If your half-hourly reactive energy (kVArh) runs above roughly 33% of your active energy (kWh) in any half-hour period, your power factor has dropped below 0.95 and you are likely being charged for it. The first move is simple: pull your MPAN-level half-hourly active and reactive data and run that ratio check before spending a pound on equipment.
TL;DR:
- For each half hour, charge only reactive energy above 32.87% of active energy, then multiply the billing period total by your regional DUoS rate.
- Check charges per MPAN, distinguish imports from exports, and investigate missing reactive readings because some distribution operators estimate them using a power factor near 0.90.
- Use fixed capacitors for steady loads, automatic stepped banks for variable demand, and detuned equipment where drives or UPS systems create harmonics.
- Use several weeks of interval data and a specialist harmonic study to size correction; fixed capacitors can create leading power factor when loads fall.
Table of Contents
- Power factor basics: active, reactive and apparent power
- The 0.95 rule: how reactive charges are actually calculated
- How to check your half-hourly data and calculate the charge
- How to avoid and mitigate power factor penalties
- How EnerlyticsAI helps you check and fix reactive charges
- Measure first, fix second
- Run your reactive-charge check with EnerlyticsAI
- FAQ
- Sources
Power factor basics: active, reactive and apparent power
Every electricity supply carries three related quantities. Active power (kW) is the energy that actually does work, running motors, lighting and heating. Reactive power (kVArh) is the energy that builds and collapses magnetic fields in inductive equipment without doing useful work. Apparent power (kVA) is the combination of the two, and the ratio between active and apparent power gives you the power factor: cos θ = kW/kVA.
A power factor of 1.0 means all the current drawn is doing useful work. Most industrial and commercial sites sit below that because of inductive loads:
- Motors and compressors, especially when running under light load
- Transformers, which draw magnetizing current even when idle
- Fluorescent and older ballasted lighting fixtures
A low power factor means your site draws more current than its useful output requires. That extra current causes higher I²R losses in cables and transformers, more voltage drop, and it occupies network capacity the local distribution network has to size for, which is exactly what Distribution Use of System charges are built to recover.
The 0.95 rule: how reactive charges are actually calculated
Network operators do not fine you for a low power factor. They charge for the reactive capacity your site occupies on the distribution network, and the trigger point is specific: when reactive energy in a half-hour exceeds about 32.87% of active energy (commonly rounded to 33%), the excess becomes chargeable. That ratio corresponds mathematically to a power factor of 0.95, since tan(cos⁻¹0.95) equals 0.328684, confirmed in DUoS charging methodology guidance.
A power factor of 0.95 is the line. Below it, you pay for the reactive units above that 33% threshold; at or above it, you typically owe nothing extra for reactive demand, according to EnergyCosts' explainer on reactive power charges.
The math itself runs every half-hour of every settlement day. For each period, the network operator compares measured reactive kVArh to the 32.87% benchmark of active kWh. Any kVArh above that benchmark is chargeable, and those chargeable units are summed across the whole billing period, then multiplied by the DUoS reactive rate in pence per kVArh set out in your region's charging statement.
It is worth repeating: this is a structural network cost, not a punitive fine from your supplier. It exists because reactive demand occupies cable and transformer capacity the network has to build for, whether or not it delivers useful energy. One practical risk to watch for: when a meter fails to record reactive data, some DNOs estimate it using an assumed power factor, often around 0.90, according to SSEN's DUoS charging statement, which can overstate what you actually owe.
How to check your half-hourly data and calculate the charge
You do not need a consultant to find out if you are paying reactive charges. You need your half-hourly data and a spreadsheet.
Data to gather:
| Item | Where to find it |
|---|---|
| HH active energy (kWh) by MPAN | Supplier portal or half-hourly data export |
| HH reactive energy (kVArh) by MPAN | Same HH data export |
| DUoS reactive rate (p/kVArh) | Your region's DUoS charging statement |
| Agreed MIC or contract capacity | Your supply contract or DNO agreement |
Once you have the export, the process is repeatable:
- Export half-hourly active and reactive readings for each MPAN at your site.
- Calculate the ratio of reactive to active energy for every half-hour period.
- Flag any period where reactive exceeds 0.3287 times active, the 33% boundary.
- Sum the chargeable kVArh (the amount above that threshold) across the billing period.
- Multiply the chargeable total by your DUoS reactive rate to get the cost.
A few things catch teams out at this stage. If your site has multiple MPANs, check whether charges are calculated per MPAN or aggregated, since aggregation can mask a single circuit with a genuinely poor power factor. Confirm whether the data distinguishes import from export, since treatment differs. And watch for half-hours with no reactive reading at all, which may have been filled in with an estimated power factor rather than a real one. Running this in a spreadsheet works for a single site; importing the data into a platform built for half-hourly analysis saves time once you are managing several meters.
How to avoid and mitigate power factor penalties
Once you know which half-hours are chargeable, the fix is usually mechanical: install power factor correction (PFC) capacitors sized to your load profile. A few choices matter more than others.
- Fixed versus automatic banks: a fixed capacitor suits a steady load; a site with variable demand needs automatic, stepped correction that switches capacitance in and out as load changes.
- Placement: correction near the main incomer corrects the whole site at the meter, while correction near individual large motors reduces losses in the cabling between them and the switchboard too.
- Detuned or filtered banks: sites with variable speed drives, UPS systems or other harmonic-generating equipment need detuned capacitors to avoid resonance and overheating.
- Active PFC: sites with variable loads or on-site generation sometimes need dynamic correction that reacts in real time rather than switching in fixed steps.
Operationally, sequencing which loads start together and when capacitor banks switch matters almost as much as the hardware. A fixed bank left connected after a shift's load drops can swing you into a leading power factor, which carries its own network penalties.
Before you commit capital, build the evidence. Use your interval-data analysis to show the scale of the problem and the savings a correction would deliver, and only then consider requesting a reduction in your agreed Maximum Import Capacity.
Pro Tip: Never size a capacitor bank off a single meter reading. Always base it on several weeks of half-hourly data and have a competent electrical specialist run a harmonic study first, since over-correction and resonance cause more disruption than the original low power factor.
If your site generates power on site, whether that is a backup generator or commercial solar panels, reactive flows can behave differently at different times of day, which is another reason to check actual interval data rather than assume a single correction factor fits year-round.
How EnerlyticsAI helps you check and fix reactive charges
Our platform is designed to make the diagnostic steps above fast rather than manual. Importing half-hourly data by MPAN gives you a visual half-hourly active and reactive comparison in minutes instead of a spreadsheet exercise. Automated bill validation and anomaly detection flag unusual reactive charges on incoming invoices, and multi-site views let you compare power factor performance across every meter you manage, alongside the carbon reporting you already need for Scope 1, 2 and 3 disclosures.
The workflow maps directly onto the process above: import your half-hourly data, let the per-half-hour reactive-to-active check run, flag chargeable periods, and export a report you can use to justify an MIC reduction request or a capital case for correction equipment.
Measure first, fix second
Small power factor gains produce outsized savings because losses scale with current squared, not current itself. The discipline that matters most is resisting the urge to buy capacitors before the interval data and a harmonic check confirm exactly what your site needs.
— Chris
Run your reactive-charge check with EnerlyticsAI
Checking your power factor exposure costs nothing but the time it takes to export your data. Our free forever plan lets you import half-hourly readings by MPAN and see your reactive-to-active ratio laid out clearly, without a procurement process or a sales call.
From there, our half-hourly data tools and bill validation features help you confirm whether a charge on your latest invoice is genuine or an estimate, so you know whether the next call is to an electrician or to your supplier.
FAQ
What runs up your electric bill the most?
For most industrial and commercial sites, active energy consumption (kWh) and demand charges drive the bulk of a bill, with reactive charges adding a smaller but avoidable amount when power factor drops below 0.95. Checking your DUoS charges alongside your unit rate shows where reactive costs sit relative to the rest of the invoice.
Why would the power factor be low?
A low power factor usually comes from inductive equipment such as motors, transformers and older fluorescent lighting that draw magnetizing current without doing useful work. Motors running well under their rated load are a particularly common cause on industrial sites.
How do I calculate the penalty for low power factor?
Compare your half-hourly reactive energy (kVArh) to your half-hourly active energy (kWh) for each period, and flag any half-hour where reactive exceeds about 32.87% of active, the equivalent of a power factor below 0.95, as explained by EnergyCosts. Sum the chargeable kVArh above that threshold across your billing period and multiply by your DUoS reactive rate in pence per kVArh.
How do I avoid power factor penalties?
Install correctly sized power factor correction equipment, whether fixed, automatic or detuned depending on your load and harmonic profile, after confirming the problem with half-hourly interval data. Routine monitoring afterward catches any new equipment or load changes that push your power factor back down.
Sources
- Reactive Power Charges Explained: kVArh And Power Factor (EnergyCosts)
- Use of system charging methodology (Commercial National Grid)
- DUoS charging statement (SSEN)