Every truck roof is a power plant.

For years, solar on vehicles sounded like a gimmick. A truck burns tens of thousands of litres a year; a few square metres of panels cannot move 40 tonnes. That is still true. But it misses where trucks actually spend their electricity.

A modern truck carries a growing list of electrical loads that have nothing to do with driving: the cab climate system during rest breaks, the tail lift, telematics, lighting, and above all the refrigeration unit on a reefer. On a diesel truck all of that energy comes from burning fuel, either through the alternator or through a separate diesel engine on the reefer. On an electric truck it comes out of the same battery that sets the range. Solar is very good at exactly this job: steady, modest power, available whether the truck is driving or parked.

Three things changed in the last two years.

• The data arrived. The EU-funded SolarMoves project put irradiance sensors on 23 vehicle types and logged 1.3 million km across Europe. Its results, published in 2026, put real numbers on trailer yields and payback.⁴⁶
• Approval paths opened. In January 2025 Sono Motors' retrofit solar kit for buses became the first vehicle-integrated PV product to get a general operating permit (ABE) in Germany. Until then, the authority had no test criteria for it at all.¹⁸
• Diesel got expensive. French diesel averaged 2.35 €/L in early October 2026, driven by refining margins and supply disruption in the Gulf.²⁵²⁶ Trailer builders Langendorf and Royal Burgers now fit IM Efficiency's SolarOnTop system as standard, and retailer Action took 250 trailers with it this year.¹⁰

The honest answer to "does it work?" is that it depends on four things: the vehicle, how it is used, where it drives, and what the solar power replaces. The rest of this guide walks through them in that order.

"Trucks" covers very different machines. Before talking about solar, we sort them into five families. What matters for solar is the flat, sun-facing surface each one offers and whether that surface stays with the energy user.

Buses are a different business from freight, but they belong here. They run fixed routes, stand in depots in daylight, and carry heavy climate-control loads. FlixBus is expanding Trailar solar mats across its coach fleet after four years of pilots.¹⁶

Two trailers with identical roofs can have completely different business cases. Three questions decide it.

Electric or diesel?

On an electric truck, solar feeds the traction battery and adds range. Fraunhofer ISE's model suggested 4,000 to 6,000 extra kilometres a year for a truck;² SolarMoves found up to 15% more daily range without charging.⁴ On a diesel truck, solar cannot move the vehicle. It replaces the fuel burned to make electricity, which is a smaller but surprisingly expensive amount (see chapter 08).

Refrigerated or ambient?

A reefer is the single largest electrical consumer in road freight. A diesel transport refrigeration unit burns roughly 2.8 L of fuel an hour on average, for an electric equivalent of about 2.9 kWh an hour.²⁷ That load runs whether the truck is moving or parked, which matches solar well. Ambient vehicles still have tail lifts, lighting and telematics, but the loads are smaller.

What route, and where does it park?

Urban stop-and-go means frequent door openings and tail-lift cycles, short distances and a lot of time parked in the open. Long haul means steady driving and long rest breaks with the cab heated or cooled. A trailer that sits in a covered yard or under a dock canopy loses part of its yield. And a trailer that is often uncoupled only helps the tractor while they are connected.

Everything starts with square metres. Modern vehicle modules deliver around 200 Wp per square metre; Fraunhofer's reference module was 21% efficient, or 210 Wp/m², with 95 to 96% of a truck roof covered.³ The rule of thumb is simple: one square metre of roof is about 0.2 kWp.

That makes the trailer the obvious winner. A 13.6 m semi-trailer roof is about 34 m², or 6.9 kWp.³ Scania's research trailer went further, with 140 m² of cells along the roof and both sides.¹¹ But side walls are vertical and often shaded: SolarMoves measured 1.3 kWh/m² a day on side sensors against 2.8 on the roof.⁵ A square metre on the side is worth about half a square metre on the roof.

Coupled or not coupled

A trailer's surface helps the tractor only while the two are connected. Fraunhofer ISE compared both setups for an electric semi.³ When the trailer could deliver power only while driving, solar covered 0.9 to 1.6% of the energy demand. When the energy was stored on the trailer and used whenever needed, the share rose to 5 to 9%. The lesson: solar on a trailer works best when the trailer has its own battery and its own consumer, such as a reefer or a tail lift.

A flat roof in Seville produces roughly 70% more per kilowatt-peak than one in Stockholm. Working from Fraunhofer ISE's simulations for a truck-mounted system, a flat vehicle roof yields about 840–880 kWh per kWp a year in Stockholm, about 975 in Freiburg and about 1,450–1,490 in Seville, before shading.³ Paris sits close to Freiburg.

Shading is the big unknown. Those simulations ignore it. SolarMoves was built partly to measure it, and its researchers name shading and driver behaviour as the two biggest gaps between models and reality.⁵ Where the truck parks matters as much as where it drives: a reefer parked in an open yard all weekend collects far more than one parked against a warehouse wall.

Standby losses matter too. A charging system that draws 25 W around the clock costs about 3% of the yearly yield. One that keeps the vehicle's 700 W electronics awake to charge wipes out about 60%.³ Good electronics wake only when there is sun to harvest.

Electrically there are three ways to connect solar to a truck. Which one you choose follows from the electric-or-diesel question in chapter 03.

A. The 24 V board net

The simplest and most common setup. Thin modules on the roof feed a charge controller that tops up the vehicle's starter or auxiliary batteries. The energy then powers the tail lift, cab loads and electronics instead of the alternator. DHL's trials with Trailar mats reported 4–8% fuel savings on rigid trucks and 2–4% on a trailer with a tail lift; installation took under six hours on a rigid and under twelve on a trailer.¹⁵ Reefer makers use the same idea at small scale: Carrier sells 50 W and 110 W panels just to keep the reefer battery charged, so the diesel engine does not have to start for it.²¹

B. A dedicated reefer battery

Here solar charges a large battery on the trailer that runs an electric refrigeration unit. This is where the big savings sit, because it replaces a whole diesel engine rather than some alternator load. Sunswap's Endurance units drew 78% of their yearly energy from the roof, up to 98% in summer and 24% in winter.¹³ The battery and the electric reefer unit are the expensive part; the solar makes them cheaper to run.

C. High voltage, into the traction battery

On an electric truck the prize is the traction battery itself. Fraunhofer ISE fed a 3.2 kWp roof straight into the 800 V battery of an 18-tonne truck through compact high-voltage power electronics. One practical lesson from the test: plug in only in the evening, so the battery has room to take the day's solar first.¹

Fixing the modules

Vehicle modules are glass-free and thin. Trailar's mats are 2 mm thick;¹⁵ IM Efficiency's panels weigh about 6 kg each, or 24–72 kg for a full system.⁸ Fraunhofer set a ceiling of 2.6 kg/m² for its truck modules,² and lightweight building modules such as Sunman's eArc sit around 4 kg/m².²⁹ Most roof systems are bonded with structural adhesive; some are mechanically clamped so they can move to another vehicle. Whatever the method, the roof skin, the adhesive and the cable entry all have to survive washing brushes, vibration, frost and 90 km/h of wind for a decade.

A panel that detaches at speed is a projectile. This is the part of the project to take most seriously, and the part where the rules are still catching up.

Height and width

In the EU, Directive 96/53/EC caps vehicles in international traffic at 4.00 m high and 2.55 m wide, or 2.60 m for refrigerated bodies with insulated walls at least 45 mm thick. Member states may allow taller vehicles for purely national transport.²³ Many trailers are built right up to 4.00 m, so the module, its frame and any edge trim must fit inside whatever millimetres remain. A 2 mm mat bonded flat usually fits; a framed panel on rails often does not. The same logic applies to side panels and the 2.60 m width.

Approval

There is still no single EU procedure for vehicle-integrated PV. The early movers created their own paths. Fraunhofer's solar e-truck was cleared for the road by TÜV and DEKRA.¹ Sono Motors defined and ran its own test programme with Germany's KBA and TÜV to win the first general operating permit for vehicle-integrated PV in January 2025, which it says should ease approval in other UNECE countries.¹⁸ In practice, plan for three checks: the mechanical fixing (adhesion and pull-off under load), electromagnetic compatibility of the charge electronics with the vehicle (UNECE Regulation 10 territory), and, for high-voltage systems, electrical safety and isolation from the traction network. Ask the supplier which approvals the specific system holds for your vehicle type and country.

Weights and dimensions are being rewritten

The EU is revising its weights and dimensions directive, mostly to give zero-emission trucks extra weight allowance for their batteries. As of May 2026 the file was still in trilogue between Parliament and Council.²⁴ SolarMoves has separately asked for vehicle solar to be counted in the VECTO model used for truck CO₂ standards and in the Renewable Energy Directive.⁴ Neither change exists yet.

Insurance and the fleet

Treat solar like any other body modification. Tell the insurer before the first trip, keep the installer's documentation and approval with the vehicle file, and agree who pays if a module is damaged by a low bridge, a branch or a wash brush. Add the system to the periodic inspection checklist: adhesive edges, cable entries and connector seals. Drivers and depot staff need one short briefing, especially on high-voltage systems.

The business case comes down to one question: what does a kilowatt-hour cost on this vehicle today? The answer depends on how the vehicle makes its electricity.

• Through the alternator of a diesel truck. The engine turns fuel into motion at a fraction of its energy content, and the alternator loses more on top: alternator efficiency ranges from about 55% for old units to 80% for the best today.²⁸ We work with about 0.35 L of diesel per kWh as a planning figure, and 0.3 to 0.5 as the realistic range.
• Through a diesel reefer engine. Using EPRI's averages of 2.8 L per hour against an electric equivalent of 2.9 kWh per hour, each kWh of cooling costs close to 1 L of diesel.²⁷ This is why reefers are the strongest case.
• From the grid, for an electric truck. Here solar is worth your charging price per kWh, plus the time saved at chargers.

At French pump prices of 2.35 €/L including VAT,²⁵ a business that recovers VAT pays about 1.95 €/L. A kWh made by an alternator then costs around 0.70 €, and a kWh of diesel refrigeration close to 1.90 €. Grid electricity is far cheaper, which is why the payback on electric trucks is slower even though the energy share is higher.

Savings that are not fuel

Flat batteries are one of the most common roadside breakdowns. IM Efficiency puts a 24 V battery replacement at 350–500 € and a dead-battery breakdown at 500–800 €, and claims solar support stretches battery life from two years to as many as six.⁸ These figures are the vendor's own, but fleet managers will recognise the costs.

Read vendor numbers as upper-end results from well-chosen vehicles. The independent studies show the same direction with more modest figures. The calculator below lets you plug in your own assumptions and see where the payback lands.

Choose a vehicle, where it drives and what the solar replaces. Every default comes from the sources above or is labelled as an assumption; change any of them to match your fleet and your quote. This is a planning tool, not an offer.

A short tour of the projects that have produced public results, from research fleets to commercial roll-outs.

Most fleets should start small: five to ten vehicles of one type, on one route, with fuel or energy metering before and after. These are the questions we work through with every fleet.

Vendor sources are marked. Figures derived by ALL IN SOLAR from a source are described as such where they appear.