Salt batteries for photovoltaics

“Salt battery” is the name commonly used in Italy to refer to sodium-ion batteries: batteries that use sodium — the element found in common table salt — instead of lithium to store energy. Heiwit manufactures them for residential and commercial solar PV systems and sells them through the Heiwit installer network. It commissioned its first sodium-ion battery on a solar PV system in June 2025. We are not aware of any earlier installations in Italy.

On this page you will find what they really are, how long they last according to field data, what determines the price and in which cases they are worthwhile compared to lithium. The technical abbreviations, such as BMS and LCOS, are explained in the sodium storage glossary.

The net cost also depends on the financial assistance available: you can find out exactly what is available, and under what conditions, in the incentives for photovoltaic energy storage.

Warning: “salt battery” refers to three different technologies

It's the most common confusion, and it's worth clearing it up right away because the three families have very different performances:

Technology What is this about? I use domestic photovoltaic energy
Sodium ions (Na-ion) Same lithium battery architecture, with sodium as the charge carrier Yes — it is Heiwit technology
Sodium-nickel-chloride (ZEBRA) High-temperature battery, works above 250 °C No, designed for industrial and transport uses
Saltwater Salt-based electrolyte dissolved in water Rarely: low density, bulky

When we talk about salt batteries here, we always mean the first one: sodium ions. It is the only one of the three that is actually entering homes, because it has size and performance comparable to lithium.

How a salt battery works

The principle is identical to that of a lithium-ion battery. During charging, sodium ions move from the cathode to the anode through an electrolyte; during discharge, they make the reverse journey, and that movement generates the current that powers the home. What changes is not the mechanism, but the material that acts as the shuttle.

The difference is felt most significantly on three fronts:

  • Raw materials. Sodium is one of the most abundant elements on Earth and can even be extracted from seawater. Heiwit cells contain neither lithium nor cobalt.
  • Thermal stability. Sodium-ion chemistry is less prone to thermal runaway, the phenomenon responsible for fires in lithium batteries.
  • Behaviour in the cold. Where lithium noticeably loses capacity, sodium withstands much lower temperatures while maintaining useful performance.

How long it really lasts: field-measured data

As for the lifespan, there are many laboratory estimates and few real figures. These come from the Heiwit fleet installed and monitored in Italy:

Parameter Measured value How was it measured
Design life per individual battery 6,500 cycles Test with 90% depth of discharge (DoD) and residual capacity above 70%
Cumulative cycles of the entire battery pack 31.972 Total aggregated across all installed units, as at 17 September 2026
Round-trip efficiency (RTE) 95% Energy returned compared with energy stored in a full cycle
Self-discharge 0.35% per month 6% lost over 539 days, from 31/10/2024 to 23/04/2026, across several batteries, at room temperature, at the latitude of Milan

The self-discharge figure is the one worth looking at most closely: it means that a battery left idle for a whole year retains over 95% of the energy stored. For anyone with a second home or a system with long periods of inactivity, it makes a real difference.

Safety and temperatures: what the datasheet and the TÜV SÜD test say

The Heiwit battery has these temperature ranges:

  • Discharge: from −25 °C to +60 °C
  • Charge: from 0 °C to +60 °C
  • Storage: from −40 °C to +70 °C
  • Permissible relative humidity: from 0 to 100 per cent

The system comprising the Virgo inverter and the battery also holds CEI 0-21 certification issued by TÜV SÜD on the basis of the type tests (document no. D 132944 0003).

In practice the battery must be installed indoors, including unheated garages or cellars. Below 0 °C it keeps supplying energy, but it must not be charged until the temperature rises above zero again.

How much does a salt battery cost

The cost of a battery storage system depends on variables that change from system to system. As a guide, the recommended retail price of the 10 kWh Heiwit battery is €4,900 VAT included. The final quotation is prepared by the installer.

What determines the final price:

  • Installed capacity. A 10 kWh and a 20 kWh do not scale linearly: as the size increases, the cost per kWh tends to fall.
  • Inverter. A Heiwit inverter is always required: the battery does not work with inverters from other brands. On an existing system a retrofit is carried out: the Heiwit inverter is connected on the AC line and the existing solar PV inverter stays in place.
  • Installation type. Single-phase or three-phase, battery position, run lengths, necessary electrical works.
  • Tax deductions. Tax relief for energy efficiency improvements has a significant impact on the actual cost and must be assessed on a case-by-case basis.

The parameter that really matters is not the purchase price, but the’LCOS (Levelised Cost of Storagehow much each kWh that the battery stores and discharges costs over its entire lifespan. A battery that costs more but withstands many more cycles can have a lower LCOS — and this is precisely the ground on which sodium competes with lithium.

Request a quote for your system providing consumption and configuration figures: we will get back to you with a quote tailored to your specific case, not a generic price list.

Salt or lithium batteries: when sodium is the better choice

Sodium ions (Heiwit) Lithium LFP
Critical raw materials Neither lithium nor cobalt Lithium, partly graphite
Low-temperature performance Full power down to −25 °C Capacity loss below 0 °C
Thermal runaway risk Significantly reduced Present, managed by BMS
Energy density Lower: for the same number of kWh it takes up more space Superior
Purchase cost per kWh Generally higher today Today's low
Recyclability High, with no lithium or cobalt Complex

Sodium makes economic sense when space is not a constraint, when the installation is in a cold or unconditioned environment, when fire safety is a priority (indoor spaces, commercial businesses, accommodation facilities) and when thinking about whole-life cost rather than initial outlay.

Lithium remains the preferred option if space is very limited or if the initial budget is the main constraint. Let’s be clear: it’s not the right technology for every situation.

For a new or existing system

The 10 kWh Heiwit salt batteries are modular and can be installed in both scenarios:

  • New system: The battery and hybrid inverter are sized together based on the actual energy consumption of the home.
  • Existing system (retrofit): the storage is added to a solar PV system that is already running, including other brands. The battery is connected to a Heiwit hybrid inverter, installed on the AC line alongside the existing inverter, which stays in place. This is the most common case among those with a system installed before battery storage became cost-effective.

Standard module features: 10 kWh capacity, 48 V, maximum power 5,000 W, response time like a UPS 10 ms, weight 130 kg, 10-year Italian warranty on the battery and inverter. Each Heiwit inverter, Virgo or Lybra, manages up to 3 modules of 10 kWh, that is 30 kWh.

Frequently asked questions about salt batteries

Are salt batteries already available in Italy?

Yes. Several articles online still claim that the technology is not commercially available, but Heiwit commissioned its first sodium-ion battery in June 2025. Since then, installers in the Heiwit network have completed systems throughout the country, from Lombardy to Sicily, both residential and commercial.

Can a salt battery catch fire?

The risk is significantly lower than with lithium, as sodium chemistry is less prone to thermal runaway. No battery is risk-free, but thermal stability is one of the main reasons why this technology is chosen for indoor environments and commercial activities.

How long does a salt battery last?

It depends on the cycles, not the years. With one full cycle per day, a design life of 6,500 cycles corresponds to approximately 18 years of operation. As at 17 September 2026, the fleet of Heiwit batteries monitored in the field had accumulated a total of 31,972 cycles.

Does it also work in winter or in the mountains?

Yes, but there is a limitation to be aware of. The battery discharges between −25 °C and +60 °C, but it only charges between 0 °C and +60 °C, and must be installed indoors. Its cold-weather performance during discharge is one of its most notable advantages over lithium batteries.

Can it be added to an existing photovoltaic system?

Yes, by retrofit, including on systems from other brands. However, the battery only works with a Heiwit inverter. Inverters from other brands do not support the voltage range of sodium batteries. The Heiwit hybrid inverter with the battery is connected on the AC line and the existing inverter stays where it is. Either the single-phase Virgo or the three-phase Lybra is used, depending on the system.

How much does a 10 kW salt battery cost?

The recommended retail price of the 10 kWh Heiwit battery is €4,900 VAT included. For installation on an existing system, the recommended price is €500. The total depends on the inverter, the type of installation and the applicable tax deductions: requesting a quote you receive an amount calculated on your system.

Do salt batteries contain lithium?

No. Heiwit sodium-ion cells contain neither lithium nor cobalt.

Insights

Heiwit S.p.A. designs and installs sodium-ion energy storage systems for photovoltaic plants. Operational headquarters in the province of Varese, with a network of partner installers throughout the national territory.