The panel says 200 W. The station shows a maximum input of 500 W. You do the maths: 1000 Wh of battery divided by 200 W gives five hours to recharge. Simple.
Except it never works out like that. Never.
I have spent weeks measuring real-world recharge times across different stations with different panels, at different hours, in different seasons. The finding is always the same: the actual time is between 1.5 and 3 times longer than the theoretical time. If you want to understand how the MPPT helps maximise your production, that is a good companion read. This is not a scam. It is physics. But if you do not understand why, you will be disappointed with your solar setup.
When a panel manufacturer announces 200 W, they are talking about power under STC (Standard Test Conditions): irradiance of 1000 W/m2, cell temperature of 25 degrees, air mass AM 1.5. These conditions exist in laboratories. Outdoors is another story.
In mainland Britain, irradiance hits 1000 W/m2 for only a handful of hours per day in midsummer, and almost never in winter. In March, I typically measure 700-800 W/m2 between midday and two in the afternoon, and 300-400 W/m2 in the morning and late afternoon.
Your 200 W panel will therefore produce 140-160 W in the best part of the day, and more like 60-80 W in the morning. Averaged across a full day of sunshine, you can bank on 60 to 70% of the nominal power. So 120-140 W on average, not 200 W.
First reality check.
You might think a solar panel loves blazing heat. Not really. Photovoltaic cells lose efficiency when they heat up. The temperature coefficient of a typical monocrystalline panel is around -0.35% per degree above 25 degrees.
In summer, the surface temperature of a panel on a campervan roof can climb to 65-70 degrees. That is 40 degrees above the reference. Which translates to -14% of power, just from heat.
And it is a vicious circle: the sunnier it is, the hotter the panel gets, the more efficiency it loses. Irradiance goes up but temperature goes up with it. The net gain is far less than you imagine.
That is why a panel on a ventilated roof (with an air gap between the panel and the roof) produces noticeably more than one bonded directly. The air circulating underneath carries away heat. In my measurements, the difference is 8 to 12% of production in summer. Not negligible.
A panel produces at its maximum when sunlight arrives perpendicular to its surface. On a campervan roof, the panel is horizontal. The sun is only perpendicular at the zenith -- and only under the tropics at that.
In the UK, the sun reaches a maximum angle of roughly 62 degrees above the horizon in summer (at the solstice) and around 15 degrees in winter. A horizontal panel therefore misses a significant portion of the available irradiance, especially in winter.
The effect is dramatic in the morning and evening. When the sun is 15 degrees above the horizon, a horizontal panel captures only 26% of available irradiance. At 30 degrees, it is 50%. That is why your panels seem to produce nothing before ten in the morning and after four in the afternoon during winter.
Some vanlifers install adjustable tilt brackets to angle their panels towards the sun. Effective -- 20 to 35% gain depending on the season -- but it adds complexity and weight on the roof.
Even if your panel produces well, electricity loses energy at each stage before reaching the battery cells.
Cables first. A five-metre run of 4 mm2 solar cable carrying 8 A loses roughly 2% of power. Seems like nothing, but it adds up. And if your cables are undersized or too long, it climbs fast. I have seen van setups with 10 m of 2.5 mm2 cable -- there you lose 6-7% in the wires alone.
The MPPT controller next. It adapts the panel voltage to the battery voltage to extract maximum power. A good MPPT runs at 97-99% efficiency. Those integrated into portable stations are generally decent, but not always at the level of dedicated units like the Victron SmartSolar. Budget 2-5% loss here.
The BMS (Battery Management System) and charge electronics add another 3-5% in losses. That is energy dissipated as heat whilst balancing cells and regulating charge.
Total chain losses: 7 to 15% of what the panel actually produces. Not the biggest factor, but one more layer stacked on top of everything else.
You park your van under a tree for shade inside. Logical. Except the shade also falls on your panels. And then it is a catastrophe.
A solar panel is not a simple surface that produces proportionally to its lit area. Cells are wired in series in strings. If ONE cell in a string is shaded, the entire string drops. Bypass diodes limit the damage by short-circuiting the shaded sections, but you still lose a disproportionate chunk of production.
A branch shading 10% of your panel surface can cause production to drop by 30 to 50%. I have measured this multiple times -- frustrating but reality.
My advice: always park in sunlight if you need to recharge. Seems obvious, but when you arrive at a wild camping spot at six in the evening and seek shade for a comfortable sleep, you easily forget that tomorrow morning your panels will be in shadow until eleven.
Let us take a concrete example. You have a 1000 Wh station to recharge from 20% to 100%, meaning 800 Wh to inject. Your panel is a 200 W unit on the van roof, in March, in southern England.
The theoretical calculation says: 800 Wh / 200 W = 4 hours. Easy.
The realistic calculation. Your panel produces an average of 120 W during the good hours (say 10 am to 4 pm, six usable hours). But it is not a constant 120 W -- it ramps up through the morning, peaks around noon, and falls off. Integrating the curve, you will inject roughly 600 Wh across those six hours. Not enough for a full charge.
You finish the charge the next morning. Total: 8 to 10 hours of useful sunshine for 800 Wh. Two to two-and-a-half times longer than the theoretical figure.
In summer it is better. Longer days, stronger irradiance. The same 200 W panel pulls 140-160 W average over 8-10 usable hours. That is 1100-1600 Wh in a day. A full 800 Wh charge is comfortably achievable.
In winter in northern England? Forget it. With three to four hours of usable sun and 60-80 W average, you are looking at three days to recharge 800 Wh. That is when a second panel or mains top-up becomes essential.
First tip: charge in the morning. The battery accepts more current when it is low. The MPPT works at its best between 20% and 80%. Above 80%, the charge algorithm switches to absorption then float -- current drops progressively. The last 20% takes as long as the first 50%. Run your hungry devices in the evening, let the battery sit low overnight, and exploit the morning sun for an efficient charge.
Second tip: clean your panels. Dust, pollen, bird droppings -- it accumulates and reduces production by 5 to 15%. A wipe with a damp cloth each week makes a real difference.
Third tip: if you have the choice, park facing due south. Obvious, but when wild camping you often pick the spot for the view rather than solar orientation. When recharging is critical, sacrifice the view.
Fourth tip: watch the station temperature during solar charging. If it exceeds 40 degrees (shown in the app), the station will throttle charge current to protect itself. In summer, place it in the most ventilated area of the van and open the windows.
Now that you understand the losses, you can size your installation correctly.
The rule I follow: plan for two to three times the panel wattage versus your theoretical calculation. If you need 500 Wh per day and want to recover it via solar, do not get a 100 W panel thinking it will do the job in five hours. Get 200 to 300 W of panels.
For a van in Britain, my baseline recommendation in March 2026: two rigid 200 W panels on the roof, giving 400 W nominal. In practice you will pull 200-300 W in summer and 80-150 W in winter. Sufficient to cover 500-800 Wh of daily consumption in summer, and to supplement significantly in winter (the rest from mains or the car charger).
Foldable panels of 100 W or 200 W as a complement, deployed when you are parked, are a solid addition for long stays in the sun. But do not rely on them as your primary source -- you will forget to set them up, you will not be bothered, and they catch a gust of wind when your back is turned. That has happened to me twice. The second time, the panel landed in a bush ten metres away. It survived, but my patience did not.
A word on foldable panels, because many beginners start with them before fitting rigid ones on the roof.
Foldable panels (see our comparison of the best panels for 2026 for specific models) have one huge advantage: you can orient them towards the sun. A well-angled foldable produces 20 to 40% more than a horizontal rigid panel on the roof. In theory, brilliant.
In practice, it means you need to be present to adjust the angle every hour or two. It means you need a stable spot to prop the panel. And it means it should not be too windy -- a 200 W foldable has a fair bit of sail area.
The real-world yield of a foldable relative to its nominal power is similar to a rigid -- same cells. However, foldable panels often have less well-ventilated cells (fabric backing versus aluminium frame with air gap), which makes them run hotter and lose a touch more to temperature.
For choosing between formats, read my foldable vs rigid comparison. My view: the foldable is an excellent supplement to a fixed roof installation. As a sole source? Workable for occasional camping, but a daily hassle in full-time vanlife.
To figure out your watt-hour needs and estimate your output with our solar calculator, you will have all the cards in hand. There you have it. Solar recharging is wonderful, but never as quick as it looks on paper. Accept it, size accordingly, and you will never be disappointed.
Because the 200 W is measured in a laboratory under perfect conditions you never encounter outdoors. In real conditions, you get 60 to 70% of the nominal power -- so 120 to 140 W in full summer sun. Panel temperature, angle of incidence, clouds, and cable losses eat the rest.
In summer in southern Britain, aim for 30 to 35 degrees from horizontal, facing due south. In winter, steepen to 50-60 degrees to capture the low sun. The trick: watch the power reading on your station and adjust the angle until you find the maximum. A well-oriented panel produces 30 to 40% more than one lying flat.
In summer in the south, you get 6 to 8 hours of usable sun per day. In Scotland or the north-west, count 4 to 5 hours. In winter, it is 2 to 4 hours in the south and 1 to 3 hours further north. These figures are for useful sun -- the first and last hours of the day produce very little.
Cedric
