The most common failure mode for a new fridge owner isn't a broken fridge. It's a dead battery on the second morning of a three-day camp, warm food, and a quiet vow to go back to coolers.
That failure is almost never the fridge's fault. It's a planning failure — usually caused by trusting a marketing runtime claim instead of doing twenty minutes of arithmetic. So this guide skips the magic numbers entirely and gives you the framework instead. Learn the math once and you can size a system for any fridge, any trip, any weather.
Why Runtime Claims Are Mostly Useless
A compressor fridge doesn't draw power steadily. The compressor kicks on, pulls the cabinet down to temperature, and shuts off. Your average draw — the number that actually drains your battery — depends on how often and how long that compressor runs. And that depends on:
- Ambient temperature. The gap between outside air and your set point is the whole job. A fridge in 95°F sun works dramatically harder than the same fridge in a 65°F shaded camp.
- How full it is. Thermal mass is your friend. A full fridge holds temperature through compressor-off cycles far better than a half-empty one.
- Lid discipline. Every opening dumps cold air. A family hitting the fridge twenty times a day is a different electrical load than a solo camper opening it four times.
- Set point. Fridge temps vs near-freezing vs running a freezer zone are different workloads entirely.
This is why any single "runs for X days!" claim is fiction. The honest answer is a range.
The Planning Number: Draw Classes
For 45-liter-class compressor fridges like the Dometic CFX3 45 and ICECO VL45, a typical averaged draw at 12V lands around 0.7 to 1.2 amp-hours per hour, cycle- and conditions-dependent. Use the table below to pick your planning scenario:
| Scenario | Planning draw | Per day (24h) |
|---|---|---|
| Mild weather, shaded, full fridge, few openings | ~0.7 Ah/hr | ~17 Ah |
| Typical summer trip | ~1.0 Ah/hr | ~24 Ah |
| Hot, sun-exposed, frequent openings, cold set point | ~1.2 Ah/hr | ~29 Ah |
When in doubt, plan with the higher number. Electrical margin is the cheapest insurance you'll ever buy.
The Battery Math, Step by Step
Here's the whole framework in four steps. Worked example: a typical summer weekend, three nights, no driving after arrival.
Step 1 — Daily load. Fridge at 1.0 Ah/hr × 24 hours = 24 Ah/day. Add your other loads: lights, phone charging, a diesel heater fan, whatever. Call the extras 6 Ah/day for a modest camp. Total: ~30 Ah/day.
Step 2 — Trip load. 30 Ah/day × 3 days = ~90 Ah needed.
Step 3 — Usable capacity, not rated capacity. Batteries don't give you their label number. As a planning rule: lithium (LiFePO4) batteries can be drawn down to roughly 80–90% of rated capacity; lead-acid types should only be cycled to around 50% to avoid shortening their life. So:
| Battery | Rated | Usable (planning) |
|---|---|---|
| 100Ah LiFePO4 | 100 Ah | ~80–90 Ah |
| 100Ah AGM (lead-acid) | 100 Ah | ~50 Ah |
Step 4 — Compare and add margin. Our 90 Ah trip load against a 100Ah lithium's ~85 usable Ah is cutting it close — workable in mild weather, marginal in heat. Against a 100Ah AGM's ~50 usable Ah, it's a guaranteed dead battery on night two. The fix is more capacity, or replenishment.
That's the entire method. Plug in your own days, your own loads, your own battery chemistry.
Solar: The Replenishment Term
Solar changes the equation from "how long until empty" to "does daily input match daily output." The honest framing:
A panel's rated wattage is a laboratory number. Real harvest depends on sun hours, panel angle, shade, and weather — a useful conservative habit is to assume a panel delivers its rated output for only a limited number of effective full-sun hours per day, and that real-world output runs meaningfully below the sticker rating. The math framework:
Daily harvest (Ah) ≈ (panel watts × effective sun hours × real-world efficiency factor) ÷ 12V
If your estimated harvest matches or beats your ~30 Ah daily load, you can hold camp indefinitely in good weather. If it covers half, you've doubled your stay-time instead. Run your own conservative numbers rather than trusting the brochure — and remember that a string of overcast days takes solar to near zero, which is why battery capacity remains the foundation and solar is the extension.
Driving does the same job: your alternator replenishes the bank whenever the engine runs, which is why fridge power is nearly a non-issue for move-every-day travel and a real project for multi-day base camps.
Practical System Notes
- Never run the fridge off your starter battery at camp. A dead house battery is an inconvenience; a dead starter battery in the backcountry is an emergency. Separate them — dual battery setup or a portable power station.
- Wiring matters. Long, thin 12V runs lose voltage, and low voltage makes compressors work harder or cut out. Use appropriately heavy cable and good connectors to the fridge circuit.
- Use the low-voltage cutoff settings. Most compressor fridges let you choose how deeply they'll discharge the supply battery before shutting down. Set it conservatively on any battery you also depend on.
- Mind the weight budget. Batteries, a power station, and a loaded fridge add up fast — run your full setup through the load checker before you find out the hard way, and anchor all of it properly (a battery box is just as dangerous loose as a fridge; see /gear for securing hardware).
The Bottom Line
Size the system in this order: estimate your daily amp-hours with the high end of the draw class, multiply by days, divide by your battery's usable capacity, then add solar or drive time to extend. No fabricated runtimes required — just arithmetic and honest margins. Full details on how we frame power numbers are in our methodology.
FAQ
How big a battery do I need to run a 45L fridge for a weekend?
Do the math for your conditions, but as a framework: at a typical ~1.0 Ah/hr planning draw, the fridge alone wants roughly 24 Ah per day. Two nights ≈ ~50 Ah plus your other loads — comfortable for a 100Ah lithium, marginal for a 100Ah lead-acid (only ~50 Ah usable). Hot weather pushes everything up; plan with margin.
Is a portable power station better than a dual battery setup?
Different tools. A power station is zero-install, portable between vehicles, and doubles as house power — ideal for first fridge setups. A wired dual-battery system charges automatically while driving and lives permanently in the rig — better for high-frequency use and bigger loads. Plenty of serious rigs run both.
How much solar do I need to run a fridge indefinitely?
Enough that your conservative daily harvest estimate beats your daily load — for a fridge-plus-basics camp around 30 Ah/day, that typically means a panel sized well above what the raw arithmetic suggests, because real-world output runs below sticker ratings and clouds happen. Size for your worst expected day, not your best.
Does a fridge use less power if I run it as a freezer before the trip?
Pre-chilling — running the fridge down on shore power and loading it with already-cold food — meaningfully reduces work during the trip's first day, since the compressor isn't fighting warm contents. Frozen items also act as thermal ballast. It doesn't change steady-state draw, but it's free margin and worth doing every trip.
The trail dispatch
Weight changes, rating corrections, and new gear run through the load model — one email when it matters, no filler.