A fridge is usually the largest continuous load in a small off-grid cabin, and it is the load that most often gets undersized. People buy panels for the fridge's rated wattage, forget that a fridge cycles on and off, ignore the battery needed for the night, and then wonder why the food spoils on a cloudy day. Sizing a solar setup for a cabin fridge is straightforward once you think in daily energy, measured in watt-hours, rather than in the wattage printed on the back of the appliance.

The first step is to find out how much energy your fridge actually uses in a day. A small, efficient 12-volt compressor fridge in a cabin typically uses around 300 to 700 watt-hours per day, depending on its size, the ambient temperature, and how often the door opens. A standard household fridge can use 1,000 to 2,000 watt-hours per day. The most reliable figure comes from the fridge's energy label or from a plug-in power meter used over a full day, since the rated running watts alone tell you nothing about how often the compressor cycles.

Once you know the daily energy, divide it by your location's peak sun hours, which is the number of hours of full-strength sunlight your panels effectively receive per day. Most temperate locations get between three and five peak sun hours, with summer higher and winter lower. Always size for the worst season you plan to use the cabin, not the best, because a system sized for July will fall short in January.

As an example, a small compressor fridge using 400 watt-hours per day, divided by four peak sun hours, needs 100 watts of panels before losses. Add a margin of 25 to 30 percent for inverter losses, wiring losses, panel heat, and dust, and you arrive at roughly 130 to 150 watts. A single 150-watt panel would cover that fridge in good sun, while a standard household fridge using 1,500 watt-hours per day would need closer to 500 watts, or two to three standard panels. If your cabin gets only three peak sun hours in winter, increase those figures by about a third.

Panels only produce power while the sun shines, but a fridge needs power all night. The battery is what bridges that gap, and it is the part most often undersized. For a fridge using 400 watt-hours per day, you need at least that much usable battery capacity for one night, and ideally two days of reserve for cloudy weather, which means 800 watt-hours of usable storage.

Usable capacity is not the same as the number printed on the battery. A lithium iron phosphate battery can safely use about 80 percent of its rated capacity, while a lead-acid battery should only be discharged to about 50 percent to avoid shortening its life. That means 800 watt-hours of usable storage requires roughly a 1,000 watt-hour lithium battery, or a 1,600 watt-hour lead-acid bank. A 12-volt direct-current fridge has an advantage here, because it runs straight from the battery without an inverter, saving the 10 to 15 percent that an inverter would waste.

If you use a standard household fridge, you need an inverter, and it must handle the compressor's startup surge, which is typically two to three times the running watts. A fridge that runs at 150 watts may briefly draw 400 to 500 watts when the compressor starts, so an inverter rated for at least 1,000 watts with a good surge rating is a safe choice for a small cabin fridge setup.

The charge controller must also match your array. Divide your total panel wattage by your battery voltage to estimate the charge current, and choose a controller rated above that figure. A maximum power point tracking controller is worth the extra cost for a cabin system, since it extracts more energy from the panels in low light and cold weather than a simpler controller, and those are exactly the conditions when a fridge system is under the most strain.

Before spending money, measure or look up your fridge's daily watt-hours, check your location's winter peak sun hours, and run the calculation: daily watt-hours divided by peak sun hours, multiplied by 1.3 for losses, gives your panel wattage. Size the battery for at least two days of fridge energy, and confirm the inverter surge rating covers the compressor startup. Keep the fridge out of direct sunlight, leave space around it for ventilation, and set the temperature no colder than necessary, since every extra degree of cooling costs energy.

A well-sized cabin fridge system is not large or expensive, but it is easy to get wrong by guessing. Measure the load, size for winter sun, and give the battery enough reserve for a cloudy stretch. Do that, and the fridge will run quietly in the background, which is exactly what a cabin fridge should do.