Understanding overlanding power needs
The direct answer for a typical rig
For a 45-watt 12V fridge, basic lighting, and electronics totaling about 528Wh a day, a roof or hood-mounted panel around 141 watts covers stationary camp days when the vehicle isn't driving and the alternator isn't charging. A 110Ah auxiliary battery at 80% depth of discharge covers 2 days of that load without any charging input at all. The NREL's PVWatts solar calculator keeps the current program details, and it is worth a look before you decide.
Why fridge duty cycle changes everything
A 12V compressor fridge cycles on and off to maintain its set temperature rather than running continuously, typically active around 35% of the time depending on ambient heat, insulation quality, and how often the lid opens. Using a fridge's full rated wattage multiplied by 24 hours, instead of its actual duty cycle, significantly overstates daily load and leads to an oversized, more expensive system than necessary. Pair this with the battery lifespan estimator to see the full picture before you commit.
Why overlanding sizing differs from a full off-grid system
Most overlanding trips involve regular daily driving, during which the vehicle's alternator recharges the auxiliary battery far faster and more reliably than solar could. Solar mainly earns its keep during stationary camp days, extended base camps, or multi-day stops where the engine sits off, which is why this calculator sizes around stationary days between drives rather than a full autonomy target like a stationary cabin would need.
Matching panel type to your rig
A roof or hood-mounted rigid panel handles rough terrain and vibration better than a portable folding panel left loose in camp, though a portable panel offers the flexibility to park in shade and still catch sun elsewhere. Either works for this calculation, since the sizing math only cares about total wattage and effective sun hours, not mounting style. DOE's solar photovoltaic technology basics covers the fundamentals if you're comparing panel types for the first time.
What to check once you have a rig sizing
Compare against the camping portable solar panel calculator if you're using a portable power station instead of a dedicated aux battery system, and check battery voltage selection if you're deciding between a 12V and higher-voltage setup for your rig. Weight matters for an overlanding build too, and the battery chemistry weight comparison tool in our battery chemistry storage tools compares LFP against lead-acid on that front directly. Production estimates throughout draw on NREL's PVWatts solar calculator, and the battery lifespan estimator in our maintenance and lifespan tools is worth checking given how differently a battery ages under vibration and heat compared to a stationary install.