What size battery for your caravan fridge?
By Sithum Imarsha Last reviewed
Battery Ah = fridge Ah a day × days with no charging ÷ the share you can use. A Dometic CFX5 45 uses about 18.2 Ah a day, so 3 days needs about 68 Ah of lithium. Solar is worked out from BOM sun hours for your town and month.
Your result
Battery you need
68Ah
18.2 Ah a day × 3 days ÷ 80% usable = 68 Ah
- Used a day
- 18.2Ah
- Charged a day
- 62.2Ah
- Daily balance
- +44Ah
- Solar a day
- 62.2Ah
- With your battery and charging
- No limit: charging covers use
- Sun hours
- 5.5 h a day, Sydney in October (BOM)
- Solar to break even
- 59W
Air temperature is 25 °C unless you type yours (Estimate), so winter output is conservative.
Dometic CFX5 45: 0.76 Ah an hour, tested at 32 °C ambient.
Topping up? See how long a charge takes.
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How we calculated this
What you use a day
Fridge makers publish an average in Ah an hour at 12 V, so no duty cycle is needed: × 24 for a day. Then add your other 12 V loads.
0.76 Ah/h × 24 = 18.2 Ah, 18.2 Ah a day in all
Solar in a day
Wh = panel watts × BOM peak sun hours × heat loss × (1 − system losses) × controller. The panel runs about 31 °C above the air in full sun and loses 0.45% a degree above 25 °C. A PWM controller gets 19% less than MPPT with a 25 °C panel and about the same at 75 °C; we draw a straight line between, which is our estimate. Ah = Wh ÷ battery volts.
797 Wh (5.5 sun hours, panel at 56 °C, 86% for heat, 14% losses, 98% controller) = 62.2 Ah
Charging while you drive
Ah = DC-DC charger amps × hours driving. The rating is the charger's output; a hot charger, a low alternator voltage or a nearly full battery give less (our estimate: no maker gives a figure).
0 Ah, 62.2 Ah a day in all
Battery you need
Ah = Ah a day × days with no charging ÷ the usable share (80% LiFePO4, 50% lead-acid).
68 Ah
How long you can stay
Days = your battery's usable Ah ÷ the daily shortfall (use − charge). If charging covers use, power is not what limits the stay.
80 Ah usable, +44 Ah a day: No limit: charging covers use
| Fridge | Size | Use | A day | Test ambient | Status |
|---|---|---|---|---|---|
| Dometic CFX5 25 | 25 L | 0.77 Ah/h | 18.5 Ah | 32 °C | Official source |
| Dometic CFX5 45 | 46 L | 0.76 Ah/h | 18.2 Ah | 32 °C | Official source |
| Dometic CFX5 55 | 55 L | 0.87 Ah/h | 20.9 Ah | 32 °C | Official source |
| Dometic CFX5 75DZ (dual zone) | 75 L | 2.45 Ah/h | 58.8 Ah | 32 °C | Official source |
| Dometic CFX5 95DZ (dual zone) | 94 L | 2.83 Ah/h | 67.9 Ah | 32 °C | Official source |
| Bushman DC85-X (upright) | 85 L | 1.25 Ah/h | 30 Ah | 25 °C | Official source |
Dometic tests at 32 °C, Bushman at 25 °C, so do not compare them side by side. Engel publishes a range of amps (0.5 to 3 A), not a daily figure, so it is not a preset. The rated input (about 9 to 10 A) is the most a fridge draws, not its average.
| Near | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sydney | 6.3 | 5.5 | 4.6 | 3.8 | 2.9 | 2.4 | 2.7 | 3.6 | 4.7 | 5.5 | 6 | 6.4 |
| Melbourne | 6.8 | 5.9 | 4.6 | 3.1 | 2.2 | 1.8 | 2 | 2.8 | 3.8 | 5 | 5.9 | 6.7 |
| Brisbane | 6.5 | 5.9 | 5.1 | 4.5 | 3.7 | 3.3 | 3.6 | 4.4 | 5.4 | 6 | 6.5 | 6.7 |
| Perth | 8.1 | 7.1 | 5.7 | 4.2 | 3.1 | 2.6 | 2.7 | 3.5 | 4.6 | 6.1 | 7.3 | 8.2 |
| Adelaide | 7.6 | 6.7 | 5.3 | 3.8 | 2.6 | 2.1 | 2.4 | 3.2 | 4.3 | 5.7 | 6.8 | 7.4 |
| Hobart | 6.4 | 5.4 | 4 | 2.6 | 1.7 | 1.3 | 1.6 | 2.4 | 3.5 | 4.8 | 5.7 | 6.5 |
| Canberra | 6.8 | 5.9 | 4.8 | 3.7 | 2.9 | 2.3 | 2.6 | 3.4 | 4.6 | 5.7 | 6.4 | 7 |
| Darwin | 5.2 | 5.4 | 5.9 | 6.1 | 5.8 | 5.5 | 5.7 | 6.3 | 6.6 | 6.8 | 6.4 | 5.7 |
| Alice Springs | 7.5 | 7.1 | 6.6 | 5.6 | 4.5 | 4.1 | 4.4 | 5.2 | 6.2 | 7 | 7.4 | 7.3 |
| Cairns | 5.8 | 5.7 | 5.5 | 5.1 | 4.6 | 4.3 | 4.6 | 5.2 | 5.9 | 6.4 | 6.5 | 6.3 |
| Townsville | 6.5 | 6.1 | 5.9 | 5.4 | 4.6 | 4.3 | 4.6 | 5.4 | 6.4 | 7 | 7.1 | 7 |
| Mount Gambier | 6.9 | 6 | 4.6 | 3.1 | 2.1 | 1.8 | 2 | 2.8 | 3.8 | 5 | 6.1 | 6.8 |
BOM data (Official source), long-term monthly means, on a flat surface. That fits a panel lying flat on a roof. A tilted panel gets more in winter; we do not model tilt. Melbourne in June is 1.8 hours, not the 5 hours some tools use all winter.
| Figure | Value | Status and source |
|---|---|---|
| System losses | 14% | Official source: NREL PVWatts v5 manual (2014): default system losses 14% (for fixed home systems) |
| MPPT controller | 98% | Official source: Victron SmartSolar MPPT 100/30 and 100/50 datasheet: maximum efficiency 98% |
| PWM against MPPT, cell at 25 °C | 81% | Official source: Victron white paper: PWM or MPPT (20 Jan 2020): PWM harvests 19% less than MPPT at a 25 °C cell |
| PWM equal to MPPT at | 75 °C cell | Official source: Victron white paper: PWM or MPPT (20 Jan 2020): about the same as MPPT at a 75 °C cell |
| Panel power loss | 0.45% per °C | Official source: Victron white paper: PWM or MPPT (20 Jan 2020): example panel −0.45% per °C |
| Panel temperature in full sun (NOCT) | 45 °C | Official source: NREL PVWatts v5 manual (2014): INOCT 45 °C for an open rack |
| Daytime air temperature | 25 °C | Estimate: Our default daytime air temperature. Type yours |
“Official source” means read in full on the maker’s own datasheet, manual or spec page, or in an official dataset (BOM). “Dated third-party source” means read on a dated third-party page, such as a product test. “Not yet checked” means one non-official source. “Estimate” means our own pick or reasoning: type your own figure.
Sources
- Victron: LiFePO4 Smart battery datasheet
- Victron: GEL and AGM batteries datasheet
- Victron: BMV-712 Smart and SmartShunt manual
- Victron: which solar charge controller, PWM or MPPT?
- Victron: SmartSolar MPPT 100/30 and 100/50 datasheet
- NREL: PVWatts version 5 manual
- Bureau of Meteorology: Climate Data Online, mean daily solar exposure
- Dometic: CFX5 45 portable fridge (AU)
- Dometic: CFX5 25 portable fridge (AU)
- Dometic: CFX5 55 portable fridge (AU)
- Dometic: CFX5 75DZ portable fridge (AU)
- Dometic: CFX5 95DZ portable fridge (AU)
- Bushman: DC85-X upright fridge
- Engel Australia: MT-V40F 40 litre fridge
- Redarc: BCDC in-vehicle DC-DC chargers
Start with the fridge’s daily amp hours
A 12 V compressor fridge does not run all the time. Its compressor cycles on and off with the thermostat. So use the maker’s average figure, not the rated amps.
- Dometic publishes an average in Ah an hour, tested at 32 °C: 0.76 Ah an hour for a CFX5 45, about 18 Ah a day.
- Bushman publishes 1.25 Ah an hour for its DC85-X, averaged over 24 hours at 25 °C.
- The rated input on the sticker (about 9 to 10 A) is the most the fridge draws. It is not its average.
Add your other 12 V loads as amps × hours: LED lights, a water pump, phone charging.
Battery size
Battery Ah = Ah a day × days with no charging ÷ the share you can use.
- LiFePO4: 80% usable, the depth of discharge Victron and Renogy rate their cycle life at.
- Lead-acid (AGM, gel, flooded): 50% usable, the default floor in Victron battery monitors.
Three days of a CFX5 45 needs about 68 Ah of lithium or about 109 Ah of lead-acid.
Solar: use your month, not a national number
The Bureau of Meteorology publishes the mean daily solar exposure for each weather station. Divided by 3.6, that is peak sun hours. On a flat panel:
- Sydney: 2.4 hours a day in June, 6.3 in January.
- Melbourne: 1.8 in June, 6.8 in January.
- Hobart: 1.3 in June.
Then take off losses. Hot panels make less power: about 0.45% a degree above 25 °C, and a panel in full sun runs about 31 °C above the air. System losses (dirt, shade, wiring) are 14% for a fixed home system in NREL’s PVWatts model, and often more for a portable panel. An MPPT controller is about 98% efficient. A PWM controller harvests 19% less with a cool panel.
Charging while you drive
A DC-DC charger adds up to its rated amps for each hour of driving. Redarc’s BCDC range comes in 25, 40 and 50 A. Real output can be lower when the charger is hot, the alternator voltage is low or the battery is nearly full.
What this calculator does not do
- It does not size cables or fuses. Follow the battery and charger maker’s installation manual.
- It uses one temperature for the day. A fridge in a hot car uses more than the maker’s test figure.
- It treats solar as a daily average. A run of cloudy days is when the battery size matters.
Questions
How many days does a caravan fridge last on 100Ah with no solar?
A Dometic CFX5 45 uses 0.76 Ah an hour at 32 °C, about 18 Ah a day. 100 Ah of LiFePO4 at 80% usable gives 80 Ah: about 4.4 days with no charging. 100 Ah of AGM at 50% gives about 2.7 days.
What size solar panel do I need for a caravan fridge?
It depends on where and when. A flat 200 W panel on MPPT covers a CFX5 45 in Sydney in June (2.4 sun hours), but falls short in Hobart in June (1.3). The result shows the panel size that breaks even for your month.
Why use BOM sun hours, not a fixed number?
Sun hours change up to 6 times across Australia and the year. BOM gives Melbourne 1.8 hours a day in June and 6.8 in January. A single winter figure of 5 hours overstates southern winters.
MPPT or PWM?
Victron’s example: a PWM controller harvests 19% less than MPPT with the panel at 25 °C, and about the same with the panel at 75 °C. So MPPT gains most in cool weather.
Does a DC-DC charger fill the battery while I drive?
It adds its rated amps for each hour of driving, at best. A 25 A charger for 2 hours adds up to 50 Ah. Lithium charges near its full rate until about 95%. Lead-acid slows from about 80%, so it rarely gets full from driving.
Why is the fridge’s rated amps not its running amps?
The rated input (about 9 to 10 A for a CFX5) is the most it draws. The compressor cycles on and off, so the daily average is about 0.76 A for a CFX5 45.