Solar Water Heater Guide: Off-Grid & Hybrid PV Boilers

Toosolar Technical Guide

Fothermo photovoltaic water heaters explained simply: off-grid PVB boilers, PVB-AC hybrid boilers, tank sizing, solar-panel selection, backup heating and installation essentials.

At a glance

Fothermo boilers use DC electricity from photovoltaic panels to heat and store domestic hot water. The panels connect directly through MC4 connectors, so normal solar operation does not require a solar inverter or battery.

Need / application Recommended starting point
Small sink or very low hot-water demand Compact 10-litre PVB-10.
Off-grid cabin, holiday property or water preheating PVB-30 or PVB-80, depending on daily demand.
Grid-connected home with reliable year-round hot water PVB-30-AC or PVB-80-AC hybrid boiler with automatic 1,500 W AC reheating.
Caravan or mobile installation Reinforced CPVB-10 with photovoltaic and 12/24 V DC operation.
Existing compatible hot-water tank ROD 1000 photovoltaic heating element.
Quick rule Choose the tank size from the number of users and daily hot-water demand, then select solar panels that stay within the boiler's voltage and current limits. Do not choose panels by wattage alone.

How photovoltaic boilers work

Solar panels produce DC electricity, which is sent directly to the boiler's heating element. An integrated MPP controller adjusts the load to the available solar power, while the water tank stores the collected energy as hot water.

  • No rooftop water or glycol circuit.
  • No circulation pump.
  • No solar inverter or battery required for normal solar operation.
  • Direct MC4 connection from the photovoltaic panels.
  • Adjustable water temperature up to approximately 65°C.
  • Three-year manufacturer's warranty.
Photovoltaic water heating is not solar thermal Traditional solar thermal systems circulate fluid through rooftop collectors. A photovoltaic boiler instead converts solar-panel electricity directly into heat, making the electrical side simpler and avoiding rooftop water pipework.

Off-grid PVB or hybrid PVB-AC?

Product family How it heats Best suited to
PVB off-grid boilers Primarily from photovoltaic panels. Compatible models can also reheat gradually from a suitable low-voltage DC source. Cabins, tiny homes, workshops, holiday properties, outdoor kitchens and preheating before another water heater.
PVB-AC hybrid boilers Solar energy is used first. If water falls below the selected minimum temperature, an integrated 1,500 W element reheats it from a standard 230 V socket. Grid-connected homes where solar savings and dependable year-round hot water are both required.
Solar-first hybrid operation The hybrid boiler can work without its AC connection and heat entirely from solar power. When connected to the grid, the thermostat controls only the minimum backup temperature; available solar energy can continue heating the water above it.

Model comparison

Model Capacity Suggested PV Backup heating Typical application
PVB-10 10 L Approx. 200 Wp Optional 12 V DC, approximately 62 W Sinks, compact cabins and very low demand.
PVB-30 30 L Approx. 450 Wp Optional 18 V DC, approximately 141 W One person, occasional showers or water preheating.
PVB-80 80 L Approx. 1,000 Wp Optional 18 V DC, approximately 141 W Larger off-grid demand and regular shower use.
PVB-30-AC 30 L Approx. 450 Wp Automatic 1,500 W / 230 V AC Small grid-connected homes and apartments.
PVB-80-AC 80 L Approx. 1,000 Wp Automatic 1,500 W / 230 V AC Households with regular hot-water demand.
Important electrical limits The PVB and PVB-AC boilers accept approximately 42 V maximum photovoltaic open-circuit voltage and use up to approximately 550 W of solar heating power. The complete array must remain within the current model's voltage, current and power limits in all temperatures.

Choosing capacity and solar panels

Tank size Practical use
10 litres Handwashing, a small kitchen or very limited demand where compact dimensions matter most.
30 litres One person, occasional showering or preheating before an instantaneous or conventional heater.
80 litres Regular shower use, more than one user or a household that needs more stored hot water.

When fully heated to 65°C, Fothermo states that the 30-litre boiler can provide approximately 72 litres of mixed water at 34°C, while the 80-litre boiler can provide approximately 180 litres. Actual usable volume depends on incoming water temperature, pipe losses and the chosen outlet temperature.

Solar-panel selection

  • Use the suggested array as a practical starting point: around 200 Wp for 10 litres, 450 Wp for 30 litres and 1,000 Wp for 80 litres.
  • A larger array can improve morning, winter and cloudy-weather performance even though boiler solar heating is limited to approximately 550 W.
  • Two or more panels are normally connected in parallel, not in series.
  • Check panel open-circuit voltage (Voc), including the increase expected in cold weather.
  • Check total parallel current, cable length, cable cross-section, MC4 compatibility and polarity.
Why panel voltage matters Series connection raises voltage and can exceed the boiler's input limit. Parallel connection raises available current while keeping the array voltage close to that of one panel.

Installation essentials

  • Mount the boiler vertically on a wall and use fixings suitable for its full water-filled weight.
  • Fill the tank completely before activating any heating source.
  • Never block the pressure-relief outlet and follow the supplied plumbing and safety instructions.
  • Keep the complete photovoltaic array within the boiler's voltage, current and power limits.
  • Use suitable MC4 connectors, cable cross-section, polarity and weather protection.
  • Follow local electrical, plumbing and building regulations.
Plug-and-play does not remove plumbing requirements The photovoltaic connection is designed to be straightforward, but safe wall mounting, water connections, pressure protection and commissioning remain essential. Use a competent installer whenever the work is outside your experience.

Common mistakes to avoid

  • Connecting panels in series and exceeding the maximum photovoltaic voltage.
  • Ignoring the increase in panel Voc during cold weather.
  • Assuming a 1,000 Wp array makes the boiler heat at 1,000 W; PVB solar heating is limited to approximately 550 W.
  • Choosing a tank that is too large for the available solar energy or too small for daily demand.
  • Expecting a standard PVB model to reheat as quickly as a PVB-AC hybrid boiler.
  • Using undersized extension cables or incorrect MC4 polarity.
  • Mounting the boiler on a wall that cannot safely support its filled weight.
  • Switching on the boiler before it is completely filled with water.

Frequently asked questions

Does a Fothermo boiler need a solar inverter?

No. The photovoltaic panels connect directly to the boiler, and its integrated controller manages the DC solar input.

Does it need a battery?

No battery is required for normal solar operation. Standard PVB models can optionally use a compatible low-voltage backup source, while PVB-AC models have integrated 230 V reheating.

How many solar panels do I need?

A practical starting point is approximately 200 Wp for the PVB-10, 450 Wp for 30-litre models and 1,000 Wp for 80-litre models. Final selection must respect all voltage, current and power limits.

Should the panels be connected in series or parallel?

Two or more panels are normally connected in parallel. This increases available current while keeping voltage close to that of one panel. Series connection may exceed the boiler's maximum input voltage.

Why use more panel power than the boiler's 550 W heating limit?

A larger array can help the boiler reach its maximum heating power earlier, later and during weaker sunlight. It does not increase the boiler's internal maximum solar heating power.

Will a hybrid boiler work during a power cut?

The solar-heating side can continue whenever sufficient sunlight is available. The 1,500 W AC backup element requires grid or alternative AC power.

Can a PVB-AC boiler replace a conventional electric boiler?

In many suitable installations it can replace or supplement a conventional boiler, but capacity, plumbing, wall mounting, pressure protection and local requirements must all be checked.

How long does the water take to heat?

Heating time depends on tank size, starting water temperature, solar irradiance, panel array and the selected final temperature. A larger tank stores more hot water but requires more energy and time to heat.

Do I need professional installation?

The low-voltage photovoltaic connection is designed to be simple, but correct wall mounting, plumbing and pressure protection are essential. Professional assistance is recommended whenever required by local rules or the installer's experience.

More questions? Visit the Toosolar Technical FAQ or contact Toosolar for product-specific support.

Why quality matters

A pressurised water heater combines electrical controls, heating elements, plumbing connections, insulation and safety components. Clear limits, reliable temperature control, documented safety devices, correct installation and manufacturer support are more important than choosing by price alone.

  • Fothermo boilers are made in Germany and supplied with a three-year manufacturer's warranty.
  • Integrated MPP control helps use the available photovoltaic energy efficiently.
  • Current manuals and datasheets make panel selection and installation safer.
  • Matched accessories reduce the risk of incompatible power sources and connections.
Toosolar focus Toosolar offers the complete Fothermo water-heating range with practical product-selection support. We prioritise reliable products, clear documentation and established manufacturer warranty support. This careful selection helps keep Toosolar's product return rate extremely low—approaching zero.
Need help selecting a photovoltaic water heater? Tell us the number of users, required hot-water volume, off-grid or grid-connected application, available solar-panel specifications, installation location and cable distance. We can help identify a suitable Fothermo boiler and avoid common tank-sizing, panel-voltage, wiring and installation mistakes.