By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After 22 years of watching fridges kill tidy solar plans, I size every system for the heatwave duty cycle, never the number on the sticker.”
— Dave. Measure twice, buy once.
ArticlesCalculatorsSite PlannerShopIf you’ve ever woken up to a warm stubby because the fridge chewed through your battery bank by 2 a.m., you know the pain. The fridge is the one load that never clocks off.
By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After 22 years of watching fridges kill tidy solar plans, I size every system for the heatwave duty cycle, never the number on the sticker.”
If you’ve ever woken up to a warm stubby because the fridge chewed through your battery bank by 2 a.m., you know the pain. The fridge is the one load that never clocks off. I’ve been wiring off-grid systems for 22 years, and in every bush block, the fridge is the first thing that sends a perfectly good solar plan sideways.
Quick answers
A fridge runs 24 hours a day, seven days a week. It doesn’t care if it’s overcast or if you’re already down to 40% state of charge. Most other loads—lights, pumps, microwave—kick on for minutes and shut up. The fridge just keeps chipping away.
The second problem is what sparkies call the locked rotor surge. When a compressor starts, it pulls three to five times its running current for a split second. That’s no drama on the grid, but an undersized off-grid inverter or a battery bank with a lazy BMS can trip before the compressor even warms up. I’ve seen otherwise tidy systems fold under a fridge start while the washing machine was finishing a spin cycle.
Then there’s temperature. Most energy labels assume 25 °C ambient. Throw the fridge into a donga that hits 45 °C in January and the duty cycle—the percentage of time the compressor actually runs—doubles or worse. A 500 Wh/day fridge in spring can easily pull 1200 Wh/day in a heatwave. That turns a comfortable battery into a dead battery in one hot night.
These are the workhorses of the bush. Almost all use a Danfoss/Secop BD-series compressor—the same guts you’ll find in an Engel, an Evakool, or a Waeco. The compressor runs at variable speed and sips DC directly from the battery, so there are no inverter losses.
A 12 V chest freezer converted to a fridge (stick on an external thermostat like an Inkbird ITC-308, ~$40 at time of writing) will average about 40–60 W while the compressor is actually spinning. In 25 °C weather the compressor might run 30–40% of the time—that’s the duty cycle. Do the sums: 50 W × 24 hours × 0.35 duty cycle = 420 Wh per day. Round it to 400–600 Wh for a 100–150 litre chest in mild weather.
Dedicated 12 V upright fridges and dual-zone camping boxes usually draw a bit more. The upright shape spills cold air every time you open the door, and the seals are less forgiving. Figure 50–80 W running, duty cycle more like 50% on a hot day, so 600–1000 Wh per day is normal.
In 40 °C heat the same 12 V chest will run a duty cycle of 60–80%. Suddenly it’s pulling 800–1200 Wh a day. If you size the solar for the sticker number, you’ll be buying a new battery by February.
A modern 240 V fridge with an inverter compressor (think Samsung, LG, Westinghouse, Haier around 200–300 litres) is a different beast to the old clunker in your nan’s garage. The inverter ramps the compressor up and down gently, so there’s no brutal start surge. Running power is typically 60–80 W, duty cycle 40–50% in moderate ambient, giving 600–900 Wh per day for the fridge itself.
The catch is the inverter you need to run it. A quality pure sine wave inverter—something like a Victron or a Selectronic—draws 15–25 W just sitting there doing nothing. Over 24 hours that’s an extra 360–600 Wh. Suddenly your daily total is closer to 1000–1400 Wh. A cheap modified-sine-wave inverter will cost less and destroy the fridge compressor within a year, so don’t even think about it.
If you’ve already got a large battery bank and a 3 kVA inverter for other loads, the 240 V fridge can be the cheaper overall install because the appliance itself is a third of the price of a name-brand 12 V upright. A decent 270 L inverter fridge might set you back $700–900 at the big green shed, whereas a similar-size 12 V Evakool upright pushes past $2,500. The inverter fridge will eventually cost you more in panel and battery capacity, but the payback on the appliance price can still work out.
Gas fridges are electrically silent—no compressor, no inverter. Instead a small flame boils an ammonia-water mixture in an absorption cycle. They use no DC power at all unless they have an internal light or piezo igniter, so they’re popular with grey nomads who park up for weeks and don’t want to mess with solar.
Fuel consumption is the dirty secret. A 150 L three-way fridge (Dometic, Thetford, old Electrolux) will burn roughly 350–450 g of LPG per day in 25 °C. When the mercury hits 40 °C, that can jump to 600–800 g per day because the absorption cycle gets lazier as the condenser struggles to dump heat. A 9 kg swap bottle holds about 8.5 kg of usable gas and costs around $30 at a servo. At 800 g per day, you’ll drain it in ten or eleven days—nearly $3 a day just for refrigeration.
They also need a permanent flue to the outside; I’ve attended a cabin where a gas fridge had been venting into the kitchen for six months. Carbon monoxide doesn’t smell like anything, but a splitting headache in the morning is a solid clue.
For a solar-centric house, gas is usually a backup, not the daily driver. It’s hard to beat the maths of a 200 W panel that makes free electricity for decades versus feeding gas bottles into a machine every week.
Cold air is heavy. Open a chest lid and the cold air lies there like a sleepy dog. Open an upright door and the bottom half sheets onto the floor in seconds. That one fact means a chest of the same internal volume will use roughly 20–30% less energy over a day, and the difference gets worse in hot weather.
I’ve met plenty of people who insisted on an upright because “a chest is like an esky, I can’t see what’s in it.” Fair enough. If you stack your baskets well and label the lid, you’ll stop buying doubles and the power saved will pay for a carton. If you seriously can’t handle a chest, buy the most efficient 12 V upright you can afford and fit the biggest battery you can carry—just don’t whinge about the numbers later.
A 215 L chest freezer from a hardware chain (around $900) with an external thermostat makes a brilliant fridge. I’ve had one running at my shed for nine years. It sips around 500 Wh a day in spring and about 1 kWh a day when the shed hits 42 °C. Total setup cost: $940 plus a couple of hours with a screwdriver.
Let’s work through a real-world off-grid fridge system so you can see where the money goes.
Take a 150 L chest fridge set to 3 °C, using a 12 V compressor. In autumn when it’s 25 °C during the day, it pulls 480 Wh per day. But we size for worst case—say 40 °C summer ambients. As a rough rule of thumb, every 5 °C above 25 °C adds about 25% to daily consumption. At 40 °C that’s three 5 °C steps, so 1.25 × 1.25 × 1.25 = roughly double. We’ll plan for 950 Wh per day to give a bit of headroom.
Panels
Assume a sunny inland site with 5 peak sun hours in summer (most of Australia away from the coast gets this). To make 950 Wh in 5 hours you need 190 W of raw panel output. Knock off 5% for wiring and controller losses, another 5% for battery charge efficiency—call it 10% overall. 190 W ÷ 0.9 ≈ 210 W of panels. Two 175 W panels or a single 250 W household panel will do nicely. At time of writing a 250 W mono panel is under $150.
Battery
950 Wh ÷ 12 V nominal = 79 Ah. A lithium iron phosphate (LiFePO4) battery can give 80% depth of discharge without sulking, so 79 Ah ÷ 0.8 = 99 Ah—buy a 100 Ah lithium battery. If you’re still running sealed lead-acid, never draw past 50%, so you’d need 158 Ah; call it a 200 Ah AGM bank. A quality 100 Ah lithium with a decent BMS runs $400–600 in 2025 Australian dollars.
Charge controller and inverter
A 20 A MPPT charge controller (Victron SmartSolar 100/20 around $140) will handle the 250 W panel at 12 V easily. If it’s a 12 V fridge, no inverter. If it’s a 240 V fridge, you’ll need roughly another 25% on the panel and battery numbers to feed the inverter’s idle draw.
That’s the system: 250 W of solar, a 100 Ah lithium battery, a 20 A MPPT controller, and a chest fridge you can trust.
I’m not endorsing any of these, just describing what’s parked under verandahs from the Flinders Ranges to the Daintree.
12 V chests and uprights
Engel, Dometic/Waeco, Evakool, Brass Monkey, Snomaster, National Luna. Engel and Evakool are probably the most common dedicated 12 V fridges I see. The Engel 40 L is close to a cult; it’ll rattle along for 20 years if you don’t suffocate the compressor with dog hair.
For the budget-conscious, a 215 L domestic chest freezer from a joint like Bunnings or Appliances Online with a Secop compressor inside—some are still Chinese clones, but many work fine—paired with an Inkbird thermostat has become the default bushie hack. It’s not pretty, but it works.
240 V inverter fridges
Samsung and LG inverter models in the 200–300 L range turn up routinely because they’re cheap and the compressors are efficient. Haier and Westinghouse sometimes badge the same compressors. The key is to pick a model that lists a low annual energy consumption, then double it for off-grid summer use.
LPG/gas
The Dometic RM series and Thetford three-ways dominate the caravan park set. Old Electrolux units show up on Gumtree for $200 and often still work, but check the burner jet and flue before you fire one up indoors.
A fridge is a heat pump. It moves heat from inside the cabinet to the outside air. The hotter that outside air gets, the harder it is to dump heat, so the compressor runs longer and the cabinet gains heat faster through the insulation. It’s a double whammy.
I said earlier: for every 5 °C above the lab-test 25 °C, add roughly 25% to the daily energy number. That stacks up fast. In a 40 °C shed, a 500 Wh/day fridge can become a 1,000–1,200 Wh/day appliance. If you’re on the edge of your battery capacity in March, November will break you.
Practical things that help:
Buying the biggest fridge “to be safe”
A 400 L upright has more surface area than a 150 L chest, so it leaks more heat through the walls regardless of how much food is inside. That means a base load of 1,500–2,000 Wh per day before you’ve even opened the door. If there’s only two of you and a slab of homebrew, you’re paying for empty litres of cold air. Buy the smallest box that holds a week’s worth of tucker and a carton, and your solar system will thank you.
Sticking the fridge in a hot tin shed with zero shade
Solar panels get shade tolerance these days; fridges don’t. I’ve measured a 30% jump in daily consumption just by shifting a fridge from an airy verandah to a west-facing metal shed.
Using a modified-sine-wave inverter on a 240 V compressor
That’s a fast way to turn a new fridge into a humming brick. The choppy waveform makes the compressor motor run hot and the control board unhappy. Pure sine wave or DC only.
Forgetting the inverter idle draw
I’ve done the sums for you earlier. If you use a 240 V fridge, add 20 W continuous to the daily tally. That’s the difference between making it through a cloudy day and firing up the generator in your jocks at 5 a.m.
Running a Peltier cooler and calling it a solar fridge
Those little 12 V thermoelectric boxes that plug into a ciggie socket are not fridges. They can cool about 10 °C below ambient and pull 40–50 W continuously without cycling. You’ll flatten a battery and still have warm cheese. They’re for keeping a six-pack from boiling in the ute on the drive home, nothing more.
Ignoring the defrost cycle on some 240 V fridges
Many inverter fridges still use a resistive defrost element that kicks in every 8–15 hours and can pull 150–300 W for 20 minutes. That’s another 50–100 Wh a day to add to the budget. Check the specs.
While many off-grid projects are achievable as DIY, certain situations require licensed professionals:
Always check local regulations and obtain necessary permits before commencing work.
Worth a watch: 8 Considerations When Buying And Designing An Off-Grid System With A New DC Refrigerator Freezer · Phocos
The article runs through a worked example of the solar-and-battery maths, and it's the kind of thing you want sorted before you spend a dollar on gear. The fridge is the hardest load in an off-grid house, so getting this wrong is exactly how you end up with warm tinnies at 2 a.m. Measure twice, buy once, that's the rule I live by.
I break this down in the chest vs upright section, and one of them is what I call an energy trap. They don't perform the same on solar, and the difference will absolutely show up in your battery bank over a season. Have a read of that section before you commit to either.
There's a whole section on temperature derating in 40°C heat because this catches plenty of people out. A fridge that sips power in mild weather can turn into a battery killer once the heat really sets in. Always size for your hottest week, not your average day.
I see the same blunders over and over, and the big one is thinking bigger is better. A bigger fridge costs more upfront, then bleeds your solar and battery every single day after that. The common mistakes section runs through the full list, worth a read before you spend a cent.
