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.
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.”
Cold food is not optional. Whether you're running a weekender shack, a full-time off-grid block, or a remote work camp, the fridge decides whether your system actually works. The catch: fridges are the single biggest energy gobbler on most off-grid solar systems, and the way the compressor cycles makes them sneaky to size. Skip the marketing brochures. Here's the hard arithmetic, the real numbers, and the traps.
Refrigerators don't draw a steady wattage like a light bulb. They draw in bursts. The compressor spins up, pulls hard for a few minutes until the cabinet hits temperature, then cuts out. That on/off ratio is called duty cycle, and it's everything.
A 12V chest fridge typically uses 400–600 Wh per day in mild weather (25 °C). In that same mild ambient, the compressor might run 30–40% of the time. That sounds efficient, until summer hits. In 40 °C heat the same fridge will run a duty cycle of 60–80%, pulling 800–1200 Wh a day. Most fridge energy labels assume 25 °C ambient. In a 45 °C environment the compressor duty cycle can double. If you size your system for the label number, you'll be short in the first real heatwave.
A chest freezer converted to a fridge with an external thermostat averages about 40–60 W while the compressor is actually spinning. A dedicated 12V upright fridge typically draws 50–80 W running with about a 50% duty cycle on a hot day, using 600–1000 Wh per day. Standard usage in practice looks like this:
| Appliance | Typical Daily Use |
|---|---|
| Chest fridge only | ~0.6 kWh/day |
| Upright fridge only | ~1.2 kWh/day |
| Fridge + freezer combined | ~2.0 kWh/day |
One often-missed spec is locked rotor surge. When a compressor starts it pulls three to five times its running current for a split second. That surge is what blows cheap inverters, undersized cabling, and marginal charge controllers.
12V DC fridges are the gold standard for off-grid. No inverter, no conversion loss, soft-start compressors designed for solar. They run cooler on average, and most modern units use variable-speed compressors that sip power.
Modern 240V inverter-drive fridges (Samsung, LG, Westinghouse, Haier, 200–300L) typically run at 60–80 W with a gentle ramp instead of a hard start surge. They're convenient because you can buy them at any Big Box retailer. But you need an inverter running 24/7, and DC-to-AC conversion wastes 10–15% of the energy. Running a standard AC household fridge off solar requires an inverter, and that conversion overhead is permanent.
3-way absorption fridges on LPG are the old-school choice for caravans. They run silent with no moving parts. Sounds good, until you check the bottle. A 3-way absorption fridge on LPG can empty a 9 kg gas bottle in under a fortnight in 40 °C heat and still requires a flue. And that flue bit is critical: a gas fridge without proper ventilation is a carbon monoxide hazard. Weep holes, clearance to combustibles, and a certified installation are not optional.
Chest vs upright: Chest-style units are the most energy-efficient form factor because cold air stays in when the lid is opened. A dedicated solar fridge or freezer draws roughly 20–25% less power than a combo fridge/freezer of the same size. If you can stand the ergonomics, chest wins on Wh per litre.
Solar sizing is just maths. The inputs: daily Wh load, peak sun hours, system efficiency. The mistake most people make is using panel rated wattage as if it produces that all day.
The three rules:
1. Add 25% to the daily kWh figure to account for typical system losses (charge controller, wiring, battery round-trip).
2. Add 10–15% more if you're running the fridge through an inverter.
3. Assume winter solar output at about half the rated panel output for worst-case sizing.
Worked example for a chest fridge drawing 950 Wh/day:
Always round up. Panel output also drops in heat and as panels age, so oversize by 20–30% if budget allows.
For smaller dedicated solar units, common pairings look like:
| Setup | Daily Use | Panel Size | Battery |
|---|---|---|---|
| Solar fridge only (30–60L) | 0.35–0.55 kWh/day | 100–200 W | 100Ah lithium |
| Solar freezer only (100–210L) | 0.40–0.55 kWh/day | 150–200 W | 100Ah lithium |
| Fridge/freezer combo (50–165L) | 0.55–0.80 kWh/day | 200–300 W | 150–200Ah lithium |
One more thing on the array side: an MPPT charge controller pulls 20–30% more energy from the same panels compared to a basic PWM controller. For a fridge-critical load, MPPT pays for itself in saved panel cost.
The fridge runs at night, so the battery bank is what determines whether you wake up to cold food or warm milk.
Size for 2 days of autonomy (daily load × 2) so a cloudy day doesn't kill your food.
The depth-of-discharge rule is non-negotiable:
Lithium (LiFePO4) is the recommended battery chemistry for a solar fridge setup, outlasting AGM batteries 2–3x. If you're starting fresh, don't waste money on lead-acid for fridge duty.
Worked example for a 1,200 Wh/day fridge with lithium:
Battery storage needed (kWh) = Daily kWh ÷ Depth of discharge (usable %). That's the formula, and it works in reverse too: if you've only got a 200Ah lithium battery (100Ah usable at 50% depth of discharge mentioned in some specs, or 80% in others, so check your battery's actual datasheet), it can run a 60L DC refrigerator for 14–16 hours overnight.
Temperature matters. Batteries should be kept above 10 °C for reliable performance. In winter, a battery box in an uninsulated shed will lose capacity fast, not because the battery is degraded but because the chemistry slows down. Insulate the box, or mount the batteries in a conditioned space.
If you're running a 240V fridge, the inverter matters more than the fridge. Two rules:
1. Pure sine wave only. Modified sine wave can damage compressor electronics and motors. The savings on a cheap MSW inverter get eaten the first time you replace a fridge control board.
2. Size for surge, not running watts. Inverter surge capacity matters more than running watts because motor loads can spike several times higher at startup. A fridge that runs at 80W might need an inverter rated at 800–1,200W continuous to handle the locked-rotor surge cleanly.
The energy label assumes 25 °C and a well-ventilated installation. Most off-grid installs fail both conditions.
A dedicated solar fridge (cooling only) holds 2–10 °C (35–10.0 °C). That's warmer than a household fridge. If you need freezer temperatures, get a dedicated freezer or accept that a cooling-only fridge won't make ice.
How many solar panels do I need to run a fridge off-grid?
For a typical 12V chest fridge drawing 0.6 kWh/day, plan on 200–300W of panels with an MPPT controller. For a larger upright or fridge+freezer combo drawing 2 kWh/day, plan on 600–800W.
Can I run a normal household fridge on solar?
Yes, but you need a pure sine wave inverter sized for at least 3–5× the fridge's running wattage to handle the startup surge, and you'll lose 10–15% to conversion inefficiency. A dedicated 12V fridge is more efficient overall.
Is lithium worth the extra cost for a fridge battery?
Yes. Lithium lasts 2–3x longer than AGM, gives you more usable capacity per Ah, and handles partial state of charge without damage. For a fridge-critical load, the premium pays back.
What temperature will my solar fridge actually hold?
A dedicated solar fridge (cooling only) holds 2–10 °C. It's not a freezer. If you need ice cream or long-term meat storage, get a dedicated solar freezer or a fridge/freezer combo unit.
How long will my battery last running a fridge overnight?
A 200Ah lithium battery with 80% usable capacity (160Ah) can run a 60L DC refrigerator for 14–16 hours overnight. Larger fridges or longer autonomy windows require proportionally bigger banks.
Do gas fridges still make sense off-grid?
For remote, low-duty applications where solar is impractical, yes. But they still need 12V for the control board, and the gas consumption in hot weather is brutal. A 9 kg bottle can be emptied in under a fortnight at 40 °C.
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.