In short. Generator and BESS screening answers three questions before any battery is priced: what job the battery must do, how many kW it must deliver at once, and how many kWh it must hold to finish that job. All three come from a measured load profile and the ratings of the existing generator, not from an assumed benefit of adding storage. A screening result shows whether a detailed study is worth commissioning; it does not size a commercial battery.
Start with the job, not the battery
Most generator and BESS enquiries begin with a battery size in mind and work backwards to a justification. Screening works the other way round: each possible battery job pushes the power rating, energy rating and control scheme in a different direction.
| Battery role | What dominates the sizing | Typical duration | Control need |
|---|---|---|---|
| Load-step and transient support | Power (kW) and response time | Seconds to minutes | Fast inverter response, droop coordination |
| Peak shaving | Energy across the peak window | 30 minutes to 3 hours | Demand forecast, state-of-charge planning |
| Generator-off operation at light load | Energy through the off period, plus recharge power | 4 to 12 hours | Grid-forming inverter, generator start on low SOC |
| Storing solar for evening use | Energy and daily cycle count | 3 to 6 hours | Solar curtailment, generator minimum-load protection |
A site may want two roles at once, for example generator-off nights and transient support by day. Energy reserved for one role is not available to the other, so the screening must say how the state of charge is shared.
Power and energy are separate numbers
Battery power, in kW, is the rate at which the inverter can deliver or absorb. Battery energy, in kWh, is how much can be delivered before the usable state of charge is exhausted. For a roughly constant load, the installed energy follows the relationship below.
Einstalled = (Pload × t) / (DoD × ηrt)
Here Einstalled is the nameplate battery energy in kWh, Pload is the average load in kW, t is the duration in hours, DoD is the usable depth of discharge as a fraction, and ηrt is the round-trip efficiency of battery and inverter together. Worked example (illustrative): the generator is to stay off for 6 hours overnight while a 60 kW average load runs. With a depth of discharge of 0.85 and an efficiency of 0.88, the installed energy is (60 × 6) / (0.85 × 0.88), about 481 kWh. The inverter must also carry the night-time peak, perhaps 110 kW when a compressor starts, so the power rating comes from that peak and not from the average.
What to measure on the existing generator plant
The generator side of the screening is usually the weaker half of the data, because a nameplate kVA is not a load profile. Collect the following first.
- Interval load data at 15 minutes or finer for at least two representative weeks, including a monsoon week and a summer week.
- The generator’s prime or standby rating, its minimum recommended sustained load (commonly around 30 per cent of rating to avoid wet stacking) and its documented load-step acceptance.
- Running hours per year on generator supply and the reason for each run.
- Fuel consumption at the loads actually seen; the generator fuel consumption guide explains why litres per kWh rise sharply below 40 per cent load.
- Where the battery would connect, and whether paralleling with the DISCOM is permitted at all.
Where the generator was sized on connected load rather than measured demand, common on older Telangana industrial sites, screening often shows it running at 20 to 35 per cent load. A battery that lets it run fewer hours at higher load then saves fuel and maintenance before any other benefit is counted. See generator sizing.
Screening arithmetic that holds up
A credible screening compares at least two operating strategies against the same load profile: the existing one, usually one generator running whenever the DISCOM is absent, and the hybrid one, in which the generator runs only to recharge the battery or when load exceeds inverter capacity. For each, tabulate generator hours, load factor, litres of diesel, starts per day and battery cycles per day; a strategy that cycles the battery twice a day roughly halves its life.
Charging is part of the load
Charging power adds to site load. A 250 kVA set carrying 120 kW and charging at 100 kW runs at about 220 kW, a healthy load factor; the same set charging at 200 kW is overloaded. State the charge-rate limit and check that the generator can accept the charging step.
Transitions and fault current
Switching between battery and generator is where hybrid plants fail at commissioning. The inverter must form the grid while the generator is off, or hand over through a short break the loads can tolerate. Inverters also contribute only about 1.1 to 1.5 times rated current into a fault, so protection that relied on alternator contribution may not operate during battery-only periods. See generator and BESS systems.
Controls decide whether the hardware pays
The energy management controller must know the state of charge, the generator’s availability and minimum-load limit, the forecast demand and any reserve requirement, and it must decide when to start and stop the generator and how to share load while both run. Poor dispatch logic erases the benefit of good hardware: a controller that starts the generator at 40 per cent state of charge and runs it until the battery is full produces more starts and a lower average load than a competent operator would. Write the dispatch rules down during screening: the low-SOC start point, the stop point, the behaviour when the DISCOM returns and the fallback on controller failure. Priority logic is covered under load shedding.
What a screening result can and cannot say
A completed screening states a probable battery role, an indicative power and energy range, the resulting generator operating pattern and the data still missing. It can say that a 400 to 500 kWh, 150 kW system would let the generator run four hours a night instead of twelve. It cannot say what that system will cost, what the fuel saving will be to the rupee, whether the DISCOM will permit the connection, or how the specific inverter and alternator will behave together. Those answers need equipment data, a site survey and a transient study. The inputs on the DG plus BESS hybrid screening tool make that boundary explicit, and the peak shaving guide covers the case where demand reduction is the main job.
Common mistakes
- Sizing the battery from the generator kVA instead of the measured load; the inverter ends up far larger than the load needs and the energy store too small for the off period.
- Quoting battery energy in nameplate kWh without depth of discharge and efficiency; delivered runtime is then 20 to 30 per cent shorter than promised.
- Ignoring charging power when the generator returns; the generator trips on overload or the battery never reaches full charge before the next off period.
- Assuming existing protection will work on inverter supply; faults during battery-only operation may not clear.
What to send us
Send the generator nameplate photograph, interval load data covering at least two weeks, annual generator running hours with the reason for each run, a single-line diagram showing where the DISCOM, generator and loads connect, and one sentence on the outcome you want. If solar is installed or planned, include its capacity. We will say which battery role fits and whether a detailed study is justified. Send your application details for a generator and BESS screening.
Questions engineers ask
Can a battery replace my diesel generator altogether?
For most Indian industrial sites, no. A DISCOM outage during a storm or a planned shutdown can last longer than any affordable battery. The generator remains the long-duration source and the battery reduces how often and how hard it runs.
How many kWh do I need to run the generator less at night?
Multiply the average night load in kW by the hours the generator should stay off, then divide by the usable depth of discharge and the round-trip efficiency. A 60 kW load for 6 hours needs roughly 480 kWh installed under typical assumptions.
Does adding a BESS let me buy a smaller generator?
Sometimes. If motor starts or short peaks set the generator size, a battery that covers those transients can allow a smaller set. The generator must still recharge the battery while supplying the site, so the reduction is modest.
Do I need DISCOM approval for a generator and BESS plant?
If the battery or generator will ever operate in parallel with the DISCOM supply, yes, and the interconnection must satisfy CEA regulations and the DISCOM’s conditions. If the system runs only while isolated through the changeover, approvals are simpler, but the isolation must be proven.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.