In short. Cold-storage continuity is a race between electrical restoration and the temperature rise the stored product can tolerate. The generator must start the compressors in sequence, carry condenser and evaporator fans, and run for the longest credible outage, which in a Telangana monsoon can be many hours. Size on compressor starting rather than running load, sequence the restart through the plant controller, link fuel autonomy to product risk, and alarm temperature and generator faults to a person.
Thermal autonomy sets the clock
Every cold store has a thermal autonomy, the time it can hold temperature after refrigeration stops. A full, well-insulated frozen store at minus 18 degrees Celsius rises slowly, often a degree or two over several hours, because the product mass acts as a thermal flywheel. A partly loaded chilled room for fruit, vegetables or seed rises much faster. A blast freezer with product mid-process has almost no autonomy, and pharmaceutical cold chains at 2 to 8 degrees Celsius have excursion limits measured in minutes rather than hours.
The acceptable outage duration therefore depends on the product, the temperature setpoint, the door discipline and the operator’s own quality rules, and it must come from the facility, not from the generator supplier. Once that number is known, everything else follows: how fast the set must start, how fast the compressors must be back, and how long the fuel must last.
Compressor starting dominates the sizing
Refrigeration load in running terms is modest. The difficulty is restart. Reciprocating compressors on direct-on-line or star-delta starters draw several times their full-load current for a few seconds; screw compressors start unloaded but still impose a step; and after a long outage every compressor and fan wants to start together while the rooms are warm and suction pressure is high. On a generator that is the event that trips the breaker.
S_start = P_motor × k_start / (η × PF)
In this expression S_start is the starting apparent power in kVA, P_motor is the compressor motor rating in kW, k_start is the starting current multiple for the starter type, η is motor efficiency and PF is the motor’s full-load power factor, since starting kVA scales with the full-load kVA. For a 75 kW reciprocating compressor started direct-on-line with k_start of 6, η of 0.93 and PF of 0.85, S_start is roughly 570 kVA for the first second or so, which a 300 kVA set cannot hold without a deep voltage dip (illustrative). The same motor on a soft starter limited to 3.5 times full-load current brings this to roughly 330 kVA; on an inverter drive the step becomes little more than the running load. This is why cold-store generator projects usually include starter upgrades or a strict one-at-a-time restart, and why the method on motor starting on generators matters more here than the kVA = kW / PF arithmetic.
A sizing illustration
A fruit and vegetable cold store near Hyderabad has three 55 kW screw compressors, 40 kW of condenser fans, 30 kW of evaporator fans and pumps, 25 kW of lighting, office and dock loads, and electric defrost heaters of 30 kW per room that are interlocked so that only one room defrosts at a time (illustrative). The running load with all three compressors loaded is about 260 kW, or 325 kVA at 0.8 PF. If the plant controller starts the compressors one at a time, unloaded, with a 30 second interval, a 400 to 450 kVA set carries the restart comfortably; if all three try to start together, the same site needs a set roughly twice that size. The sequencing feature is cheaper than the extra engine.
Sequence, prioritise and protect the right rooms
| Zone or load | Why it matters on backup | Restoration priority |
|---|---|---|
| Plant controller, PLC, temperature monitoring | Nothing restarts safely without it | UPS, no break |
| Frozen and blast rooms | Highest product value per degree of excursion | First compressor group |
| Chilled rooms, ripening rooms | Faster temperature rise, quality loss | Second group |
| Condenser fans and pumps | Compressors cannot run without heat rejection | With the first compressor |
| Evaporator fans | Temperature uniformity inside the room | With each room’s compressor |
| Electric defrost heaters | Large resistive steps, deferrable | Blocked until plant is stable |
| Dock doors, lighting, office | Operational convenience | Last |
| Ammonia detection and emergency ventilation | Safety | First, independent of product |
Where the plant uses ammonia, gas detection and emergency ventilation are safety loads and belong on the generator ahead of any product consideration. Prioritising rooms is also a legitimate way to reduce generator size: holding the frozen rooms and accepting a longer excursion in the chilled rooms can halve the compressor load the generator must restart. The priority logic is covered on load shedding automation and priority logic.
Fuel autonomy and sustained operation
A cold store’s generator may run for twelve, twenty-four or more hours during a monsoon outage or a feeder fault, and the fuel plan must match the product risk, not the day tank. A set loaded to 75 percent consumes in the region of 0.2 to 0.25 litres per kWh, so a 400 kVA set carrying 260 kW uses roughly 50 to 65 litres per hour (illustrative), or well over a thousand litres in a day. Decide how many hours of fuel are kept on site, how a refill is arranged at night or on a holiday, and who checks the level. The calculation is on fuel consumption and the storage question on fuel tank sizing. Coolant, oil and radiator cleanliness matter as much as fuel in sustained operation at high ambient, and a set that has only ever run a weekly test should be given a load run before the season.
Alarms: temperature, generator and a human
A cold store that loses power at night with nobody on site discovers the failure in the morning. The monitoring system should alarm room temperature, generator fail-to-start, low fuel and loss of mains, and the alarms should reach a named person by SMS or an app, not a hooter in an empty plant room. Remote monitoring of the generator controller is now inexpensive and is described on communications and remote monitoring. The person who receives the alarm must also know who to call for the generator, which is where a service agreement with a defined response earns its cost; see annual maintenance contracts.
Common mistakes
- Sizing the generator on compressor running kW. The first simultaneous restart trips the set and the rooms warm while the operator resets breakers.
- Leaving defrost heaters free to switch on during backup. A 30 kW resistive step lands on a set already carrying a compressor start.
- Keeping only a day tank of fuel. A long outage ends with the set stopping at 3 am and nobody aware until the morning.
- Omitting condenser fans or pumps from the standby bus. Compressors trip on high discharge pressure within minutes of starting.
- Relying on a weekly no-load test for readiness. The engine has never been asked to pick up a loaded compressor until the day it must.
What to send us
Send the refrigeration plant schedule with compressor ratings and starter types, condenser and evaporator fan loads, defrost method and interlocks, the room temperatures and the maximum excursion the facility accepts, the plant controller’s ability to sequence restarts, and the longest outage you have experienced. Photographs of the compressor nameplates, the main switchboard and any existing generator and AMF panel help. Send your cold-storage requirement.
Questions engineers ask
How do I calculate the generator size for a cold store?
Start from the compressor starting kVA for the starter type, decide whether the controller can sequence starts, add the running load of fans, pumps and the rooms already running, and allow for any defrost that cannot be blocked. Sequenced starting typically allows a set around 1.5 times the running kVA; simultaneous starting can demand two to three times (illustrative).
How long can a cold room hold temperature without power?
A full frozen store may hold for several hours with doors closed; a partly loaded chilled room may exceed limits within an hour; a blast freezer or pharmaceutical store may have minutes. The facility’s own records and product rules decide, and the generator start and restart time must be shorter than that.
Should the generator start automatically?
Yes. Cold stores lose power at night and on holidays, and an AMF panel starts the set and transfers within about 10 to 15 seconds without anyone present. Manual start depends on someone being there, awake and able to operate the changeover.
Can a rental generator be used for a cold store in an emergency?
It can, provided the set is sized for compressor starting, the cable run and connection point are suitable, and someone takes responsibility for fuel and alarms. Mobilising on a rough kVA guess usually ends with a set that cannot start the compressors.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.