Power Factor and Reactive Power: What Solar Changes

A plant manager once showed me a bill where energy consumption had dropped 30% after installing solar and a power factor penalty had appeared that had never been there before. Nothing was faulty. The interaction is real and reasonably common.

The mechanism, briefly

Inductive loads – motors, transformers, fluorescent ballasts – draw reactive power in addition to real power. Reactive power does no useful work but occupies network capacity, so suppliers bill for poor power factor, either directly or through a kVA-based demand charge.

Power factor is the ratio of real power to apparent power. A site drawing 400 kW real and 300 kVAr reactive has an apparent power of 500 kVA and a power factor of 0.8.

Why solar can make the ratio worse

Solar inverters typically supply real power at close to unity power factor. They do not, by default, supply reactive power.

So the solar offsets your real power draw while your reactive draw stays exactly the same. Real power falls from 400 kW to 150 kW; reactive stays at 300 kVAr. Your power factor has fallen from 0.8 to about 0.45, even though nothing about your equipment changed.

If your tariff bills on kVA demand or applies a power factor penalty, the bill can move in the wrong direction. The reactive component is now a much larger share of a smaller total.

What existing correction equipment does

Many industrial sites already have power factor correction – capacitor banks that supply reactive power locally. These are usually automatic, switching stages in and out based on measured demand.

Two things go wrong when solar arrives:

Sizing assumptions become stale. The bank was sized for the pre-solar profile. Under the new profile it may over-correct at some times, producing a leading power factor which some tariffs penalise just as readily as lagging.

Control point and speed. Older contactor-switched banks respond slowly. With solar output varying under passing cloud, the reactive requirement changes faster than the bank can follow.

Using the inverters

Modern grid-tied inverters can supply or absorb reactive power. Many are capable of operating at power factors down to 0.8 leading or lagging, and can be configured for a fixed power factor, a fixed reactive setpoint, or dynamic control against measured site conditions.

This is frequently the cheapest solution, because the hardware is already installed. The constraints:

• Supplying reactive power consumes apparent power capacity, so real power output may be reduced at high reactive demand. Usually only relevant near peak generation.
• Inverters only help while generating. Your reactive demand at 2 a.m. still needs conventional correction.
• Your network operator may have requirements or restrictions on reactive power injection. Check the connection agreement.

What to do

1. Check your tariff. Establish whether you are billed on kVA demand or subject to a power factor threshold, and what that threshold is. Some tariffs do not care, in which case none of this matters.
2. Measure before and model after. Get reactive demand data alongside real demand. Model the post-solar power factor across the day. This should happen at design stage, and it usually does not.
3. Review existing correction. Confirm capacity and control strategy remain appropriate under the new profile.
4. Configure the inverters deliberately. Do not accept the default unity setting without checking whether a different setpoint serves you better.
5. Re-check after commissioning. Compare actual billed power factor before and after. If it has worsened, the fix is usually configuration rather than hardware.

None of this is difficult. It gets missed because power factor sits with the electrical engineer and solar sits with the energy manager, and the two conversations happen separately.