Vaillant eloBLOCK and photovoltaics – is combining with your own electricity worth it?

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Vaillant eloBLOCK and photovoltaics – is combining it with your own electricity worth it?

When a customer today decides on an electric boiler, one question almost always comes up: "What if I have, or will have, solar panels on my roof?" It's not an unnecessary question. Combining a photovoltaic system with an electric boiler is one of those investments where economic logic truly aligns with technical reality – if designed correctly. And this is exactly where the catch lies, because many people imagine that it's enough to put panels on the roof and the boiler will heat "for free." The reality is somewhat more complex, but still very interesting.

In this article, we'll break down all the essential aspects of this combination: how surplus energy management works, what outputs are realistic, when this investment truly pays off, and what specific scenarios from our project practice show about the limits of a reasonable solution. We'll focus on boilers from the Vaillant eloBLOCK range, which are particularly well designed for this type of operation.

Why an electric boiler and photovoltaics – the basic logic of the combination

A photovoltaic system produces electricity during the day, with peak production occurring between 10:00 and 15:00. The problem is simple: at this time, most households don't consume much electricity intensively – people are at work, children at school, cooking is minimal. Surplus electricity goes into the grid, where the distribution company pays a feed-in price that today in Slovakia, for new systems (so-called small sources), is significantly lower than the price at which you buy electricity.

This is where the electric boiler or water heater comes into play: you can "store" surplus electricity as hot water or thermal capacity in the house, instead of selling it at a low feed-in price. This principle is called self-consumption of surpluses. From an economic point of view, the logic is clear – every kWh you "store" as heat instead of feeding back into the grid saves you the difference between the purchase and sale price of electricity.

In Slovakia, this difference in 2024–2025 is typically €0.18–0.22/kWh for purchase versus €0.05–0.08/kWh for sale. This means that every kWh consumed by your own boiler instead of being sold to the grid saves you a real €0.10–0.17. For a 9 kW boiler running 2 hours a day on surplus, that's €1.8 to €3.1 per day – not a negligible figure during the season.

Vaillant eloBLOCK – why it is suitable for operation with PV surplus

Not every electric boiler is equally suitable for combined operation with photovoltaics. The Vaillant eloBLOCK has several technical features that make it a good choice for exactly this type of operation.

The first key feature is modular stepped output control. eloBLOCK boilers are not simple "on/off" devices. Depending on the variant, output switches in several steps – for example, a boiler with a maximum output of 14 kW can operate in steps of 2, 4, 6, 8, 10, 12 or 14 kW (exact steps depend on the specific model and its resistive element topology). This allows better adaptation to the current PV surplus instead of one large jump.

The second important feature is compatibility with external control signals. The eloBLOCK can be connected to an external thermostat or, more importantly, to a control output from the inverter or an energy manager that starts the boiler or changes its settings according to the current surplus. Modern PV systems (e.g. SMA Sunny Home Manager, Fronius Ohmpilot, E3/DC, or universal solutions like Shelly EM) can activate the boiler via a relay or digital input exactly when there is enough of your own energy available.

The third aspect is structural simplicity. The eloBLOCK is essentially a resistive boiler with a water circuit, pump and expansion vessel. It has no complicated processors, compressor, or gas valves. This means it tolerates frequent starts and stops without wear, which is common when controlled according to PV surplus.

Overview of available outputs and their relevance for PV systems

To correctly size the PV + boiler combination, it is important to know the specific outputs of the available models:

Vaillant eloBLOCK VE 6/14 EU III – 6 kW: the smallest variant, suitable for well-insulated houses up to about 80 m² or as a supplementary heat source. For PV, it's a favorable match – 6 kW output covers a typical home PV system with a capacity of 6–8 kWp on a sunny day.
Vaillant eloBLOCK VE 9/14 EU – 9 kW: a compromise variant for houses of 80–120 m², forming a very good economic core in combination with an 8–10 kWp PV system.
Vaillant eloBLOCK VE 12/14 EU – 12 kW: for larger houses of 120–160 m², typically requiring a 10–15 kWp PV system so that autonomous operation is at least partial during the winter months.
Vaillant eloBLOCK VE 14/14 EU III – 14 kW: a powerful variant for houses of 150–200 m², where PV alone is not enough to cover all consumption but significantly reduces costs.
Vaillant eloBLOCK VE 18/14 EU III – 18 kW: the largest variant, mostly for larger houses or residential buildings with multiple units. For PV combination, you need a powerful PV array of 15+ kWp.

How eloBLOCK control using PV surplus energy works

The basic question every customer asks: "How do I actually implement this technically?" There are three main approaches, from simplest to most sophisticated.

1. Simple timer switch or thermostatic relay

The cheapest solution – the boiler is simply set to a fixed operating time between 10:00 and 14:00, when PV production is statistically highest. The disadvantage is obvious: the boiler also runs on cloudy days, when energy comes from the grid. Suitable only for customers with a minimal automation budget and in regions with high solar irradiation. In practice, I've seen this with a customer in southern Slovakia, where this primitive control worked surprisingly well – during the summer months it covered hot water heating with PV energy on about 65–70% of days.

2. Energy manager with production and consumption metering

This is the standard recommended solution today. An energy manager (e.g. SMA Sunny Home Manager, Fronius Symo + Smart Meter, Goodwe EMS, or independent devices like Shelly EM with automation) measures the energy flow in the house in real time. When the surplus exceeds a set threshold (e.g. 1.5 kW continuously for 3 minutes), it sends a signal and the boiler turns on. When the surplus drops below the boiler's minimum output, the boiler turns off. Such a system can respond to cloud cover with a delay of a few minutes.

A practical implementation for a specific customer in the Trenčín region: A 140 m² house, a 10 kWp PV system, a Vaillant eloBLOCK VE 9/14 EU boiler, a Shelly EM as a meter + Shelly 1 as a switching relay on the boiler input. Total automation cost about €85. The customer states that in the summer period, the boiler takes approximately 70% of the thermal energy for DHW heating from its own PV. In the transitional period (spring/autumn), about 40–50%. In winter, it's only 10–20%, with most energy still coming from the grid.

3. Fully integrated system with stepped output control

The most sophisticated solution uses stepped control of the boiler's output. Instead of simple on/off switching, the system regulates which power stage of the boiler is active according to the current surplus. For example, with a surplus of 4 kW, only 2 resistive elements are switched on (4 kW output), and with a surplus of 8 kW, all available elements are switched on. This requires a boiler with available stepped inputs and an energy manager that controls these stages.

The eloBLOCK allows this, but you should consult in advance with an electrician and the seller about the specific wiring, as not all models have directly available external inputs for each output stage. More technical details on the wiring itself can be found in the article "Installing the Vaillant eloBLOCK – procedure, electrical installation requirements and fuse protection".

Economic analysis: when the combination is truly worth it

This is a chapter where I must be honest: the combination of PV + electric boiler is not always economically ideal. It depends on several factors.

When the combination is most worthwhile

Underfloor or warm-air heating with a low-temperature circuit (35–45 °C): The eloBLOCK can heat for longer at lower output, which better follows PV production. More about underfloor heating in the article "Connecting the Vaillant eloBLOCK to an underfloor heating system – what you need to know".
A house with good thermal mass: Massive walls, a concrete floor, or a large volume of heated space act as a thermal storage. The boiler can run for 3–4 hours on a sunny day and the house will retain heat until evening.
Presence of a DHW storage tank: Heating domestic hot water is an excellent use of surplus PV energy. 200 liters of water heated from 15 °C to 65 °C represents stored energy of about 11.6 kWh – equivalent to the output of a good sunny noon from a 10 kWp system.
High purchase price of electricity and low feed-in price: The bigger this difference, the faster the payback.
Regional sunshine levels: Southern Slovakia (Záhorie, the Danube Lowland, the Košice Basin) has significantly higher potential than the mountainous regions of the north.

When the combination is less advantageous

A low-energy house with low heat consumption: If a house consumes only 3,000 kWh per year for heating, the savings from PV surplus will be small – we're talking about hundreds of euros a year, which can extend the payback period of the PV system by years.
Northern Slovakia with significant cloud cover: In areas with an average of only 1,400–1,500 hours of sunshine per year, PV production is 15–20% lower than in the south.
A house with an existing cheap heat source: If you have a functioning heat pump or biomass boiler with low operating costs, an electric boiler as the main heat source does not make economic sense even with PV.

Approximate payback calculation for a typical scenario

Consider a 120 m² house in central Slovakia, an 8 kWp PV system (annual production about 7,200 kWh), an eloBLOCK 9 kW as the main heat source. The house consumes about 12,000 kWh of electricity per year (heating + DHW + other). Without PV: 12,000 kWh × €0.20/kWh = €2,400/year. With PV and proper control: own PV consumption about 65% = 4,680 kWh self-consumed, saving 4,680 × 0.20 = €936/year; selling the remaining 2,520 kWh × 0.06 = €151/year. Total savings/income: ~€1,087/year. Investment in the PV system (8 kWp, installation, inverter, energy manager) about €9,000–11,000 without subsidies. Net payback without subsidies: 8–10 years. With available subsidies (Green Households II or others), the payback can be shortened to 5–7 years.

This calculation is approximate, and real figures depend on specific tariffs, climatic conditions and household behavior. For detailed planning, we recommend consulting boiler sizing – a useful guide can be found in the article "What eloBLOCK output do I need for my house – kW calculation according to area".

The winter problem: why PV isn't enough in freezing weather

One of the most common mistakes customers make is assuming that a PV system "covers" heating even in winter. The reality is different, and it should be stated openly.

In December and January in Slovakia, an 8 kWp PV system produces only 200–350 kWh per month, a fraction of summer production (800–900 kWh per month). At the same time, it is precisely in these months that heat consumption is highest – a 120 m² house can consume 1,500–2,000 kWh of heat alone per month. The math is clear: PV covers 10–20% of winter heat consumption at best.

This does not mean that the combination is unfavorable. It means that an electric boiler with PV is a system where PV helps significantly during the transitional period (October, November, March, April) and in summer for DHW heating, but winter heating still comes primarily from the grid. The overall annual savings are nevertheless real and meaningful.

For customers who want to minimize grid dependence even in winter, a solution with battery storage exists. A battery system with a capacity of 10–15 kWh can store daily PV production and release it in the evening or morning. However, the cost of battery storage is still relatively high (€4,000–8,000 for 10 kWh of capacity), which extends the overall payback of the system. For most households today, a battery combined with a boiler doesn't yet make the best economic sense, unless their primary goal is energy self-sufficiency as a matter of principle.

Practical scenarios from project practice

Scenario 1: New build in energy class A0 – PV as the primary source in summer

A customer near Bratislava, a 160 m² house in class A0, consumed only 6,500 kWh annually for heating and DHW combined. A 10 kWp PV system, Vaillant eloBLOCK VE 12/14 EU as a backup/supplementary source (the primary source being an air-to-air heat pump). The eloBLOCK was used mainly for DHW heating in the storage tank and as supplementary heating when the outdoor temperature dropped below −5 °C, when the heat pump reached the limit of its output. In this scenario, PV covered about 78% of the eloBLOCK's annual consumption, which at €0.20/kWh meant savings of over €1,000/year on this part of the system alone.

Scenario 2: Older renovated villa – main heat source

A 220 m² house from 1978, after facade insulation and window replacement, with the original gas boiler decommissioned due to age. The customer chose the Vaillant eloBLOCK VE 18/14 EU III as the main heat source combined with 14 kWp PV. During the winter months, heating was almost entirely covered by the grid, but during the transitional period (60 days in spring and 60 days in autumn), PV covered 40–60% of the boiler's daily consumption. Total annual savings from PV on heating and DHW came to about €850–1,000/year. The customer was satisfied but admitted in retrospect that for such a large house, they would consider a heat pump as the base and the eloBLOCK as a peak source.

Scenario 3: Cottage area – seasonal use

A customer with a 70 m² cottage in the Low Tatras, a 6 kWp PV system installed primarily for self-consumption of electricity, an eloBLOCK VE 6/14 EU as backup heating. The cottage is used mostly on weekends from March to November. During this period, the PV system covered most of the boiler's consumption. The customer didn't need complex automation – a simple thermostat set for weekend switching between 9:00 and 16:00 was sufficient. Total heating and DHW costs decreased by about 60% compared to the previous electric heating setup.

Technical installation requirements – what not to forget

Combining PV + eloBLOCK doesn't require any special construction work, but several technical aspects need to be considered:

Electrical connection capacity: An 18 kW eloBLOCK requires a three-phase supply with 3×32 A fuse protection. The PV inverter is also three-phase at higher outputs. The combined maximum power draw should be verified with the electricity distributor to avoid overloading the connection.
Bidirectional metering: For correct evaluation of self-consumption and grid sales, you need a bidirectional electricity meter, which distributors today standardly install with newly installed PV systems.
Energy manager and communication: If you want automatic boiler control based on PV surplus, plan for it before installation. Additional wiring afterward is possible but more complicated. More about electrical installation requirements can be found in the article "Converting to three-phase power for the eloBLOCK – when it's necessary and how to do it".
Hydraulic system: If you plan to use the eloBLOCK for DHW heating via a storage tank, supplement the hydraulics with a three-way valve or an external tank with its own heat exchanger. The eloBLOCK itself does not directly heat a DHW tank – you need a combined tank or a secondary boiler.
Surge protection: A PV system can, in some configurations, cause voltage fluctuations in the house's network. A surge arrester in the distribution board is therefore recommended, which is good practice even without PV.

Subsidies and legislation in Slovakia in 2024–2025

The current situation regarding subsidies for PV systems and electric boilers in Slovakia is evolving. The Green Households program provided vouchers for PV panels and solar collectors, but did not directly address electric boilers as such. However, the combination of PV + electric boiler can be supported via the PV part – i.e., you get a subsidy for the PV system and purchase the boiler with your own funds.

It's important to follow current calls, as the conditions and availability of vouchers change. For companies and entrepreneurs, other tools exist as well (the Slovak Recovery and Resilience Plan, cohesion funds). We always recommend that customers check the current status on the website of SIEA (Slovak Innovation and Energy Agency) before making a final decision.

Regarding electricity rates: households with a PV system are subject to a specific tariff (D5 or D6 depending on the distribution territory), where the distribution fee may be higher, but the optimization possibilities are also greater. Check the details with your electricity supplier.

Frequently asked customer questions – FAQ

Can I connect the eloBLOCK directly to the PV inverter without the grid (off-grid)?

Theoretically yes, but in practice this is not recommended as the main heating source. Off-grid operation without batteries is unstable – the boiler would start and stop according to current sunshine, causing temperature fluctuations and unnecessary mechanical strain on the device. Moreover, most off-grid inverters have different voltage characteristics. If you want maximum independence, add battery storage and have the system designed by a professional. The eloBLOCK works best in an on-grid configuration, where the grid serves as a backup.

How many kWp of PV panels do I need for the eloBLOCK to run primarily on its own energy?

As a rough guide: the PV output in kWp should be at least equal to the boiler's output in kW, ideally 1.2–1.5 times that. So for a 9 kW boiler, we recommend 10–14 kWp of PV. But note – this only applies to the summer/transitional period. In winter, the boiler will run primarily from the grid regardless of the size of the PV system. A larger PV system will help you over a longer time window (from March to October instead of just May to August).

Can the eloBLOCK itself "know" that cheap PV energy is available?

No, the eloBLOCK itself does not have a function for recognizing PV surplus. It's a standard electric boiler with a thermostat and timer. You must add the control intelligence externally – via an external energy manager, a smart relay (e.g. Shelly), or your PV inverter's proprietary system. Some customers solve this using a smart socket or smart meter with automation rules (e.g. via Home Assistant). Details on connecting control inputs can be found in the boiler's service documentation or in the article on installation and electrical wiring.

Is the eloBLOCK also suitable as a backup source alongside a heat pump + PV system?

Yes, and this is currently one of the most common configurations in new builds. The heat pump is the main heat source (high efficiency), the eloBLOCK serves as a peak source during frosts below −10 to −15 °C, when the heat pump drops to its minimum. In summer, the eloBLOCK heats DHW from PV surplus. Such a triple combination (heat pump + eloBLOCK + PV) in practice provides an excellent balance of energy independence and operating costs.

Does it make sense to combine the eloBLOCK with PV if I have a night electricity tariff?

The night tariff (HDO/off-peak) was the traditional solution for electric boilers – cheap electricity at night, heat storage. PV produces electricity during the day. These two approaches don't exclude each other, but need to be properly coordinated: at night the boiler takes cheap grid electricity via the off-peak tariff, during the day it takes surplus from PV. The hot water tank or storage vessel must have sufficient volume to handle both charging cycles. We're talking about tanks of at least 300–500 liters for a 120–150 m² house.

How long does the payback period take for the whole PV + eloBLOCK system?

At current prices (an 8–10 kWp PV system including installation and energy manager costs about €9,000–14,000, an eloBLOCK 9 kW about €900–1,200) and savings of €900–1,200/year, the total payback without subsidies comes to 8–12 years. With available subsidies for the PV system (30–50% of eligible costs in some calls), this can be shortened to 5–8 years. PV panel lifespan is stated as 25+ years, and the eloBLOCK, with proper maintenance, lasts 15–20 years. The net economic benefit after the investment is paid off is therefore significant.

Conclusion: is the combination really worth it?

Do you have a question on this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise you.