1,880-watt solar system with 2 Nexol heating elements and a controller for a hot water boiler

The configurable PV retrofit kit from Offgridtec (MPN: 025256) adds direct electric water heating using solar power to a compatible hot water storage tank. You can choose between the Nexol NEX R1, which features a single screw-in heating element, and the NEX R2, which features two heating elements and dual-zone control. Depending on the variant, three to five Trina Solar modules provide 1,410 to 2,350 Wp. The Energy Controller utilizes the PV direct current without a separate inverter and also enables demand-driven supplementary heating via 230 V. A hot water storage tank is not included in the kit.

The advantages of the PV retrofit kit

• Choice of NEX R1 with one radiator or NEX R2 with two-zone heating
• Direct PV water heating without a separate inverter
• Configurable module power from 1,410 to 2,350 Wp
• Optional supplementary heating via a 230-V mains connection
• For compatible hot-water storage tanks with a capacity of 100 to 300 L
• Includes 23 to 25 m of H1Z2Z2-K solar cable and two PV Stick sets
• Integrated MPP tracking

The system consists of:

• Energy Controller
  • 1x Nexol NEX-R1 with 1x screw-in heating element for PV and grid
  • 1x Nexol NEX-R2 with 2x screw-in heating elements for PV and the grid
• Trina Solar 470W Vertex S+ double-glass N-type i-TopCon solar module (3–5 units, depending on selection)
• Solar cable H1Z2Z2-K 1X6 WH/BK single-conductor EN50618 IEC60228 (23–25 m, depending on selection)
• 2x Weidmüller PV-Stick Set

Important Note: The 5-module configuration is intended only for locations where the minimum module temperature does not fall below −15 °C. For colder locations, such as parts of Scandinavia or alpine regions, select the 3- or 4-module configuration.

PV Power Directly for Water Heating

The Nexol Energy Controller feeds the direct current from the series-connected module string to the heating system via an integrated MPP tracker. No additional inverter is required for this. The maximum usable PV power is 1.5 kW. A higher installed module power does not increase the maximum heating output but can provide the system with more available PV power when solar irradiance is low. The optional grid-connected backup heater operates at 230 V and 1.5 kW. The touchscreen display allows you to monitor and adjust the temperature, PV output, and heating cycles. A configurable high-temperature cycle supports regular heating to temperatures exceeding 60 °C. Regardless of this, the hygiene requirements of the specific drinking water system must still be observed.

Compatibility Check for the Existing Hot Water Storage Tank

The existing storage tank must have a volume of 100 to 300 liters and a suitable socket for the NEX R1 or two matching sockets for the NEX R2. A 1½-inch thread, a socket length of no more than 90 mm, and at least 400 mm of available installation space are required. The radiators must be installed horizontally. The lower socket should be positioned as low as possible; for the NEX R2, the upper socket must be located in the upper third of the tank. Other heat sources must not heat the tank to more than 80 °C. If suitable inspection openings are available, a compatible adapter flange plate may be used as needed. The storage tank, flange plate, and radiators must be inspected by a qualified contractor for mechanical and hydraulic suitability prior to installation.

Select the NEX R1 or NEX R2 to match the storage tank

The NEX R1 is suitable for hot water storage tanks with a compatible free threaded socket. Its combined PV and mains-powered heating element is mounted as low as possible in the storage tank and heats the water from the bottom up. The maximum heating output is 1.5 kW.

The NEX R2 requires two suitable threaded sockets and enables two-zone heating. The combined PV and grid-connected heating element is mounted in the upper third of the storage tank. It first heats the upper portion of the tank, from which hot water is drawn. Once the set temperature is reached, the system switches to the lower heating element—which is powered exclusively by PV electricity—and heats the remainder of the storage tank’s contents. The designation of the R2 variant with two heating elements does not mean that 3 kW is permanently available at the same time. The controller uses a maximum of 1.5 kW of PV power and controls the two storage zones sequentially.

• Retrofitting Existing Systems – Compatible with 1.5" E-Sleeve
• Ensuring a hot water supply
• Hot water production using solar power
• Smart touch-display control
• Heating support
• Legionella prevention program

470 W Trina Solar Vertex S+ double-glass modules

Thanks to modern technologies such as high-density cell interconnection, the spacing between solar cells is minimized, allowing more sunlight to be harnessed. Multi-busbar and N-type i-TOPCon technology further ensure higher light conversion efficiency, lower energy losses, and better current conduction. With an efficiency of 23.5%, the module reliably delivers energy even on cloudy days or during periods of shade. The robust double-glass construction provides optimal protection for the panel against weather conditions and fire.

• High-Density Cell Interconnection Technology
• Double-glass for optimal reliability
• Low series resistance, improved current conduction
• Excellent resistance to fire and environmental influences
• 5,400 Pa shear load and 4,000 Pa wind load (test loads)
• High compatibility with inverters

Technical Specifications

Nexol Energy Controller

• Maximum PV open-circuit voltage: 300 V
• Maximum PV short-circuit current: 15 A
• PV connection: MC4
• Mains connection: 230 V, 50–60 Hz
• Required fuse rating: 10–16 A
• Display: 2.8-inch color touchscreen
• Ambient temperature: 5 to 40 °C
• Protection rating: IP21
• Weight: 2.4 kg
• Dimensions: 339 × 177 × 104 mm

Nexol Screw-In Heater

• Quantity: 1 for NEX R1 or 2 for NEX R2
• Maximum heating power of the active heating element: 1.5 kW
• Heating element length: 350 mm
• Installation orientation: horizontal
• Screw-in thread: 1½ inches
• Unheated area: 100 mm
• Maximum socket length: 90 mm
• Required installation space: at least 400 mm
• Protection rating: IP65
• Dimensions of the PV/grid heater: 89 × 89 × 500 mm

Trina Solar Vertex S+ 470 W

• Rated power per module: 470 Wp
• MPP voltage: 46.1 V
• MPP current: 10.20 A
• Open-circuit voltage: 54.6 V
• Short-circuit current: 10.89 A
• Module efficiency: 23.5%
• Junction box protection rating: IP68
• Weight: 21 kg per module
• Dimensions: 1762 × 1134 × 30 mm

Installation by qualified professionals

Tank conversion, radiator installation, ground wire connection, PV cabling, grid connection, and commissioning may only be performed by appropriately qualified professionals. The storage tank must be completely filled and vented before electrical connection. Operation of the radiators without water is not permitted. The Energy Controller must be mounted upright in a dry indoor location. Its IP21 protection rating and the permissible ambient temperature range of 5 to 40 °C must be taken into account when selecting the installation location.