In short. Peak shaving uses an on-site source, usually a diesel generator or a battery, to cut the demand drawn from the DISCOM during the hours when demand is highest. In India it pays mainly by avoiding contract-demand penalties and time-of-day charges, not by producing cheaper energy, because diesel power costs more per kWh than most HT tariffs. Whether it works for a site depends on when the peaks occur, how long they last and what the DISCOM permits.
What peak shaving is for
An HT consumer in Telangana or Andhra Pradesh pays for electricity in two parts: an energy charge per kWh and a demand charge based on the recorded maximum demand against the contract maximum demand. Exceeding the contracted figure in any 30-minute block attracts penal rates on the excess, and time-of-day tariffs add a surcharge in evening peak hours. Peak shaving targets those two costs. The on-site source carries part of the load for the short periods when the DISCOM demand would otherwise exceed the limit, so the recorded maximum stays below the threshold. Nothing about this reduces the energy a site consumes; it changes who supplies it and when.
That distinction matters because a diesel generator rarely produces energy more cheaply than the grid. At 0.27 litres per kWh and a representative diesel price, the fuel component alone often exceeds an HT energy charge before maintenance is counted. Peak shaving with diesel therefore makes sense only when the avoided penalty or demand charge per hour of shaving is larger than the generation cost per hour.
Read the demand profile before anything else
A single maximum demand reading from a bill says nothing about duty. The screening needs the 15-minute or 30-minute demand data that the DISCOM meter or a sub-meter records, covering at least one billing month in each season. From that data, establish four things.
- How often the demand crosses the shaving threshold: twice a month or every working day.
- How long each excursion lasts: ten minutes while a chiller and a compressor overlap, or three hours through the evening shift.
- How large the excess is in kW above the threshold, both typical and worst case.
- Which loads create the peak, and whether any of them could simply be rescheduled or interlocked instead.
The product of duration and excess, in kWh per month, is the energy the shaving source must deliver. The worst-case excess is the power it must deliver. Sites are often surprised to find that the peak is a handful of short overlaps that a staggered start on two large motors would remove at no cost, which is the cheapest form of peak shaving there is.
Generator, battery or load management
| Approach | Suits | Main constraint | Approval need |
|---|---|---|---|
| Rescheduling and interlocks | Short overlaps of a few large loads | Process flexibility | None |
| Diesel generator carrying isolated loads | Long peaks, loads that can be segregated onto a generator bus | Fuel cost per kWh, manual or automatic transfer of a load block | None for non-parallel operation |
| Diesel generator in parallel with DISCOM | Smooth shaving of the whole site demand | Synchronising, protection, DISCOM technical conditions | DISCOM approval under CEA regulations |
| Battery energy storage | Daily peaks of 30 minutes to 2 hours | Energy capacity and cycle life | Depends on connection and export arrangement |
The non-parallel generator method, in which a defined block of load is transferred to the generator for the peak window through a changeover, avoids the paralleling approvals but gives a step change rather than a trimmed profile. The parallel method is smoother and more controllable, but it brings reverse-power, loss-of-mains and protection coordination requirements that are covered under utility paralleling. A battery is the quietest option and the only one that can also absorb energy in off-peak hours; the sizing logic is in the generator and BESS screening guide.
The arithmetic of a shaving decision
Saving per month = (Avoided demand or penalty charge) + (ToD surcharge avoided × kWh shifted) minus (kWh generated × generation cost per kWh)
Generation cost per kWh for a diesel set is the fuel consumption in litres per kWh multiplied by the diesel price, plus a maintenance allowance per running hour divided by the kW carried. Worked example (illustrative): a factory exceeds its contract demand by about 150 kW for two hours on 20 working days a month, so it generates 150 × 2 × 20 = 6,000 kWh a month on a 250 kVA set. At 0.28 litres per kWh that is 1,680 litres of diesel. If the penal demand charge avoided is larger than the cost of that fuel plus roughly 40 hours of running maintenance, the scheme pays; if not, raising the contract demand with the DISCOM may be cheaper than running the generator. The comparison must use the site’s actual tariff order, not a generic figure, and the fuel consumption guide explains why litres per kWh rise if the generator runs lightly loaded during the shaving window.
Operating consequences for the generator
A standby set that is pressed into daily peak shaving is no longer a standby set. It accumulates 400 to 600 hours a year instead of 50, which changes the rating that applies, the service interval and the overhaul horizon. Several points follow.
- The set should be applied at its prime rating with the manufacturer’s permitted variable-load profile, not its standby rating, and generator sizing must be revisited on that basis.
- Load during the shaving window must stay above about 30 per cent of rating; shaving a 60 kW excess with a 500 kVA set causes wet stacking and injector fouling.
- Fuel storage, delivery logistics and the day-tank arrangement need to suit daily use rather than occasional outages.
- New or replacement sets up to 800 kW must meet CPCB IV+ norms, and a set that runs daily within city limits attracts more attention on noise and stack emissions than one that runs only during outages.
- Where the peak-shaving need is seasonal, for example only in the summer months when cooling load rises, a rented set for the season may be more economic than owning a second machine.
The broader system view, including how peak shaving fits with solar and storage on one site, is covered under peak-shaving power systems.
Common mistakes
- Deciding on the basis of one maximum demand figure from a bill; the generator then runs far longer or far shorter than the business case assumed.
- Comparing diesel generation against the average tariff instead of the marginal penalty or peak charge; the scheme looks viable and loses money every month.
- Paralleling a generator with the DISCOM without approval or proper loss-of-mains protection; the site risks disconnection and the generator risks damage on an out-of-phase reclosure.
- Running a large standby set at very light load for the shaving window; fuel per kWh rises and the engine suffers long-term damage.
What to send us
Send six to twelve months of electricity bills showing contract demand, recorded maximum demand and any penalties, the 15-minute or 30-minute demand data if your meter or BMS records it, the nameplate photograph and running hours of any existing generator, and a note on which loads you believe create the peak. We will tell you whether rescheduling, a generator, a battery or a higher contract demand is the better answer for your profile. Send your demand data for a peak-shaving assessment.
Questions engineers ask
Is it cheaper to run a diesel generator than to pay the DISCOM at peak hours?
Usually not on energy alone. Diesel generation typically costs more per kWh than an HT energy charge even with a time-of-day surcharge. It becomes worthwhile when it avoids penal demand charges for exceeding contract demand, or when the alternative is a costly increase in contracted capacity.
Can I run my generator in parallel with the grid for peak shaving?
Only with the DISCOM’s written approval and an interconnection that meets CEA technical standards, including synchronising equipment, reverse-power and loss-of-mains protection. Without that, the generator must carry an isolated block of load through a changeover.
How much generator capacity do I need for peak shaving?
Enough to carry the excess above your threshold at the worst-case peak, with the set loaded to at least 30 per cent of its rating during the window. If the excess is small compared with any available generator, consider load management or a battery instead.
Does peak shaving change my generator maintenance?
Yes. A set running daily accumulates hours quickly, so oil, filter and injector intervals arrive several times a year rather than once, and the overhaul horizon shortens. The service plan must be rebuilt around prime duty.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.