Sizing solar for a 12V fridge comes down to four numbers: what the fridge uses in a day, what your panel puts back, how much stored capacity carries it overnight, and the margin you keep for a cloudy stretch. Whether solar can run a fridge at all is a separate question; this guide takes the answer as yes and concentrates on picking the sizes. Work the four numbers in that order, because each one sets the next. The steps below cover daily draw, panel harvest, and battery buffer, then run the whole method on one fridge's published figures.
Step 1: Find the Fridge's Daily Draw
Start in watt-hours per day, not watts. Use the maker's published daily consumption figure rather than the wattage printed on the label. Compressor fridges cycle, so rated draw overstates continuous consumption.
The label number tells you what the compressor pulls while it is running. The daily figure already accounts for the share of the day it spends switched off, which is the number your panel and battery have to cover. If a maker publishes a runtime instead, divide the pack's watt-hours by the rated hours and you have the same thing from the other direction.
That share moves with ambient heat, the temperature you set, and how often the lid is opened, so read any single figure as a planning estimate rather than a fixed cost. A fridge that is filled once and left shut behaves differently from one opened every twenty minutes at a tailgate, and the daily figure is an average of whatever you actually do. Write down both numbers - running watts and daily watt-hours - and keep them separate. Everything downstream is sized from the daily one.
Step 2: Estimate What a Panel Actually Harvests
A panel's rating is a laboratory figure taken under laboratory light, and the number you plan around is the energy it returns where you park. Panels deliver about 5-7 peak sun hours in summer across most of the US, depending on region, and fewer under cloud or with the array lying flat instead of angled at the sun. Planning on five is the cautious end of that band.
From there the working shortcut is one line: panel watts times 5 hours times 0.8 system losses = realistic daily watt-hours. On that arithmetic a 100 W panel returns roughly 400 Wh a day and a 200 W panel roughly 800 Wh. Set the result beside the Step 1 daily draw.
Where you plan within the 5-7 band is a judgment call rather than a calculation. Take the low end if the panel will sit flat, spend part of the afternoon in shade, or be used outside high summer, and the high end only if you will move and aim it through the day. Pick the figure that matches how the panel will be used, not the one that makes the arithmetic work.
Two things to check before you accept the number. The panel has to clear the fridge's draw with something left over, because a panel that merely matches the daily figure never refills what the night emptied. And the panel's output voltage has to sit inside whatever the charging input will accept, which is a hardware limit rather than an arithmetic one.
Step 3: Size the Battery Buffer
Panels harvest by day and a fridge runs around the clock, so stored capacity is what carries it from sunset to morning - and through any day the sun does not show. Size the bank for at least two days of fridge load - the conservative floor of the standard two-to-six-day autonomy range. Multiply the Step 1 daily figure by two and that is your target in watt-hours.
Then adjust twice. Usable capacity is smaller than the label on lead-acid chemistry, so nameplate watt-hours and available watt-hours are not the same number. And anything else you run off the same stored energy - phones, lights, a fan - comes out of the fridge's buffer, so count those loads here rather than discovering them on the third overcast morning.
Finish by setting the four numbers in one row: daily draw, panel harvest, buffer target, and the difference between harvest and draw. If harvest clears draw with something to spare, and storage holds two days of draw, the setup is sized. If either check fails, the fix is more panel, more storage, or a fridge with a lower daily draw - which is why Step 1 comes first.
Step 4: Run the Method on a Real Fridge
Daily Draw and Panel Size
Take the Explorer Bear UR35W, a 37 qt / 35 L compressor fridge-freezer. Its published energy use is under 0.5 kWh per day, even in MAX mode, and that is the Step 1 number. It lists solar as a power source alongside AC 100-240V, DC 12V/24V, and battery, so a panel feeds the system directly.
The panel size comes from the part that does the charging. The Explorer Bear 170Wh Portable Battery is a fridge accessory for the Explorer Bear UR series refrigerators, and its own specification recommends a 90~120 W solar panel, caps panel output at 30 V, and states that charging input power must be higher than the refrigerator's working power or the battery will not charge.
Run the Step 2 arithmetic on the top of that recommended band and 120 W times 5 hours times 0.8 gives about 480 Wh a day, against a stated daily ceiling of under 0.5 kWh. At the bottom of the band, 90 W returns about 360 Wh. So the recommended panel covers an ordinary day and leaves little spare for banking, which is precisely why Step 3 exists.
The Buffer, in Real Battery Capacity
Each pack holds 170 Wh and is rated for up to 25 hours at a 40°F setting, or up to 8 hours at -4°F. Divide 170 Wh by 25 hours and the implied average is under 7 W, against the 38~40 W the fridge pulls while the compressor runs. That gap is the Step 1 cycling effect measured in one product's own figures: the running wattage alone would predict about four and a half hours from the same pack.
The UR35W runs on two of those packs, rated together for up to 48 hours at 40°F, or 16 hours at -4°F. Forty-eight hours at fridge temperature clears the two-day floor from Step 3 on stored capacity alone. At the freezer setting the pair covers 16 hours, well short of two days, so freezer duty wants more panel, more packs, or a shore-power day.
Charging closes the loop. The packs install and charge through the UR series while the fridge is powered by 110V AC or 12V/24V DC, and they can also charge through the fridge while it is running from a solar panel, so the panel, the fridge, and the buffer share one path instead of a separate charging station taking up vehicle space. Each pack also carries Type-C and USB power bank outputs, which is convenient - and, per Step 3, a load to count against the same watt-hours.
The Numbers Behind Each Step
| Figure | Number | Where it comes from |
|---|---|---|
| Daily draw | Under 0.5 kWh/day, MAX mode | UR35W energy use spec |
| Running draw | 38~40 W | Refrigerator working power, 170Wh battery spec |
| Peak sun hours | 5 hours as the planning figure | Cautious end of the 5-7 summer band |
| Panel size | 90~120 W | Panel recommendation, 170Wh battery spec |
| Panel harvest | About 480 Wh/day | Step 2 rule applied at 120 W |
| Panel voltage ceiling | 30 V maximum output | Charging limit, 170Wh battery spec |
| Buffer target | Two days of the Step 1 figure | Two-day autonomy floor |
| Pack capacity | 170 Wh per pack | 170Wh battery capacity spec |
| Pack runtime | Up to 25 h at 40°F; up to 8 h at -4°F | 170Wh battery runtime spec |
| Two-pack runtime | Up to 48 h at 40°F; 16 h at -4°F | UR35W runtime spec |
Final Thoughts on Sizing a Fridge Solar Setup
The deciding number is daily consumption, because the panel, the buffer, and the cloudy-weather margin are all sized from it. Work it out first, size the panel to clear it on a five-peak-sun-hour day with something spare, then hold at least two days of it in storage. Where a maker publishes both a running wattage and a runtime, the gap between them tells you how much of the day the compressor is really off, and that gap is usually the difference between a setup that works and one that runs down by Wednesday. Explore the published specifications for the Explorer Bear UR35W and the 170Wh battery if you want a full set of numbers to rerun against your own route.
