. Chillers, compressors, pumps, fans and lighting ballasts all contribute the same lagging reactive load, and the more of them run part-loaded, the worse it gets.

If reactive power is necessary, how do you avoid paying to haul it?

You generate it locally instead of importing it from the power station. This is the whole logic of power factor correction, and it works because reactive power comes in two opposite flavours that cancel.

An induction motor draws lagging reactive current. A capacitor draws leading reactive current, exactly out of phase with it. Put a capacitor near the motor, or a bank of them at your main switchboard, and the capacitor supplies the magnetizing current the motor needs. The reactive power now sloshes back and forth over the short loop between capacitor and motor, inside your own installation, instead of being dragged all the way from TNB's generators through every cable and transformer in between.

The reactive power still exists. The motor still needs its field. Nothing about the physics changed. What changed is the length of copper the reactive current has to travel through, and therefore the I²R losses, the consumed capacity, and the surcharge. The utility now sees mostly working current at your meter, your power factor climbs back above 0.85, and the penalty disappears. That is why the fix is capacitor banks rather than, say, buying different motors, and it is the subject of Part 23 on power factor correction.

The point worth holding onto: you never eliminate reactive power, because you cannot. You relocate where it is produced. A well corrected plant is not one that stopped needing magnetic fields. It is one that stopped renting them from three hundred kilometres away.

This is Part 8 of 23 in Cobler's Electricity Fundamentals series. Previous: kW, kVA and kVAR: The Power Triangle, Explained. Next: AC vs DC: Why the Grid Alternates and Your Phone Does Not.

Reactive power that never leaves your walls does not show up on your bill. CobiNeural's energy monitoring tracks power factor and reactive load equipment by equipment, so you can see which lightly loaded motors are costing you before the surcharge does. Book a demo and we will show you where your reactive current is really going.