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Enter estimated daily amp-hour draw, desired days of autonomy and battery chemistry below to see recommended battery bank capacity and solar panel wattage.
How This Is Calculated
Battery bank (Ah) = daily draw × days of autonomy ÷ depth of discharge. Solar array (watts) = daily draw (Ah) × 12V ÷ peak sun hours ÷ 0.8 (system-loss factor for charge-controller and wiring inefficiency).
Depth-of-discharge settings from Lifeline's AGM technical manual (50% average) and Battle Born's LiFePO4 guidance (80–100% usable; 80% used here). The 0.8 loss factor is a planning assumption.
This is a sizing estimate for planning purposes. Real-world draw varies by ambient temperature, inverter efficiency and actual usage pattern — size with margin above the calculated minimum rather than at the exact figure.
TL;DR
- Battery size = daily draw × days without charging ÷ usable share (about 50% for AGM, 80% for LiFePO4).
- Peak sun hours depend on place and season: NLR data for a flat panel range from about 8.0 in Moab in June to 0.84 in Seattle in December.
- For a 34 Ah daily load, that means roughly 64 W of panel in Moab in June but about 607 W in Seattle in December.
- Hot panels lose output: Victron's BlueSolar panels lose 0.45% per °C above 25°C.
- Size the battery with margin; an undersized array only recharges more slowly.
Why Battery Chemistry Changes the Recommended Size
Lifeline, an AGM maker, says to rate a battery at twice the capacity the load needs, which keeps the average depth of discharge at 50 percent. Battle Born says its LiFePO4 batteries can use 80 to 100 percent. Worked through the calculator's defaults (30 Ah fridge plus 15 Ah other, 3 days), 135 Ah must come out of the battery: that means about 270 Ah of AGM, or about 169 Ah of lithium used to 80 percent. Weight follows capacity. Lifeline's 105 Ah GPL-31T weighs 64 lb and Battle Born's 100 Ah BB10012 weighs 31 lb, so that AGM bank would weigh more than three times as much as the lithium one.
Getting the Daily Draw Right
The fridge is usually the biggest load. Dometic lists annual energy use for each CFX3 model; divided by 365 days and 12 volts, the CFX3 45 comes to about 18 Ah a day, the 55IM about 19 Ah, and the 75DZ and 95DZ about 25 Ah. These are standardized test figures, so a hot vehicle and frequent opening push real use higher. For other devices, multiply the rated current by hours of use. An inverter left switched on adds its idle draw: Victron's Phoenix 12/800 uses 6.5 W with no load, about 13 Ah a day, or about 1 W in ECO mode.
Peak Sun Hours by Place and Season
Panels are rated at Standard Test Conditions, 1,000 W/m² of sunlight at a 25°C cell temperature, so one "peak sun hour" is one kWh of sunlight per square meter. NLR's (formerly NREL's) solar resource data give the daily average for any US location. For a flat, horizontal panel, like one on a roof rack:
| Location | Annual average | June | December |
|---|---|---|---|
| Phoenix, Arizona | 5.78 | 8.31 | 3.18 |
| Moab, Utah | 5.24 | 8.02 | 2.45 |
| Denver, Colorado | 4.83 | 7.21 | 2.26 |
| Great Smoky Mountains (Tennessee–North Carolina) | 4.15 | 5.85 | 2.19 |
| Seattle, Washington | 3.46 | 5.99 | 0.84 |
Values are kWh/m² per day (peak sun hours) from NLR's Solar Resource Data API. Shade from trees or a campsite wall cuts them further, and tilting a panel toward the sun raises them: NLR's figures for a panel tilted at the site's latitude are 5.53 in Phoenix in December, against 3.18 flat.
What That Means for Panel Size
Using the calculator's formula for a 34 Ah daily load (a CFX3 55IM plus 15 Ah of other use), the recommended array changes by a factor of ten depending on where and when you camp:
| Where and when | Peak sun hours | Recommended array |
|---|---|---|
| Moab, June | 8.02 | ≈ 64 W |
| Calculator default | 4 | ≈ 128 W |
| Moab, December | 2.45 | ≈ 208 W |
| Seattle, December | 0.84 | ≈ 607 W |
Heat, Controllers and the 0.8 Loss Factor
The formula divides by 0.8 to allow for losses between the panel's rating and the battery. That is a planning assumption, and two published figures show where losses come from. Victron's BlueSolar panels lose 0.45 percent of their power for each degree Celsius above 25°C, so a panel whose cells reach 65°C in summer sun produces about 18 percent less than its rating. The controller matters too. Victron explains that a PWM controller works as a switch that pulls the panel's voltage down to near the battery's, while an MPPT controller adjusts it to harvest more; Victron puts the MPPT advantage at 10 to 40 percent when cells are cold (below 45°C), very hot (above 75°C) or in weak light, and says PWM is a low-cost option for small systems at moderate cell temperatures.
Solar Alongside Alternator Charging
Most builds use both. A DC-DC charger refills the battery while driving at a fixed rate — 18 A or 30 A on Victron's 12 V Orion-Tr Smart models — while solar keeps a parked camp going. On a winter or forest trip where the table above calls for several hundred watts of panel, a day's drive does more of the work.
Why Sizing With Margin Beats Sizing to the Minimum
The output is the minimum for the numbers you enter. A device left out of the tally, a hot day that makes the fridge run more, or an extra day at camp all use more than planned. Undersizing the battery means the fridge stops; undersizing the array only means the battery recovers more slowly on the next sunny day. The 12V battery runtime calculator checks how long a battery you already own will last.
How We Research This
Depth-of-discharge rules come from Lifeline's and Battle Born's battery manuals, fridge figures from Dometic's CFX3 manual, sunlight data from NLR's Solar Resource Data API (queried September 2026), and panel and controller figures from Victron Energy. See how we rank for the site's full research method.
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Sources
- Lifeline Batteries (Concorde) — Technical Manual for Lifeline AGM batteries, Doc. 6-0101 Rev. F — Rate AGM at twice the load (50% average depth of discharge) and cycle life vs depth of discharge.
- Battle Born Batteries — AGM vs. Lithium Batteries — LiFePO4 usable depth of discharge of 80–100%.
- Battle Born Batteries — 100Ah 12V LiFePO4 battery (BB10012) manual — BB10012 weight (31 lb) and usable depth of discharge.
- Lifeline Batteries — GPL-31T AGM battery specifications — GPL-31T weight (64 lb) and 105 Ah capacity.
- Dometic — CFX3 Mobile Cooling Operating Manual (CFX3 25/35/45/55IM/75DZ/95DZ/100) — CFX3 rated annual energy use by model (79, 82, 108 and 111 kWh/year) used for the daily amp-hour figures.
- Victron Energy — Inverter VE.Direct 250VA–1600VA datasheet — Phoenix 12/800 no-load draw of 6.5 W and about 1 W in ECO mode.
- NLR (formerly NREL) — Solar Resource Data API (NSRDB-based solar resource averages) — Monthly and annual average solar resource (kWh/m² per day, flat and latitude-tilt) for Phoenix, Moab, Denver, the Great Smoky Mountains area and Seattle, queried September 2026.
- Victron Energy — BlueSolar Monocrystalline Panels datasheet — Standard Test Conditions of 1,000 W/m² and 25°C cell temperature, and the −0.45%/°C power temperature coefficient.
- Victron Energy — Which solar charge controller: PWM or MPPT? — PWM pulls panel voltage down to the battery's; MPPT gains 10–40% at cell temperatures below 45°C or above 75°C or in weak light; PWM suits small systems at moderate temperatures.
- Victron Energy — Orion-Tr Smart DC-DC Charger Isolated manual: Specifications — Orion-Tr Smart 12 V models deliver 18 A or 30 A continuously.