There has been a relatively low uptake of solar in shipping, which, if one considers the extent to which it has rapidly expanded on shore, is a surprising realisation.

Coupled with cheaper battery production, new solar grew enough to meet all power-demand growth on land globally in 2025, according to global energy think tank Ember.

But at sea, progress has been considerably slower, limited to a handful of working prototypes. Over six months, Mahi Two, a tiny autonomous boat, made a 4,300nm crossing from Spain to Martinique on solar power alone, ending its crawling six-month voyage in April 2022.

In 2025, Silent 62, a pleasure yacht, crossed from Gibraltar to Antigua, 3,800nm using 72% solar power. To put this into perspective: Solar Impulse 2, a solar-powered aircraft, circumnavigated the globe in 2016.

The fact that solar generation is only now being adopted more widely on ships is notable; perhaps more notable, however, is that this development is not being led by China, despite its dominant position in the three industries central to maritime solar power: terrestrial solar energy, batteries and shipbuilding.

Instead, in July 2023, the Netherlands-based Wattlab installed its first system on Vertom Cyta at quayside in Rotterdam. This was followed in 2024 by a solar ‘flatrack’ system on a recently launched 7,280dwt general cargo vessel.

Vertom Anette, was fitted with a Wattlab five-panel SolarDeck modular rack system, secured to the ship’s hatches with twist-lock fittings. Not installed in drydock in Dalian, but alongside at Nijmegen. In the seven months that followed, the system generated 496kWh of energy, leading to a fuel saving of 123litres.

“The test results show that SolarDeck performs well in the tougher environment of coastal shipping,” said Wattlab CEO Bo Salet in early 2025. “Because salt water can drain freely from the solar panels, there’s no chance of a salt crust forming. As such, SolarDeck generates the expected power output levels.”

Asked why it has taken solar panels so long to find their way onto commercial ships, Salet told RINA: “I believe the main reason is that the maritime environment is extremely demanding. Equipment must withstand salt water, wind, vibration, vessel movement and heavy operational use, so everything needs to be thoroughly tested and proven before it can be deployed at scale.”

A larger subsequent installation on Vertom Tula, incorporates some 44 flatracks with 176 individual panels, generating up to 79kWp. This was followed, in July 2025, by a 35kWp solar panel system on HGK Shipping and Salzgitter AG’s Blue Marlin.

Although not on racks, the installation features 192 individual panels, which, on a sunny day, can contribute enough power to substitute one of the vessel’s four diesel generators.

“Furthermore, in situations where the ship is lightly loaded and travelling downstream, we anticipate that it may even sail using only solar power for limited periods,” claimed Wattlab co-founder and COO David Kester.

“The practical integration into the vessel’s daily operations is equally important. Close involvement from the crew is crucial: their experience and feedback help us understand how the system performs in real-life conditions and how it can be improved without interfering with cargo operations or other activities on deck.

“Developing a reliable maritime solar system therefore takes more than simply placing conventional solar panels on a ship,” Salet explained.

For example, newer Wattlab systems seek to address loose cabling, visible on the deck of Vertom Tula. “This was part of an earlier system iteration,” he said. “We agreed that the cable routing needed to be improved, and this has already been addressed in our latest design. The new iteration has a much cleaner and more integrated cabling solution.”

Though pioneering, Wattlab has since been joined by other suppliers. As part of ambitious retrofit programme, Western Baltija Shipbuilding, part of BLRT group, installed some 1,600m² each of solar panels on sister vessels Trans Hav and Trans Sol, part of a brace of upgrades which also included rotor sails, and a new weather deck to expand each vessel’s cargo capacity.

According to the author’s calculations, each solar installation could produce around 1,000kWh on an average North European day – contributing a not insubstantial share of the ship’s hotel load. Taking the panels together with other fuel-saving technologies, including rotor sails, Trans Hav and Trans Sol are expected by owner Sea Cargo to achieve 50% lower CO₂ emissions post-retrofit.