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Reactive power

What is reactive power?

Of all the technical concepts on a UK business electricity bill, reactive power is the one that confuses people the most. Real power (kW) is the useful work the electricity does, lighting your lights, turning your motors, heating your kettle. Reactive power (kVAr) is the back-and-forth energy that sloshes between the supply and inductive equipment without doing useful work but still loads up the network. Apparent power (kVA), which is what the network has to physically deliver, is the vector sum of the two. The relationship between them (the power factor) decides how much of the kVA your bill is calculated on actually does useful work and how much is just reactive churn. For UK industrial sites with heavy motor loads, managing reactive power can produce material capacity charge savings.

Reactive power matters mostly on larger business electricity connections, where it can show up as a separate line item on the bill if your site’s power factor slips.

Reactive power is invisible on most UK business bills, particularly NHH supplies. It becomes relevant when a site has a poor power factor (the ratio of real power to apparent power), because that produces higher capacity charges, larger DUoS bills, and (on some larger industrial supplies) explicit kVAr penalties. Power factor correction equipment addresses the underlying reactive power problem.

What reactive power actually is

Reactive power is the energy that flows back and forth between source and load in an AC system without doing useful work.

• Measured in kVAr (kilo-volt-amperes reactive).
• Distinct from real power (kW) which is the useful work.
• Required for inductive equipment to function (motors, transformers, fluorescent lighting).
• Returned to the source each AC cycle, then drawn again, in a continuous oscillation.
• Still occupies network capacity even though it does no useful work.

If you ran only purely resistive loads (heaters, incandescent bulbs), there would be no reactive power. The moment you add motors and transformers, reactive power appears.

What causes reactive power

Reactive power is a property of inductive loads.

• Electric motors. Industrial motors, compressors, pumps, fans, conveyors.
• Transformers. Both site-level and inside electrical equipment.
• Fluorescent and HID lighting. Older lighting with magnetic ballasts.
• Welding equipment. Significant reactive power draw.
• Arc furnaces and induction heaters. Major industrial sources of reactive power.

Capacitive loads (some electronic equipment) produce the opposite effect, sometimes partly cancelling out inductive reactive power. Most UK business sites are net inductive.

The power triangle. KW, kVAr, kVA

The relationship between the three power quantities forms a right-angled triangle.

• kW (real power). Horizontal side. The useful work.
• kVAr (reactive power). Vertical side. The back-and-forth energy.
• kVA (apparent power). Hypotenuse. What the network actually delivers.

The formula is. KVA² = kW² + kVAr². A site with 100 kW of real power and 50 kVAr of reactive power has 111.8 kVA of apparent power. The bill calculation uses kVA.

Power factor and its impact

Power factor is the ratio of real power to apparent power. Power factor = kW / kVA.

• Unity power factor (1.0). No reactive power. KW equals kVA.
• Good power factor (0.95+). Slightly inductive but well-managed.
• Typical commercial power factor (0.8 to 0.95). Common for mixed loads.
• Poor power factor (below 0.8). High proportion of reactive power, inefficient.

For a site running at 200 kW with a power factor of 0.7, the kVA demand is 286 (200 ÷ 0.7). The capacity charge is calculated on the 286, not the 200. Improving the power factor to 0.9 reduces the kVA to 222, saving 64 kVA worth of capacity charges.

How reactive power affects the bill

Reactive power affects UK business bills in three ways.

• Capacity charges. Based on kVA, not kW. Sites with high reactive power pay more capacity charge per kW of useful work.
• DUoS unit charges. Some DUoS bands and tariffs include kVA-based components, particularly for HH-metered sites.
• kVAr penalties. Some larger industrial UK supplies have explicit reactive power penalties built into the contract terms.

The bigger the site and the worse the power factor, the more meaningful these impacts become. For small NHH sites, the reactive power impact is largely absorbed into the standing charge.

How reactive power is measured

HH-metered UK supplies measure all three power quantities.

• The meter records kW (real power) directly.
• It also records kVAr (reactive power).
• kVA (apparent power) is calculated from the two.
• The data is available to the customer via the supplier portal.

NHH meters typically only measure kWh (cumulative real energy). The reactive power impact is implicit in the bill but not directly visible to the customer.

Power factor correction

Power factor correction (PFC) equipment is the standard solution to high reactive power.

• Capacitor banks installed at the site provide a counterbalancing reactive load.
• The capacitive reactive power cancels the inductive reactive power produced by motors and transformers.
• The net effect is that the power factor approaches unity (1.0).
• The kVA demand at the meter drops toward the kW level.

PFC equipment is typically a one-off capital investment (£10,000 to £100,000+ depending on size) with payback periods of 1 to 5 years from capacity charge savings on supplies with materially poor power factor.

kVAr penalties on large supplies

Some UK industrial supply contracts include explicit reactive power penalties.

• The contract specifies a target power factor (often 0.95).
• If actual power factor falls below the target, a kVAr penalty applies.
• The penalty is calculated on excess kVAr per half-hour and can be material.
• Power factor correction equipment is often essential on these sites.

Penalties are more common on direct-contract HH sites and on industrial supplies with significant motor or transformer loads.

Worked example

Illustrative example. A commercial site with a heavy motor load runs at 300 kW of real power. Initial measurement shows 200 kVAr of reactive power.

After PFC equipment brings power factor to 0.95.

Example only. Real PFC equipment cost, payback, and savings depend on site specifics. The point is the principle of how reactive power and power factor affect bill economics.

Practical implications for businesses

For UK business customers, reactive power matters most when one or more of these apply.

• Site is HH metered with significant motor or transformer loads.
• Capacity charges are material on the bill (large supplies).
• Power factor measurements are below 0.9.
• kVAr penalties appear on bills.
• Site is approaching its ASC and faces excess charges or reinforcement costs.

For office and retail supplies with predominantly resistive and electronic loads, reactive power is rarely material enough to justify PFC investment. For industrial supplies with heavy motor loads, it usually is. Related entries. kVA, Authorised Supply Capacity (ASC), half-hourly meter, DUoS, DNO, kWh, bill validation.

When reactive power matters and when it does not

For small office and retail supplies, reactive power management rarely justifies investment. For motor-heavy industrial sites, power factor correction equipment is usually a no-brainer at scale.

Common reactive power pitfalls

• Ignoring poor power factor until the bill hurts. The capacity charge impact builds quietly. Annual review of power factor against the bill catches it before it becomes large.
• Over-correcting with too much PFC. Installing too much capacitor bank capacity produces a leading power factor with its own issues. Properly sized PFC equipment is essential.
• Confusing peak demand with reactive power. A site can have low average reactive power but high peak demand at certain moments. PFC equipment needs to handle the peak as well as the average.
• Forgetting that motor variable-speed drives change the picture. Modern motor controls with VSDs (variable speed drives) usually reduce reactive power compared to fixed-speed motors. Equipment upgrades affect the PFC need.

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