Power cuts and commercial refrigeration: protecting stock and compressors

How long stock stays cold

Widely used food-safety guidance for household appliances is that a refrigerator with its door kept shut keeps food cold for about 4 hours, and a full freezer holds its temperature for about 48 hours (about 24 hours if half full) if the door is not opened. Commercial cabinets vary — thinner walls, glass doors and hot rooms shorten those times, while thick insulation and a full load lengthen them — so treat the figures as a starting point and measure your own units.

Two things matter most: keeping doors closed, and how full and how cold the unit was when the power went. A full chest freezer, already at −18 °C or colder, rides out an outage far better than a half-empty glass-door merchandiser.

Before the outage: prepare

• Know which units hold the most valuable or sensitive stock and plan to protect those first.
• Keep a probe thermometer that does not depend on mains power, and check its accuracy regularly.
• Keep frozen gel packs or ice in the freezer to help chill insulated boxes if you must move stock.
• Fit temperature alarms or data loggers with battery backup to high-value units, so you know how warm they got and for how long.
• If cuts are scheduled, run freezers full and at their normal set point in advance; do not crank thermostats far colder, which can freeze chilled products and stress the system.
• Write the plan down: who decides when to move or discard stock, and where it goes.

During the outage

• 1. Keep doors and lids shut. Put a sign on each unit.
• 2. Note the time the power went off.
• 3. For long outages, switch large units off at the socket or MCB so they do not all restart at once on a weak returning supply.
• 4. If the outage looks likely to exceed what your units can ride out, move the most sensitive stock to a working unit elsewhere or to insulated boxes with ice packs.
• 5. Do not open units to check unless you need to; use external displays or loggers if they have battery power.

When power returns

• 1. Wait for the supply to stabilise. Flickering lights and repeated short cuts damage compressors.
• 2. Switch units back on one at a time, a minute or two apart, so starting currents do not overload the supply or generator.
• 3. Let each unit's restart delay protect its compressor — do not toggle power to force a quick start.
• 4. Check that compressors and fans start normally and that controllers have kept their settings.
• 5. Record the outage duration and the temperatures of food and cabinets.

Deciding whether food is safe

Measure the food, not just the air. Check the temperature of representative items with a probe thermometer. Your food-safety plan should define what happens to chilled food that has been above its limit for a given time; widely used guidance discards perishable chilled food such as meat, poultry, fish, eggs, milk and cooked dishes that has been above about 5 °C for more than two hours. Frozen food that still contains ice crystals or is at refrigerator temperature can generally be refrozen, though quality may suffer. When in doubt, discard — and never judge by smell or taste. Pharmaceutical and vaccine stock must follow its own programme's excursion procedure.

Backup power

Generators and inverter systems can keep refrigeration running, but compressors draw several times their running current at start-up. Ask an electrician to size backup power for the starting load of every unit it must support, not just their rated wattage, and to provide a clean, stable output. Units started on an undersized inverter can hum, fail to start and overheat. If backup capacity is limited, prioritise freezers and the units with the most sensitive stock.

Protect against bad power on return

Power that returns with very low or very high voltage, or in rapid on-off bursts, is a common cause of compressor and controller failures. Voltage protection devices with a built-in delay, or correctly sized stabilisers, disconnect the unit until the supply is within range and then restart it after a safe interval. Ask your technician or electrician which suits your units and supply.

Safety notes

Never run a generator indoors or near windows and air intakes. Switch off at the MCB rather than pulling plugs from wet or damaged sockets. If a unit makes unusual noises, smells of burning or trips repeatedly after power returns, leave it off and call a technician; for units charged with hydrocarbon refrigerants such as R290, use one trained for flammable refrigerants.