Reactive power (Q), measured in volt-amperes reactive (VAr), is the portion of AC electrical current that sustains magnetic and electric fields in motors, transformers, and similar devices without performing net work. For event power, that matters immediately: reactive power forces you to size generators and feeders to apparent power in kVA, not just the watts on your equipment list, and an uncorrected reactive load can destabilise a generator’s voltage regulation or trigger a nuisance trip mid-show. IEEE Standard 1459 formalises the notation: S = P + jQ, where S is apparent power (VA), P is active power (W), and Q is reactive power (VAr).
Three things to do right now:
- Request kVA sizing, not just kW. Ask your generator supplier to confirm the kVA rating and the power factor the unit is rated at (typically 0.8 lagging), then check your load list covers apparent power, not just active watts.
- Specify on-site power monitoring. Require a three-phase power quality analyser at the generator output and main distribution board throughout the event, including rehearsals.
- Request phase-balanced distribution. Unbalanced phases amplify reactive effects; ask your electrician to confirm phase allocation across all feeder circuits before doors open.
Key takeaways
Reactive power (Q, measured in VAr) is the single most commonly overlooked factor in event generator sizing, and correcting for it starts with specifying kVA rather than kW.
| Point | Details |
|---|---|
| Size to kVA, not kW | Always specify generators in kVA at a stated power factor (e.g. 0.8 PF lagging) to account for reactive demand. |
| Measure during rehearsal | Log three-phase P, Q, S, and PF at the generator output and main board throughout at least one full rehearsal. |
| Leading PF is the hidden risk | Modern LED drivers and UPS bypass can produce leading reactive current, which can cause generator overvoltage or a field trip. |
| Apply local PFC carefully | Switched capacitor banks placed near inductive loads reduce reactive demand; fixed banks risk over-correction at low load. |
| Jakspartypower | Provides kVA-rated generator hire, on-site monitoring, distribution board segmentation, and standby service across Sussex. |
Table of Contents
- What is reactive power in events, explained simply?
- Active, reactive, and apparent power: how the power triangle works
- Common causes of reactive power at events
- Why reactive power matters for generator-based event supplies
- How to measure reactive power on site
- Practical steps to manage reactive power at events
- What to specify when hiring a generator or event electrician
- How Jakspartypower handles reactive power at events
- Jakspartypower: get your event power right from the start
- Sources
- FAQ
What is reactive power in events, explained simply?
Think of reactive power like the froth on a pint of beer. The froth takes up space in the glass and the barman has to pour it, but you cannot drink it. In AC electrical systems, reactive power (Q) flows back and forth between the source and inductive or capacitive loads every cycle, occupying conductor capacity without delivering usable energy to your PA, your lights, or your catering equipment.
The physics behind it is straightforward. Inductive loads, such as motors and transformers, cause current to lag behind voltage. Capacitive loads cause current to lead. Either way, voltage and current fall out of step, and the portion of power associated with that phase shift oscillates rather than transfers. AllAboutCircuits describes it as “wattless” power: energy is stored in a magnetic or electric field during one half-cycle and returned to the source in the next, so the net transfer over a full cycle is zero.
The mathematical representation is S = P + jQ. Q sits on the imaginary axis of the phasor diagram, measured in VAr, while P (watts) sits on the real axis. Apparent power S (VA) is the vector sum of both. IEEE’s coverage of reactive power confirms this notation and the distinction that Q must be measured separately from P and S, which is why a simple wattmeter is not enough on a temporary event supply.
Active, reactive, and apparent power: how the power triangle works
The power triangle puts three quantities in one picture. Active power P (watts) is what your equipment actually converts into light, sound, or heat. Reactive power Q (VAr) is the oscillating component that sustains fields. Apparent power S (VA) is the hypotenuse: the total current demand your generator and cables must carry.
Power factor (PF) is the ratio PF = P ÷ |S|. A PF of 1.0 means all current is doing useful work. A PF of 0.8 means a significant portion is active and some is reactive. In practice, a generator rated at a given kVA at 0.8 PF delivers correspondingly less active power. If your load list totals a certain active power but the combined PF is lower, you need more kVA of generation than those active watts alone suggest. That gap is where events get into trouble.
| Quantity | Symbol | Unit | Phase relationship | Does net work? |
|---|---|---|---|---|
| Active power | P | Watt (W) | In phase with voltage | Yes |
| Reactive power | Q | Volt-ampere reactive (VAr) | out of phase | No |
| Apparent power | S | Volt-ampere (VA) | Vector sum of P and Q | Partially |
Common causes of reactive power at events
Most event loads are inductive, which means they draw lagging reactive current. Some modern electronics flip that and draw leading current, which carries its own risks (more on that below).
Common sources of reactive power at events:
- Induction motors in ventilation fans, HVAC units, and backline equipment (compressors, refrigeration) are the largest lagging reactive loads on most sites.
- Moving-head luminaires and followspots contain transformer-based ballasts and motor-driven yokes that draw significant reactive current, particularly during movement cues.
- Transformer magnetising currents in stage transformers and isolation transformers add a baseline reactive draw even at light load.
- Large LED wall drivers and switch-mode power supplies can present a mix of lagging and leading behaviour depending on their internal power factor correction design.
- UPS systems in bypass mode can present a highly capacitive (leading) load to the generator, which is one of the most dangerous reactive conditions for a temporary supply.
- Dimmer packs using phase-cut control introduce harmonic distortion that inflates apparent current and distorts power factor readings.
- Compressors and refrigeration units in catering areas are consistently inductive and often overlooked in early load estimates.
A realistic scenario. Consider a mid-scale festival stage with a 10-metre LED wall driven by twelve switch-mode processors, plus six motorised chain hoists flying the roof structure. During a cue change, the hoists accelerate simultaneously while the LED wall dims to a low-brightness state. The hoists spike inductive reactive current; the LED processors at low brightness may shift toward leading behaviour. The combined reactive demand changes character within a single second, and a generator without adequate automatic voltage regulation (AVR) headroom can respond with a voltage dip or, in a worst case, a trip.
Leading reactive power deserves a specific mention. Modern LED drivers, inverter-driven equipment, and some UPS configurations can produce leading (capacitive) reactive current. Schneider Electric’s guidance is explicit: sustained leading reactive current can cause a generator to lose voltage regulation entirely, producing destructive overvoltage transients. That risk is often greater than the more familiar lagging-PF scenario. See also the role of lighting control systems for how dimmer and LED control choices affect reactive demand.
Why reactive power matters for generator-based event supplies
Reactive power increases the RMS current flowing through every conductor and every connection on your temporary supply. AllAboutCircuits confirms that reactive current still causes I²R losses in cables even though it does no net work, which means cables run hotter, voltage drop increases, and available active power at the load end falls.
For generators, the consequences are more acute than for a permanent mains supply. HeatSpring’s analysis of distribution operations shows that uncorrected reactive power increases voltage drop and reduces the real power-carrying capacity of feeders. On a temporary site without the redundancy of a building supply, that reduction is felt immediately.
Generator-specific risks include:
- Loss of voltage regulation. When reactive demand exceeds the AVR’s compensation range, terminal voltage drifts, affecting sensitive audio and lighting equipment.
- Leading power factor trips. As noted above, leading reactive current is particularly hazardous. A generator’s capability curve (the D-curve) defines the safe operating envelope; operating outside it risks a field collapse.
- Nuisance overcurrent trips. High apparent current from a poor power factor can trip protective devices even when active power is within limits.
- Overvoltage transients. During UPS bypass or sudden load shedding with a capacitive load, the generator can overshoot voltage dangerously.
The NATF’s guidance on generating unit reactive capability adds an important nuance: the usable reactive capability of a hired generator depends not just on the machine nameplate but on the associated cabling, transformers, and auxiliary systems. A generator rated at a given D-curve may deliver less reactive support than expected once the hire cable run and distribution transformer are factored in. Always request supplier confirmation of the full system capability, not just the generator plate rating.
How to measure reactive power on site
Measurement is where planning meets reality. A wattmeter alone tells you nothing useful about reactive demand; you need instruments that capture all three quantities simultaneously.
Instruments to use:
- A three-phase power quality analyser (such as a Fluke 435-II or equivalent) is the standard tool. It logs P, Q, S, PF, voltage, current, and harmonic distortion on all three phases simultaneously.
- A true-RMS clamp meter with PF measurement is adequate for spot-checking individual circuits but insufficient for logging transient events.
- A data logger set to capture at least one complete rehearsal and the first hour of the live event gives you the transient picture that a spot check misses.
Where to measure:
- At the generator output terminals, to capture total apparent power demand and PF presented to the machine.
- At the main distribution board incoming, to confirm phase balance and identify any phase carrying disproportionate reactive load.
- At critical sub-feeders (stage power, LED wall, audio racks) to isolate the worst reactive contributors.
How long to log:
Capture at minimum one full technical rehearsal and the first 30 minutes of the live event. Reactive transients during cue changes, motor starts, and UPS switching are the events most likely to cause problems, and they only appear in a logged record.
| Power factor reading | Interpretation | Recommended action |
|---|---|---|
| PF ≥ 0.95 | Acceptable | Continue monitoring; no correction needed |
| PF typically around 0.8 | Monitor closely | Consider local power factor correction (PFC) |
| PF typically below 0.8 | Investigate | Apply PFC; review load list and phase balance |
Pro Tip: Log harmonic distortion (THD) alongside PF. Dimmer packs and switch-mode supplies generate harmonic currents that inflate apparent current and make PF readings misleading. Log at the highest available sample rate to catch fast transients.
For a broader inspection framework, the event electrical equipment inspection guide covers pre-event testing protocols that complement on-site power monitoring.
Practical steps to manage reactive power at events
Mitigation works best when it is designed in, not bolted on at the last minute. The steps below follow a logical sequence from specification through to live operation.
- Size to kVA from the start. Build your load list in VA, apply diversity factors, and specify generators in kVA at the expected power factor. Translating watts to kVA with a realistic PF estimate is the single most effective mitigation step.
- Apply local power factor correction (PFC). Shunt capacitor banks placed close to the largest inductive loads (motors, transformer-heavy rigs) reduce the reactive current flowing back to the generator. Local correction is more effective than a single bank at the generator output.
- Use switched capacitor banks, not fixed. Fixed capacitors correct for one load condition; switched banks can be staged in and out as the load profile changes through the day. This avoids over-correction during low-load periods.
- Consider dynamic support for complex rigs. Where load profiles change rapidly (large automated rigs, frequent scene changes), a STATCOM or SVC can provide sub-cycle reactive response that switched capacitors cannot match.
- Balance phases deliberately. Assign loads across phases to minimise reactive imbalance. An unbalanced three-phase supply amplifies neutral currents and increases total reactive demand.
- Stage load roll-outs. Bring large inductive loads online sequentially rather than simultaneously. Motor starting currents are several times the running reactive demand; simultaneous starts can overwhelm AVR response.
- Segment distribution boards by load type. Isolating motors, dimmers, and sensitive audio on separate feeders limits the propagation of reactive spikes. See the event distribution board installation guide for segmentation principles.
A critical warning on over-correction. Adding too much capacitance shifts the load from lagging to leading. As covered above, leading power factor is often more dangerous than lagging on a generator supply. Never apply fixed capacitor correction without confirming the minimum load condition the site will see, typically during a sound check or changeover.
Pro Tip: During rehearsal, switch PFC banks in and out deliberately and log the generator’s voltage response each time. This rehearsed switching test reveals whether your correction level is appropriate before the audience arrives. If voltage rises when you add capacitors, you are already at or past unity — reduce the bank size.
What to specify when hiring a generator or event electrician
A well-written hire specification prevents reactive-power problems before they start. Include these items in every generator or event power enquiry:
- Specify kVA, not just kW. State the required kVA rating and the power factor at which it is rated (e.g. “100 kVA at 0.8 PF lagging”).
- State the acceptable PF range. For example: “Supply must maintain stable voltage regulation across a load power factor range of 0.8 lagging to 0.95 lagging.”
- Request the generator D-curve or capability confirmation. Ask the supplier to confirm the reactive capability of the complete hire unit, including cable runs and any transformers, not just the generator nameplate.
- Require on-site monitoring. Specify that a three-phase power quality analyser must be present throughout the event, with logged data available on request.
- Require phase-balanced distribution. Ask for written confirmation of phase allocation across all circuits before the event.
- Ask for standby service. A qualified electrician on site during the event is the last line of defence against reactive-power incidents.
Sample wording for a hire request:
“Please supply a generator rated at [X] kVA at 0.8 PF lagging, with confirmation of reactive capability across the full hire system. On-site three-phase power quality monitoring is required throughout the event, including all rehearsals.”
For a complete procurement framework, the event power planning stages guide walks through each stage from initial load survey to live monitoring.
How Jakspartypower handles reactive power at events
Jakspartypower brings over 40 years of combined electrical contracting experience to temporary event supplies in Sussex, which means reactive power is not an afterthought — it is part of the pre-event conversation.
Services relevant to reactive power management:
- Pre-event power surveys. Site visits to assess load lists, identify reactive-heavy equipment, and recommend kVA sizing before any equipment is hired.
- kVA-based generator hire. Generators specified and supplied at the correct apparent power rating for the confirmed load profile, with power factor taken into account from the outset. Browse the generator hire options for available ratings.
- On-site monitoring. Three-phase monitoring at generator output and distribution boards throughout the event, including rehearsals.
- Distribution board segmentation. Boards designed to isolate motor loads, dimmers, and sensitive audio on separate feeders, reducing the risk of reactive spikes cascading across systems.
- Power factor correction equipment. Where the load profile warrants it, PFC modules can be incorporated into the distribution design.
- Standby service. A qualified electrician on site for the duration, able to respond to reactive-power incidents in real time. Details on the standby service for events.
Contact Jakspartypower for a pre-event power survey and to confirm that your generator specification accounts for reactive demand.
A note from the field
The measurement step is where most event power plans fall short. Organisers specify watts, suppliers quote kilowatts, and nobody checks the power factor until something trips. The single most useful thing you can do before any event with significant motor or LED-wall load is to log the power quality during a full technical rehearsal, not just a static load test. A static test at full load tells you the steady-state picture; a rehearsal log catches the transients during cue changes and motor starts that actually cause problems on the night.
Jakspartypower: get your event power right from the start
Reactive power is one of those topics that sounds abstract until a generator trips 20 minutes before showtime. Jakspartypower’s approach is to resolve it at the specification stage, not during the event. With kVA-rated generators, segmented distribution boards, on-site three-phase monitoring, and a standby electrician throughout, the reactive-power risks covered in this guide are managed before they become your problem.
The team covers events across Sussex, from private weddings to large outdoor shows, with equipment and experience scaled to the job. Every hire includes a pre-event conversation about load profiles and power factor, so the generator that arrives on site is sized for what you are actually running, not just the watt total on a spreadsheet.
To request a pre-event power survey or get a quote for generator hire with monitoring included, visit Jakspartypower or go directly to the generator hire page.
Sources
- Reactive Power | IEEE Technology Navigator
- Reactive Power and Transmission & Distribution Operations – HeatSpring Magazine
- Calculating Electrical Load for Event Power Distribution – TechBullion
- True, reactive, and apparent power | AllAboutCircuits
- Reporting and verification of generating unit reactive power capability for synchronous machines — NATF
FAQ
What is reactive power in simple terms?
Reactive power (Q, measured in VAr) is the portion of AC electrical current that flows back and forth sustaining magnetic and electric fields in motors and transformers, without delivering usable energy. It occupies conductor and generator capacity without doing net work.
What happens if reactive power is high at an event?
High reactive power forces your generator to carry more apparent current (kVA) than the active load (kW) alone would require, increasing cable losses, voltage drop, and the risk of the generator tripping or losing voltage regulation mid-show.
What are some examples of reactive power at events?
Induction motors in fans and refrigeration, motorised moving-head lights, stage transformers, large LED-wall switch-mode drivers, and UPS systems in bypass mode are all common reactive loads at events. Leading reactive current from LED drivers and UPS bypass is particularly hazardous for generator supplies.
What is the difference between active and reactive power?
Active power (P, watts) is what equipment converts into light, sound, or heat. Reactive power (Q, VAr) oscillates between source and load each cycle and does no net work, but it must still be carried by conductors and generators, which is why apparent power (S, VA) is always larger than active power alone.