A refrigerated container is happy to sit in a field, a festival car park or a half-built site — right up until you look for somewhere to plug it in. Unlike a dry store, a reefer only earns its keep while the cooling unit is running, and that unit needs a continuous, fairly heavy electricity supply. On a rural events site or an early-stage construction plot there is usually no three-phase mains within reach, and that single fact — not the hire cost — is the number one blocker for anyone trying to keep stock cold off-grid. A generator sized to the job is the standard answer.
Why mains is the real constraint
A reefer’s cooling unit connects to a three-phase 380/400 V supply through a CEE 32 A five-pin socket; only some of the smallest units run on 230 V single-phase. That is a commercial-grade connection most rural venues, remote farms and greenfield sites simply do not have. Bringing a permanent three-phase supply to a temporary location is rarely worth it for a weekend, so the practical route is to generate power on site — either a standalone diesel genset or a clip-on unit that bolts to the front of the container.
How much power a reefer actually draws
Sizing starts with the load. Running consumption depends on the container size and the set point — pulling down to −18 °C costs more than holding +2 °C. As a rough UK guide a reefer uses somewhere between 70 and 170 kWh a day. The per-size figures:
| Container & set point | Running draw | Energy per day |
|---|---|---|
| 10 ft, +2 °C | 2–3 kWh/h | 50–70 kWh |
| 20 ft, +2 °C | 3.5–4 kWh/h | 85–95 kWh |
| 20 ft, −18 °C | 4–4.5 kWh/h | 95–110 kWh |
| 40 ft HC, +2 °C | 7–8 kWh/h | 170–190 kWh |
| 40 ft HC, −18 °C | 8–9 kWh/h | 190–215 kWh |
Those are averages over a running day. The figure that decides your generator is the peak power the unit pulls, which runs from about 5 to 12 kW across the range. Watch the units: kW is the instant power draw, kWh is the energy used over time — they are not interchangeable.
Sizing the generator
Two things push you towards a bigger set than the running numbers suggest. First, the compressor draws several times its running current for a moment each time it starts, so the genset needs real headroom above that 5–12 kW peak to swallow the surge without stalling or browning out. Second, a set run flat-out all weekend wears faster and burns more per unit than one loafing at half load. Sizing comfortably above the peak, not at it, is the safe rule — an undersized generator that trips on every compressor start will quietly let the load warm up.
You will meet two formats. A clip-on genset bolts onto the container’s front and powers only that unit; it is neat and self-contained, which suits a single reefer parked out of the way. A separate towable diesel generator is more flexible — it can also feed lighting, catering or a second reefer — but needs cabling and somewhere to stand. Ask your supplier which the hire can include.
Diesel versus mains
Where a three-phase supply already exists — an established depot, a site with commissioned power — mains electricity is cheaper per unit, quieter and needs no refuelling. Off-grid, none of that is on offer, so the honest comparison is diesel-or-nothing. The cost that matters then is fuel. A genset has to deliver that same 70–170 kWh over a running day, and under a 5–12 kW load a diesel set burns on the order of a few litres an hour. Over a multi-day event the fuel bill, plus the logistics of a bunded tank and top-ups, usually outweighs the generator hire itself. Plan your fuel autonomy around the run time you need between refuels, not just the price per day.
Planning an off-grid booking
Before you commit, pin down three things: the container size, your set point, and how many hours it must run unattended between refuels — those decide both the reefer and the generator you need. UK hire starts at roughly £12 a day ex VAT on longer bookings, though a minimum term of around 90 days is common, so a short festival slot is worth asking about specifically. You can see typical hire and purchase bands on our prices page, or work through the rest of our reefer guides for temperature, classes and delivery. When you are ready, tell us your dates and location and we pass the request to specialist suppliers, who get back to you directly if they can meet your needs.
Frequently asked questions
What size generator do I need for a 20 ft reefer?
A 20 ft reefer holds temperature on roughly 3.5 to 4.5 kWh an hour, but its peak power draw sits within the 5–12 kW band that covers the whole range. Because the compressor surges well above that each time it starts, size the generator comfortably above the peak rather than at it — an undersized set trips on start-up and lets the load warm. Ask your supplier to match the genset to the exact unit.
Can I hire a reefer with a generator included?
Often, yes. Many suppliers offer a clip-on genset that bolts to the container front, or a separate towable diesel set that can also run lighting and catering. It is not universal, so name it when you enquire: give your dates, location and set point, and ask whether the hire can bundle a generator and fuel. We pass your request to specialist suppliers who reply directly if they can meet your needs.
How much diesel does an off-grid reefer use?
Fuel tracks the energy the unit needs, which is roughly 70 to 170 kWh a day depending on size and set point. Under its 5–12 kW running load a diesel genset burns on the order of a few litres an hour, so plan fuel and tank capacity around the run time you need between top-ups. Over a multi-day booking, fuel usually costs more than the generator hire itself.
Is mains power cheaper than a generator?
Per unit, yes — mains electricity is cheaper, quieter and needs no refuelling. The catch off-grid is that a reefer needs a three-phase 380/400 V supply through a 32 A CEE socket, which rural events sites, remote farms and new construction plots rarely have. Bringing that in for a short booking rarely pays, so a generator is usually the only practical option.