A provision shop in Yola sells drinks all day from two chest freezers and loses a day of stock when the grid goes off for six hours. A household with one 250 litre chest freezer in Jos has a much easier problem, even though both are described as refrigeration. Sizing solar for either one starts with the compressor, and the compressor is the part everybody forgets: it does not draw a steady 200 watts. It draws nothing, then draws several times that for a second or two, over and over, all day.

What solar refrigeration actually has to do

A refrigerator is a heat pump, not a cold store. A fluid evaporates inside the cabinet at low pressure, absorbing heat from the food and the air; a compressor raises its pressure so the heat can be given up to the room; and a thermostat starts the compressor when the cabinet gets too warm and stops it when it is cold enough. Every watt the cabinet removes is delivered to the room as heat, plus the heat the motor itself makes. So a 200 W compressor in a warm kitchen may put 250 W of heat into the room, and the solar installation has to supply all of it.

That is the first fact of solar refrigeration sizing. A cold cabinet is a continuous, day-and-night load, and it does not stop when the sun goes down. It is the worst case for a battery bank and the best case for a good compressor, which is why the difference between a working and a disappointing system is usually the machine rather than the panels.

Compressor start surge

Like any induction motor, a compressor draws a locked rotor current at start, several times its running current, before it reaches speed. For a small domestic chest freezer that might be 200 W running and well over 600 W momentarily. For a commercial unit it can be several kilowatts. The surge lasts a second or two, so it does not drain the battery, but it will trip an inverter that has no headroom, and a compressor that trips repeatedly is a compressor whose life is being shortened every time.

The vapour compression cycle also has a thermal matching problem. A compressor designed to run in a 32 degree kitchen struggles in a 40 degree one, and the only response it has is to run longer. In the harmattan and in the dry months in the north, ambient temperature is the single biggest factor nobody accounts for when quoting for cold storage.

Freezer, fridge or cold room: they are not one thing

Three categories get quoted as if they were interchangeable, and the difference between the cheapest and the most expensive is often a factor of ten in running cost.

Type	Typical use	What the design demands
Domestic chest freezer	Home or small shop, one to two units	Modest steady load, tolerant of a few hours of outage
Display fridge or upright freezer	Retail, drinks and dairy, glass front	Glass doors mean heat gain whenever the shop is open
Deep freeze or cold room	Restaurant, butcher, pharmaceuticals	Insulated panels, forced air, tight temperature band

A cold room is a different proposition again, and the difference is not the compressor but the envelope. A chest freezer has a thick insulated lid and a small internal volume, so heat leaks in slowly. A cold room has doors that people walk through all day, and it may hold fish, meat or vaccine at a temperature where a short excursion ruins the stock. Vapour compression refrigeration handles the cold room; a chest freezer and a cold room are not the same purchase with a different size label.

Compressor size is the decision that matters most

Within any category, the compressor is chosen to match the volume and the ambient temperature, and the temptation is always to fit a larger one. Resist it. An over-large compressor cycles on and off in short bursts, which wastes energy, raises the starting current and shortens its life. A slightly small compressor runs longer and cooler, which in a hot climate is often the better answer, provided it is not so small that the cabinet never reaches temperature. So the compressor decision, not the panel decision, is where good solar refrigeration sizing is won.

For a household chest freezer in a hot compound, the practical question is whether you can add a second, smaller freezer rather than replacing the first. Two 100 litre units in parallel generally cost less than one 250 litre unit of the same total volume, because heat gain scales with surface area, and less exposed surface per litre holds temperature more easily. If one then fails, you still have somewhere to keep food, which in Nigeria is worth more than the efficiency gain.

Freezer and fridge are not the same machine

A fridge is at positive temperature, a freezer at negative, and the thermodynamics are not symmetric. Making something cold is easy; making it cold enough to freeze water is a different demand on the same refrigerant. Fridges also open more often, so they gain more heat and cycle more. If you are quoting for a business, ask whether the units are rated for the temperature you need, and not merely the volume. A unit sold as a fridge is not a freezer, and the label on the box is the only place that will tell you.

Autonomy: how long can the cold hold?

The sizing question is the same one as any other load, with one extra twist: what happens when the answer is no. Add the compressor running watts across all units, multiply by the hours, and divide by the usable share of the bank. A shop with two 200 W chest freezers drawing about 400 W together will use roughly 0.4 kWh an hour. From a 5 kWh bank with 3.6 kWh usable, that is nine hours if the bank is full when the power goes, and rather less in practice.

That figure needs two qualifications before you rely on it. The first is that a full bank is rarely full, and a bank that has been running the house all evening is nearly empty by nine. The second is that the compressor keeps cycling after a threshold, so the first food to spoil is not random: it is whatever is in the unit with the worst seal, and in a shop the unit nearest the door. This is why solar water heater sizing feels more forgiving than solar refrigeration does: nobody minds a cold shower, everybody minds spoiled stock.

Which raises the honest point. If the stock is genuinely perishable, a refrigeration load is a poor candidate for a small battery bank, because covering a long outage with stored energy is expensive. Three options are worth comparing honestly. Accept the outage and hold only what can be sold quickly and replaced cheaply, which suits drinks far better than it suits meat. Add a thermal mass such as a frozen water container or a phase change pack, which buys hours at a cost of a few naira and a freezer drawer. Or accept a generator as the backstop for the cold room specifically, which the inverter against generator comparison covers. All three are legitimate; what is not legitimate is sizing a small bank and implying a long outage is covered.

Frequently asked questions

How much solar do I need for one chest freezer?

A small chest freezer with a 150 to 200 W compressor will typically draw around half a kilowatt hour a day in a hot climate, more in the harmattan. Divide that by your peak sun hours for the array, and add ten to fifteen per cent for dust and heat. The solar load list is where this figure belongs, alongside everything else in the house.

Can a solar system run a cold room?

It can, but it is a much bigger design and it needs a proper calculation, not a rule of thumb. A cold room for a restaurant or butcher is usually a three-phase or large single-phase commercial load with a compressor that draws a great deal more than a domestic unit. Plan that with an engineer who has sized cold rooms before, and expect the compressor decision to matter more than the panel decision.

Why does my freezer warm up during an outage even before the battery is flat?

Usually because the battery is not as full as the inverter display suggests, or because the fridge is sharing the bank with the house and the compressor is being cycled rather than run. A second cause is ambient heat: a freezer in a hot room loses temperature faster, so the compressor runs more often and reaches the reserve sooner. Relocating the unit to a shaded, ventilated space genuinely helps.

Does the inverter need a special output for the compressor?

Not a special output, but genuine surge headroom and a continuous rating above the running load. A 5 kVA unit has ample room for a 200 W freezer; a 5 kVA unit has no room at all for a commercial cold room compressor starting in the same inverter. Size the inverter from the largest motor you intend to run, not from the sum of everything else.

Key Takeaways

• A fridge is a heat pump, so the room ends up with all the heat the cabinet removed.
• Compressor start current is several times running current, and it decides the inverter.
• Ambient temperature drives running hours, so hot compounds push the running cost up.
• A chest freezer and a cold room are different purchases, not one item in two capacities.
• Two smaller freezers usually beat one large one on cost and on resilience.
• Covering a long outage with stored energy is expensive, and pretending otherwise costs you stock.

Refrigeration is the load where a bank that is too small shows up in money you cannot get back, so read battery bank sizing before the freezers are delivered.

Sources: refrigeration and how heat is moved out of a cabinet; the vapour compression refrigeration cycle; vapour compression systems as used in cold stores; compressors and their starting behaviour; inrush current at start; cold stores and their construction; the evaporator inside the cabinet.