No cooling systems, no bulky containers, and no lithium: Dutch startup Moonwatt is rethinking battery energy storage systems (BESS) for hybrid solar power plants. Its sodium-ion systems operate without active cooling and can be distributed directly across solar fields, unlocking new efficiencies in design, cost, and performance.
In this interview, Valentin Rota, CCO of Moonwatt, explains, among other things, why a specialization in storage technology provides greater advantages than the typical "Swiss Army knife" generalist approach.
Moonwatt was founded in 2024 by Zukui Hu, Guillaume Mancini, and me, Valentin Rota. The three of us met in 2016 while working at Tesla in Amsterdam. Each of us previously worked at companies such as Siemens Wind Power, EDF, and Scatec Solar. In 2016, energy storage was still at an early stage: projects were small, applications were emerging, and costs were high. However, we clearly saw the long-term potential of hybrid power plants with solar and storage, particularly in remote and overseas territories.
Today, PV panels and batteries are relatively inexpensive. The main cost drivers in projects are now the electrical balance of plant and grid connections. At the same time, grid congestion has become a major barrier to deploying new solar capacity. Solar capture rates are also falling, and solar price cannibalization is threatening the economic viability of stand-alone solar plants. This combination creates strong value for hybrid PV and storage systems, particularly when the storage is integrated directly on the low voltage DC side, which allows significant simplification and savings on the balance of plant infrastructure.
These insights and our work experience at different companies led us to found Moonwatt. From the start, we wanted to focus on hybrid solar storage solutions, with sodium-based batteries as a key enabler.
Instead of relying on large, containerized systems, Moonwatt develops small, modular enclosures that are easier to manufacture at scale and simpler to install, maintain, and replace.
Moonwatt’s smaller battery storage units are distributed throughout the PV plant. This is possible because the batteries operate using fully passive cooling. We do not use HVAC or any form of active cooling, which has traditionally driven the adoption of larger container formats. Without active cooling, we can design compact storage units that sit directly next to solar panels.
This physical proximity allows us to connect directly on the DC side, both for new and existing PV plants, without redesigning the entire electrical architecture or adding medium-voltage infrastructure. The result is a simpler, more efficient hybridization of solar assets.
Most battery systems on the market are designed as “Swiss Army knife” solutions. They aim to serve many use cases – front-of-the-meter, behind-the-meter, or pv co-location – which leads to AC-coupled designs and compromises in efficiency and cost when applied to pv plants.
With no active cooling, our systems have no auxiliary power consumption, which results in higher round-trip efficiency. In addition, our DC-coupled architecture enables charging from solar to battery to grid with approximately 2–3 percent higher efficiency compared to conventional, multi-purpose systems.
We selected sodium‑ion chemistry, in particular the NFPP subfamily (sodium iron phosphate–pyrophosphate). NFPP plays a similar role in the sodium-ion family as LFP plays in the lithium-ion family. This decision is based on several factors:
Temperature tolerance
Sodium‑ion cells operate efficiently across a very broad temperature range. Lithium‑ion batteries typically require active cooling to reach a temperature close to 25 degrees Celsius, but sodium‑ion cells tolerate both high and low temperatures, making them ideal for passively cooled systems.
Cycle life and degradation
Testing shows that sodium‑ion cells deliver strong cycling performance, in many cases better than lithium iron phosphate (LFP). The chemical structure of sodium‑ion cells is more stable, which allows a higher number of cycles with lower degradation. Current results indicate that NFPP sodium‑ion cells can exceed 12,000 cycles.
Safety
Although sodium‑ion cells have lower energy density, this reduces thermal risk and makes the cells less prone to fire. We are working to demonstrate that sodium‑based systems can achieve a higher safety level than conventional LFP solutions.
Cost
Cost competitiveness can be assessed in two ways: First, in absolute price per kilowatt‑hour, and second, in levelized cost of storage (LCOS), which is the total system cost divided by the total energy throughput over the battery lifetime.
Moonwatt expects sodium‑ion NFPP batteries to reach absolute price parity within two to three years, and LCOS parity sooner, most likely within 12 to 18 months, because NFPP cells provide higher cycle life than LFP.
Market trends support this trajectory, with lithium‑ion prices rising and sodium‑ion costs declining. It is also important to note that for Moonwatt’s solution, a large share of total savings is achieved at the system level, beyond the chemistry‑to‑chemistry cost comparison.
Sustainability
Sodium‑ion NFPP batteries use a simpler and more sustainable materials set, reducing dependence on scarce raw materials and reinforcing the long‑term cost advantage of this technology. Sodium-ion NFPP batteries do not use lithium, cobalt, nickel, manganese, or copper. Sodium is one of the most abundant elements on Earth and is about 1,000 times more common than lithium.
The battery industry has historically been focused on reducing upfront capital expenditure. As storage systems are now expected to operate reliably for twenty years or more, priorities have shifted toward long‑term performance, availability, and total cost of ownership.
Our team comes from the BESS sector and has seen many systems fail in the field. In most cases, failures are linked to complexity, particularly liquid cooling systems, pumps, and auxiliary components that leak, degrade, or break over time. As a result, we designed our product around the principle that fewer parts lead to better reliability.
Performance, for us, is primarily measured by availability. By eliminating active cooling and minimizing moving parts, we significantly reduce failure points and downtime. This improves lifetime availability and lowers the levelized cost of storage when downtime and maintenance are properly accounted for.
All engineering and intellectual property of Moonwatt are based in Western Europe. The first manufacturing phase takes place in Asia in order to scale production quickly and reach the required volumes efficiently.
In a second phase we will focus on establishing dedicated production capacity in Europe, concentrating on Moonwatt’s core value creation: its unique and proprietary system integration.
This European capability would be developed in parallel with the existing Asian supply base, enabling us to serve customers closer to their projects while maintaining a resilient and globally diversified supply chain.
Our first deployment takes place in the Netherlands in mid-2026. In early 2027, the company will start commercial‑scale deployments across Europe. Our core initial markets are Germany, the Netherlands, Spain and the UK. Beyond these, we are seeing more opportunities to implement our systems for hybrid power plants in markets such as France, Denmark and Romania.
We design our systems as a standardized solution, meaning the same technical architecture can be deployed across different countries without modification. This standardization supports large-scale manufacturing, with units produced by the thousands to reduce costs. In parallel, we plan to establish a European storage and service hub to ensure fast access to replacement parts.
Our goal is to take a pan-European approach within the first three years, followed by expansion into the Middle East and Africa, and subsequently into global markets such as Australia and the United States.
