PlantmarginMaximum demand: why one bad half-hour sets the month's demand charge
On most industrial tariffs, part of the bill depends not on how much energy the plant uses, but on the highest average demand in any 30-minute window of the month. One bad half-hour sets the charge for all the others.
How it is worked out
The meter averages demand over each half-hour. The highest half-hour of the month, in kVA, is multiplied by the demand rate.
Example: a plant normally peaks at 820 kVA. One morning, after a power cut, all the motors restart together and the half-hour average reaches 960 kVA. At an example demand rate of KSh 700 per kVA:
extra charge = (960 − 820) × 700 = KSh 98,000 for that month
The plant used almost no extra energy. It simply used it at the same time.
Common causes of the peak
- Simultaneous restart after an outage or a shift change.
- Starting large motors direct-on-line.
- Batch processes (mixers, kilns, boilers) all heating up together.
- Poor power factor: kVA rises even when kW does not.
- Seasonal loads such as cooling on hot afternoons.
Ways to keep it down
- Stagger restarts. A simple restart sequence after outages often removes the worst peak.
- Soft starters and drives on the largest motors.
- Schedule batches so heating cycles do not overlap.
- Correct the power factor so kVA stays close to kW.
- Alarm before the limit. A live view of the running half-hour lets the shift supervisor hold back a load before the peak is set, not after.
- Batteries can shave a predictable peak where the numbers work.
Measure first
The fix depends on what caused the peak. A meter that records every half-hour, alongside which sections were running, turns the demand charge from a surprise into a decision.
Plantmargin tracks the running half-hour live, warns before a new peak is set, and shows which sections caused each month's highest half-hour.