Solar for car camping: panel size, shade and expectations
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The short answer
Solar on a car is a top-up, not a guaranteed daily charge. Rated watts come from lab conditions, and real output falls with cloud, shade, low winter sun and heat. Size your panel from your own daily energy use, leave a margin, and check the charge controller's limits before you connect anything.
What rated watts mean
Panel makers quote a rated power in watts. EcoFlow UK says this figure is measured under Standard Test Conditions, and that real conditions rarely match them. EcoFlow UK: how much energy a solar panel produces
Watts measure output at one moment. Watt-hours measure energy over time, which is the figure you need for planning a day. Use the rating to compare panels, not to predict what you will get on a campsite.
Why real output varies
Several things pull output below the rating. EcoFlow lists sun hours, winter, cloud cover, the sun's angle through the day, shade and high temperatures. EcoFlow UK
The Energy Saving Trust says panels still generate on cloudy days, and that stronger sunshine produces more electricity. It describes an unshaded, south-facing roof as the ideal setup. Energy Saving Trust: solar panels
A roof panel is fixed to the car, so you cannot turn it towards the sun, and a shaded parking spot can cut what it makes. Renogy UK notes that portable panels can be adjusted to maximise sunlight whatever way the vehicle is parked. Renogy UK
Charge controllers and built-in MPPT
A charge controller sits between the panel and the battery and decides how the panel's power reaches the battery. Two types matter. Renogy says an MPPT controller tracks the point where the array produces the most usable power, and that PWM controllers suit small systems whose panel and battery voltages are closely matched. Renogy: MPPT vs PWM charge controllers
MPPT is usually the more flexible choice when panel voltage is higher than battery voltage.
Check the limits before you wire anything. Victron explains that its model names show the maximum PV open-circuit voltage and the maximum charge current. Victron Energy: solar charge controllers Renogy also notes that an MPPT controller still has a maximum PV input voltage. Renogy Read the input limits in your controller's or power station's manual.
Portable or fixed panels
Fixed roof panels are bolted to the roof and need no setting up. Renogy UK lists roof-mounted panels as more convenient than portable ones. Renogy UK
Portable panels can be angled to the sun regardless of how you park. The trade-offs are that they need setting up every time, cannot safely charge while unattended because of theft and weather risk, usually cost more per watt, and give less output than roof-mounted systems. Renogy UK Renogy is a seller of both types, so weigh its comparison against your own use.
If a roof tent is already on the roof, read leaving a roof tent on your car before you add anything else up there. For roof space and fit, see will a roof tent fit my car.
Sizing from your own daily energy need
Start with demand, not the panel. Power on the road shows how to add up the watt-hours you use in a day and the longest stretch you plan to go without charging.
EcoFlow gives a rough daily estimate: rated watts multiplied by average sun hours multiplied by 0.75. EcoFlow UK Use sun-hour figures for your own area and season, not a summer figure for a winter trip.
Made-up example (not from any source, for arithmetic only): you use 600 Wh a day, you are planning a 200 W panel, and you assume 3 average sun hours. The estimate is 200 × 3 × 0.75 = 450 Wh a day. That covers part of your use, so the rest would need to come from a drive, a hook-up or a bigger panel.
The estimate is rough. Work out your own figures, then leave a margin. For how charging while driving can fill the gap, see charging while driving. For the wider list of kit to plan around your power, see the overlanding kit list.
FAQs
Will a panel on my car charge on a cloudy day?
The Energy Saving Trust says solar panels can still generate on cloudy days. Energy Saving Trust EcoFlow UK says cloud cover reduces generation, so expect less on an overcast day. EcoFlow UK
Why does my panel make less than its rated watts?
Ratings come from standard lab conditions, which EcoFlow UK says rarely happen in real life. Sun hours, winter, cloud, angle, shade and heat all reduce output. EcoFlow UK
Does the charge controller matter?
Yes. It controls how the panel's power reaches the battery, and Renogy says MPPT suits larger or higher-voltage arrays while PWM suits small, closely matched systems. Renogy
Can I leave a portable panel out while I am away?
Renogy UK says a portable panel cannot safely charge while unattended, because of theft and weather exposure. Renogy UK
Next: work out your daily energy need
Add up your daily watt-hours with the power on the road guide before you choose a panel.
More in Power
Sources
- Energy Saving Trust: panels still generate on cloudy days, more sunshine means more electricity, and an unshaded south-facing roof is the ideal setup (checked )
- EcoFlow UK: rated power is measured under Standard Test Conditions, real output is often lower, output falls with sun hours, winter, cloud, angle, shade and heat, and gives a rough estimate of rated watts x average sun hours x 0.75 (checked )
- Victron Energy: MPPT and PWM controllers, and model names that show the maximum PV open-circuit voltage and maximum charge current (checked )
- Renogy: MPPT tracks the array's operating point, PWM suits closely matched panel and battery voltages, and an MPPT controller still has a maximum PV input voltage to check (checked )
- Renogy UK: portable panels can be angled to the sun regardless of parking, need setup each time, should not be left charging unattended, cost more per watt, and give less output than roof-mounted systems (checked )