In short. Hospital backup power is designed around clinical consequence, not building area. Loads are classified by how long each can be without supply, operating theatres and ICUs get UPS or medical IT supplies in front of the generator, the generator and AMF panel restore the essential bus within seconds, and fuel autonomy, bypass-isolation transfer switches and tested restart sequences keep that protection intact through long outages and maintenance.
Start with clinical consequence, not kVA
A hospital does not fail like an office. When the DISCOM supply drops in a multi-speciality hospital in Hyderabad, a ventilator, an anaesthesia workstation, a cath lab and a blood bank refrigerator all lose power in the same instant, and each has a different tolerance. The design question is therefore which clinical and support functions must continue, for how long, and in what order they must come back. Only after that list exists does generator capacity mean anything. The hospital’s own engineering and clinical teams own that classification; our role is to turn it into a generator, transfer and distribution arrangement that can be proven to meet it.
Indian hospital projects generally draw on the National Building Code Part 8 for essential supply, IEC 60364-7-710 for medical locations, and the consultant’s own specification. The requirements of those documents, and any licensing conditions for the facility, must be confirmed by the project team. What follows is the engineering logic that sits underneath them.
Three supply tiers inside one hospital
| Tier | Typical loads | Tolerable break | Supply arrangement |
|---|---|---|---|
| No-break | Operating theatres, ICU and NICU equipment, cath lab, surgical lights, patient monitoring, servers | None, or well under a second | UPS or medical IT system with insulation monitoring, fed from the essential bus |
| Short-break | Theatre and ICU air handling, medical gas plant, lifts, fire pumps, emergency lighting, blood bank and pharmacy cold rooms, laboratory analysers | Roughly 10 to 15 seconds | Generator through AMF or ATS onto the essential bus |
| Long-break or shed | General wards’ comfort cooling, kitchens, laundry, parking, non-clinical offices | Minutes to hours, or restored manually | Load shed on outage, restored when capacity allows |
The point of the tiers is that the generator carries the second tier plus the UPS input of the first. The third tier decides how large the set would have to be if everything stayed on, and in most hospitals that figure is two to three times the essential load. Deciding what to shed is the single largest cost lever in hospital standby design. See critical-load classification.
What the generator must actually carry
Hospital essential loads are dominated by motors and by UPS rectifiers. Theatre air handling units, chillers or VRF condensers serving critical areas, medical air and vacuum compressors, lifts and fire pumps all draw starting current, while the UPS systems for theatres and ICUs present a non-linear load and a recharge block after every outage. A running-load total understates the duty. Reddy Generators sizes hospital sets from the restart sequence: which motors the AMF panel and BMS allow to start in each step, what voltage dip the theatre lighting and monitors will accept, and how much recharge the UPS draws. The method is in generator sizing and motor starting.
Lightly loaded sets are the other hospital problem. A generator sized for the whole building but carrying only essential loads during a two-hour outage may run at 20 per cent load, which over time causes wet stacking and injector fouling in CPCB IV+ engines with aftertreatment. Two smaller sets in N+1, or one set matched to the essential bus with a shedding scheme, is usually the better architecture than one large machine.
Fuel autonomy and long outages
Running out of diesel during a prolonged outage is a design failure, not a generator failure. The autonomy requirement should be stated by the hospital in hours at the expected essential load, and the tankage, refuelling arrangement and fuel quality regime should be built to meet it.
Autonomy (h) = usable fuel (L) / consumption at expected load (L/h)
Diesel generator calculator
Fuel use, runtime, tank size and cost
How this is calculated
Real power is kVA × power factor. Fuel use is estimated as L/h ≈ 0.04 × rated kW + 0.22 × load kW, which follows the usual shape of modern diesel gensets: about 0.26 L per kWh near full load, rising towards 0.38 L per kWh at a quarter load because the engine's fixed losses are spread over less output.
If you enter the full-load figure from your engine datasheet, the curve is scaled to match it. Runtime is usable fuel ÷ L/h. Tank size is L/h × hours, plus the reserve margin, divided by the usable fraction.
These are planning estimates, typically within 10 to 15 percent of a healthy engine. Actual use depends on the engine model, its condition, altitude, ambient temperature and how the load varies. For a figure you can rely on, use the manufacturer's fuel curve or a metered test.
Estimates only, worked out in your browser. For a checked selection, send us your load list and site details or call 098852 01608.
Usable fuel is the tank volume less the unusable heel and any reserve, and consumption is read from the manufacturer’s curve at the real load rather than at full rating. A worked example (illustrative): a hospital with a 320 kW essential load on a 500 kVA set draws roughly 85 to 95 litres per hour at that load. A 2,000 litre usable day tank gives about 21 to 23 hours before refuelling. If the hospital’s brief requires 48 hours, bulk storage of around 4,500 litres or a guaranteed refuelling contract is needed, and the stored fuel must be kept clean, because standby diesel stored for months degrades and grows microbial sludge. See fuel tank sizing and fuel cleaning and polishing.
Transfer, maintenance and testing without clinical risk
Transfer switches with bypass
An ATS that cannot be maintained without taking the essential bus down becomes the single point of failure in the whole chain. Hospitals should specify bypass-isolation transfer switches for the essential bus so the automatic switch can be serviced or replaced while load is carried on the bypass. Closed-transition transfer, where the DISCOM permits it, avoids the second break when supply returns and is valued in theatres. See bypass-isolation ATS.
Testing under load
A weekly no-load run proves the engine starts. It does not prove the AMF logic, the voltage seen by the theatre UPS, or the restart order of the air handling units. Monthly transfer tests under real load, at a time agreed with the clinical departments, and an annual load bank test to full rating are the minimum. Every test should record transfer time, the lowest voltage and frequency, and the loads that failed to restart.
Maintenance without a dark hospital
Overhauls and switchgear work need a written plan that states which circuits are affected, what temporary source covers them, and who restores the approved configuration. A rental set with a temporary changeover is the usual bridge; see rental power for planned outages.
Common mistakes
- Sizing the generator for the whole building and skipping load classification. The set costs far more than needed and then runs lightly loaded for every real outage.
- Powering the clinical equipment but leaving theatre air handling or medical gas compressors off the essential bus. The theatre has power but cannot be used.
- Accepting weekly no-load runs as proof of readiness. The first transfer under real load exposes an AMF setting or a restart conflict during an actual emergency.
- Installing a single ATS without bypass on the essential bus. Routine maintenance on that switch requires a hospital-wide shutdown, so it is never maintained.
- Specifying fuel autonomy by tank size instead of hours at expected load. The tank looks large on paper and runs dry on the second night of a long outage.
What to send us
Send the essential load schedule with the tier assigned to each load, the single-line diagram showing the essential and non-essential buses, UPS ratings for theatres and ICUs, and the consultant’s autonomy requirement in hours. For an existing hospital, add photographs of the generator nameplate, the AMF or ATS panel and the fuel storage, and the hours run and service history of the set. Send your application details and we will review the arrangement against the clinical classification before any equipment is proposed.
Questions engineers ask
How quickly must a hospital generator take over?
Short-break clinical and life-safety loads are generally expected back within about 10 to 15 seconds of mains failure, which a well-maintained set with an AMF panel achieves. No-break loads such as theatre and ICU equipment cannot wait that long and sit on UPS or a medical IT system in front of the generator.
Does the hospital need two generators?
For a facility with theatres and ICUs, N+1 is the usual recommendation so that one set can be serviced or fail while the essential bus remains protected. A small clinic or diagnostic centre with no life-support loads may be adequately served by a single set and a rental agreement for maintenance periods.
Should air conditioning be on the generator?
Theatre, ICU, pharmacy and laboratory air handling should be, because they maintain pressure, filtration and temperature that the clinical function depends on. General ward and public area comfort cooling is usually shed and restored only if capacity allows.
How much fuel should a hospital keep on site?
Enough for the autonomy stated in the hospital’s brief at the expected essential load, commonly 24 hours or more, calculated from the consumption curve rather than tank size. Stored fuel should be tested and polished on a schedule so it is usable when needed.
Can we test the transfer without disturbing patients?
Yes, if the test is scheduled with clinical departments, the UPS systems are confirmed healthy beforehand, and the transfer is initiated at the AMF panel. The short break is then seen only by loads designed to accept it.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.