How long does a refrigerated container stay cold without power?

A refrigerated container is a powered machine, not a passive cool box. The moment the electricity stops, a clock starts: with the doors shut it coasts for a while on stored cold, but that window runs in hours, not days, and it shrinks every time someone lifts the handle “just to check”. If the load has any value, the useful question is not whether the power will ever drop — it is how much time you have when it does, and what catches the problem before your stock does. This guide covers the hold time you can realistically expect, when a standby generator earns its keep, the alarms that buy reaction time, and the continuity plan that ties them together.

Thermal inertia: what closed doors buy you

The same thick insulation that makes a reefer’s interior smaller than a dry container of the same size — deep panels on the walls, ceiling and floor — is what slows the warm-up once the compressor stops. A container packed with already-cold stock carries a large thermal mass and holds far better than a half-empty one. The honest way to picture the reserve is as an order of magnitude — hours rather than minutes, and never days — not a figure you can schedule around. Nobody can hand you a precise number, because it moves with the load, the set point and the weather on the day.

What shortens the hold time

Several things swing that window, and they are worth knowing before an outage rather than during one:

Frozen goods at -18 °C forgive an interruption more readily than a chilled +2 °C load already sitting near its safe upper limit. The one measure that costs nothing is discipline at the door: during a cut, keep it shut. Every opening dumps the cold reserve you are trying to stretch.

When a standby generator earns its keep

For anything off-grid — a construction site, an event field, a remote depot — or for stock you simply cannot afford to lose, a standby generator is the established fallback. A full-size reefer draws 5 to 12 kW and needs three-phase 380/400 V through its 32 A CEE plug, so the generator has to match that with comfortable headroom over the compressor’s start-up surge, which sits well above the running load. It also has to run continuously: a long stop drops you straight back into the inertia problem. Have the sizing confirmed by the generator supplier and an electrician, and put diesel in the operating budget rather than treating it as an afterthought.

Alarms and monitoring: buying reaction time

Thermal inertia is only useful if someone acts inside the window, and that is what monitoring is for. Most modern refrigeration units — Carrier, Thermo King, Daikin, Star Cool — can raise a high-temperature or power-loss alarm, and remote monitoring pushes it to a phone rather than a display no one is watching at 2 am. The value is reaction time, not a logged number: an alert that reaches a named person who can restart a generator, close a door or move the load is worth more than a perfect record nobody reads. Decide in advance who receives the alarm and exactly what they do next.

Building a continuity plan

A written plan turns a scramble into a routine. At a minimum it should name:

• the hold time you assume — conservative, doors shut, on paper;
• the alarm chain — who is alerted, on what device, and their backup;
• the fallback power — a generator on site, or one that can reach you, with fuel arranged;
• the escalation — the point at which you move stock to another unit or refuse further door openings;
• the after-action check — a temperature review, plus a PTI (Pre-Trip Inspection) test on a used machine if the unit itself is in doubt.

None of this is regulatory advice: where HACCP or food-safety rules apply they vary by country and premises, so confirm your obligations with the competent authority. Note too that ATP concerns refrigerated transport, not a container standing still — a static store is a separate matter.

Matching the unit to the risk

Resilience starts with the right machine. A reefer works anywhere across -25 °C to +25 °C and burns 70 to 170 kWh a day in the UK depending on size and set point, so power belongs in the sums from day one — the rest of our reefer guides and current prices put that in context. On the hire market, indicative long-term rates start around £12 a day ex VAT, with VAT at 20%, though a 90-day minimum term is common. To buy, used units are observed at £4,500 to £7,500 ex VAT and new one-trip units at £18,995 to £22,500 ex VAT — see what is for sale. Set out your temperature, volume and power situation once in a quote request and we pass it to specialist suppliers, who contact you directly if they can meet your need.