In short. A standby power generator is a diesel set that stays idle while the DISCOM supply is healthy and takes over automatically when it fails. It is rated for emergency standby duty: variable load, a limited number of hours a year, no sustained overload margin. Selecting one is less about peak kVA than about the whole sequence of detection, start, transfer, restoration surge, retransfer and cooldown, and about keeping the set ready through months of idleness.
What standby duty actually means
Manufacturers rate the same engine and alternator at several levels depending on use. The emergency standby rating (ESP in the ISO 8528-1 convention) is the highest. It assumes the set runs only while the normal supply is unavailable, that the load varies, that average load over 24 hours is well below the rating, and that annual hours stay within the stated limit. Typical published limits are around 200 hours a year at an average load factor of about 70 percent with no overload capability, but the exact figures must be read from the rating sheet for the specific model.
The consequence is that a 500 kVA standby-rated set is not a 500 kVA machine for every purpose. Run it through a DISCOM shortfall for several hours every day and it operates outside its rating, wearing faster than the maintenance schedule assumes. For that duty the correct comparison is a prime power generator, which carries a lower nameplate kVA on the same engine but is intended for long hours.
The operating sequence, step by step
A standby installation is judged by how it moves through a chain of states, not by how it performs on a test bench. The chain below is what an AMF panel or a controller with AMF logic executes, and every step has a setting behind it.
| Step | What happens | Typical setting or check |
|---|---|---|
| Mains monitoring | Voltage and frequency on all three phases are compared against limits. | Under and over voltage windows, phase loss, phase sequence |
| Failure qualification | The abnormality must persist before a start is ordered, so a momentary dip does not crank the engine. | Start delay of about 1 to 5 seconds |
| Crank and start | Starter engages, with retries if the engine does not fire. | Three crank attempts, rest between attempts, fail-to-start alarm |
| Stabilisation | Voltage and frequency must settle within limits before transfer is permitted. | Warm-up timer, generator available signal |
| Transfer | Load is moved from the mains contactor or breaker to the generator. | Mechanical and electrical interlock proved |
| Load support | Set carries the load, including the restart surge. | Step-load capability, kW and kVA within rating |
| Return qualification | Mains must be healthy for a set time before retransfer. | Return delay of a few minutes, so a flickering supply is ignored |
| Retransfer and cooldown | Load returns to mains; engine runs unloaded to cool the turbocharger and then stops. | Cooldown timer of 2 to 5 minutes |
How the transfer and retransfer are done, with an open-transition break, a delayed transition or a brief closed-transition overlap, is covered under automatic transfer switches, and the control logic that drives the sequence under AMF panels.
Why the restoration surge sets the size
The hardest moment for a standby generator is not normal running; it is the first thirty seconds after transfer. When the generator breaker closes, every motor that was spinning before the outage wants to start again, chillers and compressors reload, and every UPS rectifier recharges its battery while carrying its load. The combination can briefly demand far more than the steady running figure an engineer measured on a normal afternoon.
Peak restoration kVA ≈ running kVA of non-motor load + Σ (starting kVA of motors that restart together) + UPS recharge kVA
Here running kVA is the measured steady demand of lighting, heating and electronic loads, starting kVA is each motor’s locked-rotor demand (roughly six to seven times running kVA for a direct-on-line motor), and UPS recharge kVA is the rectifier input at full battery charge, usually 110 to 125 percent of the UPS rating. A commercial building with 180 kVA of non-motor load, two 30 kW chillers on direct-on-line starters and a 60 kVA UPS would see a restoration peak well above 500 kVA if everything returned at once, although its running load is only about 300 kVA (illustrative). The answer is not a 600 kVA set but a staged restart: delay each chiller by a few seconds and current-limit the UPS charger, and the peak falls to what a 400 kVA set can handle. The generator sizing guide and the restart sequencing page work through this in detail.
Readiness: the part that fails in practice
In our experience across Hyderabad and the surrounding industrial belts, the standby sets that fail to perform are rarely the ones with a worn engine. They are the ones that did not start. A set that sits for weeks between outages is exposed to a short list of silent failures, each cheap to prevent and expensive to discover during a blackout.
- Starting batteries that have lost capacity because the charger failed or the electrolyte dried out in summer heat. A monthly check of battery voltage under crank is the most valuable single test.
- Diesel that has aged, absorbed water during the monsoon or grown microbial sludge in the tank bottom. Fuel older than about a year should be tested or polished.
- Coolant that has dropped below the sensor, or a radiator fouled with dust, so the set starts and then trips on high temperature once load arrives.
- A controller left in manual or off mode after the last service visit, so the AMF logic never issues a start.
- Rodent damage to sensing wires in a panel that nobody has opened for a year.
A weekly no-load run proves the engine turns over and little else. A monthly run under real building load, or a periodic load bank test to at least 50 percent of rating, shows whether the cooling system, fuel supply and transfer switch will carry the site. A structured annual maintenance contract should include these transfer tests, not only filter changes.
Related photographs and videos
Common mistakes
- Buying a standby-rated set for a plant that runs on generator for several hours every day; the engine accumulates hours far beyond its rating basis and overhauls arrive years early.
- Sizing on the running load and ignoring the restoration surge; the set trips on under-frequency or under-voltage every time it takes the building.
- Testing only on no load; the first real outage reveals a clogged radiator or a weak fuel pump that a loaded test would have caught months earlier.
- Setting the mains return delay too short; the set retransfers to a supply that drops again within a minute, cycling the load repeatedly.
What to send us
Send a load list or the last twelve months of electricity bills with the maximum demand, a note of the largest motors and how they are started, the UPS rating if one is installed, and photographs of the incoming panel and the proposed generator location. Tell us how often and how long the DISCOM supply typically fails at the site, since that decides whether standby or prime rating is the right basis. Use the generator sales enquiry and we will come back with a rating recommendation and a transfer scheme.
Questions engineers ask
How many hours a year can a standby generator run?
Most manufacturers limit emergency standby duty to a few hundred hours a year, commonly stated as around 200 hours at an average load of about 70 percent, with no overload allowance. If outages at your site add up to more than that, ask for the prime rating of the same set and size against it instead.
How long does a standby generator take to pick up the load?
With a healthy battery and a warm engine the usual figure is 10 to 15 seconds from mains failure to load transfer, made up of the start delay, cranking, voltage build-up and the transfer time. Loads that cannot tolerate this gap need a UPS in front of them.
Does a new standby set in Telangana need to be CPCB IV+ compliant?
Yes, for new diesel gensets up to 800 kW the CPCB IV+ emission norms apply, and the set must carry the type approval and conformity marking. Larger sets follow the separate norms for their class, and state pollution control board consent is still required for installation.
How often should a standby generator be run?
Run it weekly for 10 to 15 minutes to prove starting, and monthly under real load for at least 30 minutes so the engine reaches working temperature. A loaded test is the only one that exercises the cooling system and the transfer switch.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.