Protherm Ray and Photovoltaics – How to Use Surplus Energy from Solar Panels

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Protherm Ray and Photovoltaics – How to Use Surplus Energy from Solar Panels

Combining the Protherm Ray electric boiler with a photovoltaic system is nowadays one of the most common solutions I encounter when installing systems in Slovak households. And that's no coincidence. When solar panels produce more electricity on a sunny summer day than the household consumes at that moment, that surplus energy has to go somewhere. You can either sell it to the distribution company at a feed-in tariff (which in Slovakia today is usually significantly lower than the price you pay for electricity), or you can use it directly – for example, for heating or preparing domestic hot water. This is exactly where the Protherm Ray comes in, an electric boiler that is exceptionally well suited for this kind of cooperation with photovoltaics.

In this article, we'll go through in detail how this cooperation works technically, what you need on the electrical installation side, how to properly set up the whole system, what power outputs make sense for what size of photovoltaic plant, and where the most common mistakes people make in practice are. If you're wondering whether such a solution is worth it for you, this article will give you a realistic technical and economic picture without any sugar-coating.

Why Protherm Ray Specifically for Using Solar Surpluses

Not every electric boiler is suitable for cooperation with photovoltaics. The key difference compared to cheaper electric boilers lies in how the boiler can regulate its power input. The Protherm Ray has stepped power regulation – individual resistive heating elements (rods) switch on and off, or are connected in various combinations, allowing the boiler to operate at different power stages. For example, the Protherm Ray 9KE operates at stages of 2 – 4 – 6 – 9 kW, while the Protherm Ray 12KE operates at 2 – 4 – 6 – 8 – 12 kW. This feature is key for photovoltaics.

The second reason is support for the OpenTherm protocol and the ability to connect an external controller. Most quality solar management systems (SolarEdge Home, Fronius Solar.web, SOLAX, Victron, as well as Slovak solutions such as iDM or WATTrouter) communicate with the boiler either via OpenTherm, via switched inputs (digital inputs), or via an external thermostat. Protherm Ray supports all of these methods, which means you can genuinely integrate it into practically any home energy management system.

The third advantage is a built-in domestic hot water (DHW) tank in some models, or a direct connection to an external tank – hot water acts as a thermal battery in which you store solar energy during the day and use it in the evening.

Basic Diagram: How Photovoltaics and the Boiler Work Together

Before we dive into details, let's look at the principle of the whole system. A household has a photovoltaic power plant (PVP) connected to an inverter, which converts direct current from the panels into 230/400 V alternating current. The inverter is connected to the house's distribution board and also to a surplus meter – either directly via the inverter's internal meter, or via an external energy manager (e.g. WATTrouter, Carlo Gavazzi, SENEC, or similar). This manager measures how much electricity is currently flowing into the grid (i.e. how much is surplus), and sends instructions to the boiler accordingly.

The boiler therefore doesn't control itself – it's controlled by the surplus manager. In practice, it looks like this: at 11:30 in the morning, the panels produce 6 kW, the household consumes 1.5 kW for regular appliances, and 4.5 kW would flow into the grid. The manager registers this and switches the boiler to the nearest lower power stage – for example, 4 kW. The boiler starts heating water in the tank. An hour later, production is 9 kW, consumption is the same, and the manager increases the boiler to 6 kW or 8 kW. This way, the boiler's power input is dynamically adjusted throughout the day so that as little energy as possible is exported to the grid.

PVP Output vs. Boiler Output – How to Match Them Properly

One of the most common questions I get is: which boiler should I choose for my photovoltaic system? The answer isn't simply "the bigger the boiler, the better." It depends on several factors: the installed PVP output, the household's average daily consumption, the size of the DHW tank, and of course the heat loss of the house.

PVP up to 5 kWp: The realistically usable surplus on sunny days is 2–4 kW. The Protherm Ray 6KE is a good fit here, operating at stages of 2–3–4–6 kW. A larger boiler would be running at full power most of the time and would also draw electricity from the grid.
PVP 5–10 kWp: Surpluses of 3–7 kW. The ideal choice here is the Protherm Ray 9KE or the Protherm Ray 12KE. The 12KE has a sufficient range of power stages to nicely adapt to fluctuating solar production.
PVP 10–15 kWp: Peak surpluses of 5–12 kW. Here it makes sense to choose the Protherm Ray 14KE or the Protherm Ray 18KE. Larger houses with higher heat loss will also appreciate the 18 kW for winter heating from the grid at night.
PVP over 15 kWp: This is where dual-boiler solutions or higher-output boilers (Ray 21KE, 24KE) belong, though a stronger electrical supply is also needed.

Important note: the boiler size must correspond to the house's heat loss, not just the PVP output. It makes no sense to install an 18 kW boiler in a well-insulated house with a heat loss of 6 kW just because you have a large photovoltaic system. In that case, most of the output would be unnecessarily oversized. If you're looking for specific recommendations based on floor area and heat loss, please refer to the article How to Choose an Electric Boiler Protherm Ray – Output According to House Area in this Knowledge Center.

Integration Methods – From Simple to Sophisticated

In practice, there are several levels of integrating the Protherm Ray with photovoltaics. Each has its own advantages, disadvantages, and implementation costs.

1. Switched Contact (ON/OFF) – The Simplest Method

The most basic method: the surplus manager monitors production and, upon reaching a set threshold (e.g. 2 kW of surplus), simply switches on the boiler via an external thermostat input. The boiler starts up at the lowest power stage and continues heating. This solution doesn't require any special configuration – you just need to correctly wire the external thermostat input on the Protherm Ray and set the thresholds in the surplus manager.

The downside is obvious: the boiler either turns off completely or runs at minimum power, without fine adjustment. If you have a 5 kW surplus but the boiler is running at 2 kW, you're unnecessarily giving away 3 kW to the grid. However, for simple applications (heating the DHW tank), this is a sufficient solution.

2. Multi-Stage Switching via Digital Inputs

The Protherm Ray allows switching power stages via digital inputs (on some models and configurations). This way, the surplus manager can control the boiler at multiple power levels – for example, 3 kW surplus → 2 kW on the boiler, 5 kW surplus → 4 kW, 8 kW surplus → 6 kW, etc. This is a considerably more efficient solution, and I'm seeing it used more and more often in practice.

The popular WATTrouter Mx from the Czech company SmartGate fully supports this – it can control the Protherm Ray at individual power stages via relay outputs and a surplus meter. Configuration takes about an hour of work, but afterward the system operates autonomously and the savings on surpluses are significant.

3. OpenTherm – The Smartest Integration

OpenTherm is a bidirectional communication protocol through which an external controller not only switches the boiler on and off, but also directly sets the required water temperature and reads back the current status of the boiler. Some more sophisticated energy management systems (and especially smart home systems such as KNX, Loxone, or iDM Navigator) can use OpenTherm to set the boiler's required water temperature according to the current surplus – for example, if the surplus is 4 kW, the system sets the required water temperature to 55 °C, and if the surplus is 8 kW, it sets it to 65 °C. The boiler itself regulates its power input to reach the required temperature, and thereby naturally absorbs the surplus.

The topic of OpenTherm is covered in more detail in the article Setting Up and Programming the Protherm Ray – Control, Weather Compensation Curve, OpenTherm.

Hot Water Tank as a Thermal Battery – An Essential Part of the System

One of the most practical and financially advantageous things you can do with solar surplus is heating domestic hot water. The DHW tank acts as a thermal battery: during the day, when the panels are producing, you heat the water to 60–70 °C, and this thermal energy remains in the tank for several hours without significant losses. In the evening and at night, you use it without having to run the boiler from the grid.

For a typical family of 4, a 200–300 liter tank is sufficient. Heating a 200-liter tank from 15 °C to 60 °C consumes about 12.8 kWh. This corresponds to about 2–4 hours of full solar output from an average photovoltaic plant. In other words – on one good sunny day, you can fully heat the tank and have free hot water for the whole day and part of the next.

It's important to size the tank correctly – too small a tank heats up quickly and the boiler has nowhere to "store" further surpluses. Too large a tank, on the other hand, takes a long time to heat and may have greater heat losses. For most households with a PVP up to 10 kWp, 200–300 liters is the optimum.

Electrical Installation Requirements When Connecting with Photovoltaics

When combining the Protherm Ray with photovoltaics, you need to think not only about correctly sizing the boiler, but also about the electrical installation. Here are a few principles that I repeatedly see being violated:

Protherm Ray 6KE requires a single-phase 230 V / 10 A circuit breaker supply. When combined with a PVP, the inverter must also be sized for the same phase, or the manager must know which phase the boiler is running on.
Protherm Ray 9KE, 12KE, 14KE, 18KE are three-phase, requiring a 400 V / 3×16 A or 3×20 A circuit breaker supply. The PVP inverter must be three-phase (or the surplus must be measured separately on all three phases).
The surplus manager must have an energy meter (CT clamp or meter in the electricity meter) installed in the correct location – ideally on the supply line from the electricity meter, before the house's distribution board. If the meter is behind the distribution board, the system won't measure the export correctly.
When connecting via external thermostat contacts on the boiler, keep in mind that this must be a voltage-free contact (230 V on the thermostat input for the Ray would damage the boiler – see the documentation or the article Connection and Installation of the Protherm Ray – Requirements for Electrical Installation and Circuit Protection).
If the DHW tank is connected directly to the boiler via a circulation pump, you need to consider hydraulic separation – without it, circuits may interfere with each other and cause uncontrolled mixing of temperature zones.

Practical Examples from Real Installations

Example 1: Family House, 8 kWp PVP, Protherm Ray 12KE

The customer had a new-build house with underfloor heating, a heat loss of 8.5 kW, and an 8 kWp photovoltaic system (three-phase Fronius Symo inverter). A 250-liter DHW tank. We used a WATTrouter Mx with measurement on the main supply line. The WATTrouter controls the Ray 12KE at stages of 2–4–6–8–12 kW according to the current surplus.

Result after the first summer: in June and July, the DHW tank was heated almost exclusively from solar surpluses – monthly savings on electric DHW heating of about 30–40 kWh/month. The total export savings from May to September were 680 kWh, which at a feed-in tariff of €0.06/kWh represents €40.8 not paid out, but at a self-consumption value of €0.19/kWh it's a saving of €129. The real financial benefit compared to exclusive export is therefore ~€88 per season, plus the convenience of free hot water.

Example 2: Older Building, 5 kWp PVP, Protherm Ray 9KE

The customer had an insulated apartment building from the 1980s, with a heat loss of 11 kW (without renovation) and a 5 kWp single-phase PVP (Deye inverter). The Ray 9KE boiler replaced the original gas boiler. Since the 9KE is three-phase, we had to solve the surplus measurement issue – the inverter was single-phase on L1, but the boiler was three-phase. The solution was a WATTrouter with three-phase measurement and priority DHW heating set on the L1 phase.

Result: with the larger house, the boiler still relied on grid power in winter (as expected), but in summer and transitional periods, DHW heating was almost fully covered by solar power. The customer was satisfied – the monthly electricity bill in summer dropped by 25–30%.

Example 3: Cottage, 3 kWp PVP, Protherm Ray 6KE

At the cottage without a gas connection, the customer wanted a solution with minimal costs. 3 kWp panels on a steep, south-facing roof, single-phase inverter, Ray 6KE at 230 V. A simple switched relay contact – when the surplus exceeds 1.5 kW, the boiler switches on at 2 kW (minimum stage). Since visits to the cottage are irregular, the system works without more complex control.

Result: at the cottage, hot water is practically free from April to October. In winter, the customer tops up heating from the grid – but that consumption is small, since the cottage is rarely visited.

Summer vs. Winter Use – The Reality of Seasonality

It's important to be honest about what you can expect from the combination of Protherm Ray + PVP in different seasons. Photovoltaics and heat demand are, after all, exactly opposite – when production is highest (summer), heat demand is lowest, and vice versa.

June–August: Maximum solar production, minimum heating demand. 100% of heat demand (DHW) can be covered by solar power. Surpluses are large, and the boiler runs for long periods during the day.
May, September: Still good production, slightly higher heat demand. 70–90% of DHW covered by solar, with occasional grid top-up in the evening.
April, October: Variable production, more significant heating demand. The PVP covers only part of daily consumption. The boiler runs on grid power at night and in the morning.
November–March: Little or no solar production. The boiler runs primarily on grid power. The PVP doesn't directly help with heating, but reduces overall daytime consumption (lighting, appliances).

It follows that the combination of PVP + electric boiler is economically most advantageous in a house with low heat loss (passive or low-energy house), where winter consumption is small and summer surpluses can be meaningfully used. In an older, more poorly insulated house, the benefit will be smaller, because winter electric heating costs will dominate. In such cases, it sometimes pays to combine the Protherm Ray with a heat pump or another backup source.

Setting Up the Boiler for Optimal Cooperation with PVP

Once the hardware is correctly connected, the boiler itself also needs to be configured properly. A few settings are key for photovoltaic use:

DHW tank temperature limit: Set the maximum tank temperature to 65–70 °C. A higher temperature "stores" more energy, but increases tank heat losses and anode wear. 65 °C is the hygienic minimum for eliminating legionella and is also a realistically achievable temperature after several hours of sunshine.
Circuit priority: Set the DHW tank as the priority circuit – the boiler heats it before the heating circuit. During the day, when there is production, the tank is filled. Heating tends to take place more during the night/morning hours from the grid or from accumulation in the system.
Weather compensation control: If you have an outdoor temperature sensor, turn on the weather compensation curve. The boiler will then automatically lower the water temperature in the heating circuit in warmer weather. In summer, the boiler will operate at a lower water temperature, which is also more efficient for solar control.
Frost protection: Leave it on, but set the threshold to a minimum (e.g. 7 °C). In summer, no unnecessary energy will be drawn from the grid due to system overheating.
Time schedule: If you don't have a smart surplus manager, you can configure a daily schedule – the boiler will heat more intensively during daytime hours (10:00–16:00) and be dampened at night. This isn't optimal (it doesn't react to actual production), but it's better than nothing.

Economics and Payback – Real Numbers

Let's calculate this using a specific example. A family of 4, a house with a heat loss of 10 kW, an 8 kWp PVP, a 250 l tank, Protherm Ray 12KE boiler.

Annual production of an 8 kWp PVP in Slovakia (southern central Slovakia): approx. 8,200–8,800 kWh/year. Of that, direct household daytime consumption: 2,500–3,000 kWh. Remaining surplus: 5,200–5,800 kWh/year. Of this surplus, the boiler can effectively use (with proper manager configuration): 60–75%, which is 3,100–4,350 kWh/year absorbed by the boiler.

At an electricity price of €0.19–0.22/kWh, that's a saving of €590–957/year. Additional costs for a surplus manager (e.g. WATTrouter Mx): €250–400. Amortization of the manager alone: less than 6 months. In subsequent years, it's pure savings.

The total investment in PVP + boiler + manager typically ranges from €12,000–18,000 (depending on size and configuration). Annual savings including direct consumption from solar panels: €1,200–1,800. Payback period: 8–12 years, which is a very decent result given the 25-year lifespan of a PVP.

What to Watch Out For – The Most Common Mistakes in Practice

From my own experience with custom installations, here are the most common mistakes that unnecessarily reduce the efficiency of the whole system:

Incorrect placement of the measuring transformer (CT clamp): The meter must be on the supply line before all appliances, not behind the distribution board. If it's behind the distribution board, it won't measure export to the grid correctly and the manager will make mistakes.
Undersizing the tank: A tank that's too small heats up to maximum by 11:00, and the remaining 4 hours of sunshine send energy into the grid. A somewhat larger tank is better.
DHW heating not activated on the boiler: It happens that a customer has the boiler set only for heating, the DHW tank is connected, but the boiler doesn't heat it because the relevant circuit isn't activated in the settings. Check in the boiler menu whether the DHW circuit is active.
Manager set too aggressively – the boiler starts too often: Frequent short-term starts (e.g. every 5 minutes) cause wear on the relay in the manager and unnecessary thermal cycling of the boiler. Set a hysteresis or a minimum boiler run time of at least 15–20 minutes.
Ignoring three-phase imbalance: If the inverter is single-phase and the boiler is three-phase, the surplus manager must measure each phase separately. Otherwise, it can happen that there's a surplus on L1, but power is being drawn from the grid on L2 and L3 – and the overall balance is calculated incorrectly.
Insufficient hydraulic balancing: When using the boiler simultaneously for heating and DHW without a hydraulic separator, temperature conflicts can occur in the system. The result is insufficiently heated DHW as well as insufficient heating.

Combination with Battery Storage – Does It Make Sense?

A common question: is it worth adding battery storage (an accumulator) in addition to the boiler? The answer depends on the specific situation. Battery storage systems are still relatively expensive (typically €6,000–12,000 for 5–10 kWh). By comparison, the Protherm Ray fulfills the function of a thermal battery for a fraction of the price.

A water tank is essentially a thermal battery with a lower energy density than a lithium-ion battery, but much cheaper. A 250-liter tank heated from 20 °C to 65 °C holds about 14.5 kWh of thermal energy, at a tank cost of around €300–600. Compare that to a 10 kWh lithium battery costing €6,000. From a purely economic standpoint, the tank is far more advantageous.

Battery storage makes sense in cases where you want to cover evening electricity consumption as well (not just heat), or if you have peak electricity tariffs and want to reduce grid consumption during more expensive hours. But that's a different topic – for a solution focused solely on heating and DHW, a tank with the Ray boiler is a much better investment.

Frequently Asked Questions (FAQ)

Can the Protherm Ray work with any photovoltaic inverter?

Yes, because the Protherm Ray is a standard appliance – it connects to the 230 V or 400 V grid, and the inverter doesn't even "see" what's running behind it. Integration takes place via an energy manager (e.g. WATTrouter, SolarEdge Home, SOLAX Smart Load), which is independent of the inverter brand. The only important thing is that the manager can measure the output at the inverter's outlet or at the house's supply line, and that it has an output compatible with controlling the boiler (relay contacts or OpenTherm).

Do I need a DHW tank, or can I use the surplus only for heating?

It's not mandatory, but without a tank, you lose the simplest and most efficient way to store solar energy. In summer, when you don't need heating, a boiler without a tank has nowhere to "store" surpluses, and the whole point of the integration is lost. A DHW tank is therefore practically essential when combined with a PVP – at least 150–200 liters, even for a smaller household.

What boiler output should I choose if I have a 10 kWp PVP?

The boiler output must primarily correspond to your house's heat loss, not the PVP output. The PVP affects how much energy you'll draw from the grid, but the boiler must cover the entire heat demand of the house in winter. For a house with a heat loss of 10 kW and a 10 kWp PVP, the Protherm Ray 12KE or Protherm Ray 14KE is suitable. The article What Output Protherm Ray Do I Need – 6, 9, 12, 14, 18, 21, or 28 kW? will also help you with the choice.

Can I set the boiler to heat the tank only from solar energy and never from the grid?

Technically, yes – the surplus manager can be instructed to activate the boiler on the DHW circuit only when the surplus is higher than a given value. The boiler turns off the DHW circuit when the surplus drops. The problem arises in cold and overcast weather – the tank isn't heated for days or even a week, which poses a hygienic risk (legionella). We therefore recommend also setting a backup nightly legionella protection cycle at least once a week – the boiler heats the tank to 60 °C regardless of the PVP.

Is combining the Protherm Ray with a PVP worthwhile even in an old

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