Electric boiler ELÍZ and photovoltaics: is the combination worth it

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Electric boiler ELÍZ and photovoltaics: does it really pay off?

The combination of an electric boiler and photovoltaic panels has, in recent years, become one of the most frequently discussed topics among owners of family houses. It is a topic where energy technology, economics, building physics and real-world system operation intersect – and where it is easy to come across misleading information, marketing shortcuts and contradictory experiences. In this article we will look at the matter objectively and from several angles: what this combination technically involves, when it makes sense, when it doesn't, what the real savings are, and what needs to be addressed before the installation itself.

ELÍZ electric boilers are interesting precisely because they are straightforward electric heating devices without unnecessary complexity – which makes them ideal candidates for integration into systems with renewable energy sources. If you're interested in how to choose the right model or what output you need for your house, I also recommend checking out related topics in the Knowledge Center: How to choose an ELÍZ electric boiler: what to focus on before buying and What output of ELÍZ electric boiler do I need for my house.

Basic principle: why an electric boiler and photovoltaics belong together

A photovoltaic system produces electricity when the sun is shining. The problem is that a household may not consume as much electricity at that same time as is being produced. Surpluses flow into the grid – and under Slovak conditions, the purchase price for surplus electricity is either zero (community energy, new legislation since 2024) or significantly lower than the price at which you buy electricity. Feeding surpluses into the grid therefore does not have the same economic value as self-consumption.

This is where the electric boiler comes into play as one of the simplest and cheapest ways to "consume" surplus electricity directly in the household – converting it into heat. The boiler requires no batteries, no special power control in terms of frequency regulation, and can work practically with any surplus that an intelligent controller or energy manager sends to it.

The ELÍZ electric boiler is simple in this respect: it has a resistive heating element, operates on mains voltage of 230 V or 400 V (depending on model and output), has no moving parts, no compressor or other sensitive component that would be prone to frequent switching. You can turn it on and off a thousand times a year without wearing it out any more than if it ran continuously.

Technical integration options: how the ELÍZ boiler connects with a PV system

There are several ways to connect an electric boiler to a photovoltaic system. Each has its advantages, costs and limitations.

1. Simple relay switching (on/off)

The simplest method: an energy manager (e.g. Fronius Smart Meter, SolarEdge Home Monitor, Shelly EM or others) monitors the energy flow at the main fuse. When the surplus reaches a set value – for example 2 kW – the manager starts the boiler. When the surplus drops, the boiler switches off. This approach is cheap and works, but has a drawback: the boiler always runs at full power, so it switches on/off according to current conditions, which may not lead to fully optimal use of the surplus.

2. Power regulation via SSR or PWM

A more modern solution uses a solid state relay (SSR) or PWM regulation, which can smoothly vary the boiler's output from zero to maximum. The boiler therefore doesn't have to run at 100% all the time – if the surplus is only 1.5 kW, the manager gives it 1.5 kW of output. This solution is the most economically efficient and minimizes grid draw. However, it requires an energy manager with this function (e.g. Loxone, HEMS from PV inverter manufacturers, or specialized devices such as Enerles, EV-BOX for heat, etc.).

3. Integration via HTTP/Modbus API

Some modern solutions allow direct communication between the inverter and the boiler or via a smart home system. For ELÍZ boilers, which have a simpler design, this integration is usually handled at the level of external control (thermostat, timer switch, GSM relay), not via a direct boiler protocol. This is actually an advantage – the boiler is not dependent on a specific ecosystem and can be connected to almost any smart solution.

4. Combination with battery storage

A battery system stores surpluses for later – ideally for evening consumption. An electric boiler and a battery are not in conflict, but prioritization depends on system settings. Generally speaking: a battery is a more expensive investment and makes sense for evening/night peak hours. A boiler is a cheap "heat accumulator" – if you have a sufficiently large storage tank (e.g. 500–1000 liters), you can heat water to 75–85 °C during the day and use it in the evening for both heating and domestic hot water preparation without drawing from the grid.

Economics of the combination: figures from real practice

Let's break it down using a specific example. A family house with heat consumption of 15,000 kWh per year (a fairly passive standard, insulated house, about 150 m², new build or complete renovation). The electric boiler covers both heating and domestic hot water preparation.

Without photovoltaics: at an electricity price of €0.22/kWh (average including all fees, including distribution), the annual energy cost is €3,300. This is with full draw from the grid.

With a 10 kWp photovoltaic system (approx. 40–45 panels, real annual production in southern Slovakia 9,500–10,500 kWh, in Bratislava or Košice): by optimizing consumption for self-generated power, you can cover 50–65% of the boiler's consumption with solar electricity from April to September. In the winter months, when PV production is lowest and heating consumption is highest, the effect is smaller – but never zero.

A real case from practice: a customer in Nitra, a 180 m² family house, a 9 kW electric boiler + 800-liter storage tank + 8 kWp PV. Result after the first year: annual energy savings of €1,400–1,700, almost 100% of domestic hot water covered by self-generation in the summer months, heating in the transitional period (September, October, April, May) largely covered. Investment in the PV system (including inverter and installation, excluding subsidy) approx. €12,000, payback period 7–9 years.

As for the ELÍZ boiler itself, its price is a marginal item compared to the PV system – meaning the boiler itself does not represent a barrier to the project's economics. For example, the ELÍZ EQH 802 is a compact, straightforward device, where the boiler's price alone is a fraction of the total investment in the system.

Impact of the storage tank on economics

The storage tank is a key element that significantly increases the efficiency of the combination. The logic is simple: the sun shines during the day, when the house is mostly not being heated (or only minimally). Surplus electricity from PV heats the tank – and this heat is used later, when the sun shines less or not at all. The tank functions as a thermal battery.

300 l tank – suitable for basic domestic hot water, smaller house
500 l tank – combination of domestic hot water + underfloor heating, average house
800–1000 l tank – full-fledged accumulation for a larger house, allows heating a supply for 12–24 hours

With an 800-liter tank filled to 80 °C and a boiler output of 9 kW, heating takes about 6–7 hours. This is a realistic length of a sunny day from May to August. In practice, you can therefore heat the entire tank during one sunny day and draw almost nothing from the grid.

When the combination makes economic sense and when it doesn't

Not every situation is the same. Let's look at the conditions under which this combination is truly advantageous.

The combination pays off when:

You own a family house (not an apartment in an apartment building)
The house is well insulated and heat consumption is below 15,000 kWh/year
You have enough roof area with the right orientation (south ± 45°, pitch 30–45°)
You don't have or want gas heating (or want to replace it – see also the topic Converting from gas heating to an ELÍZ electric boiler: what you need to know)
You plan to live in the house long-term (at least 8–12 years, due to payback)
You can take advantage of the Green Households subsidy or other schemes
You have or plan to have an electric vehicle (synergy of self-consumption)

The combination is less worthwhile or not worthwhile when:

The house is energy-inefficient (old windows, significant thermal bridges) – here insulation should be the priority, not the heat source
You plan a short-term stay or to sell the house
The roof faces northeast or north, or is shaded
You have an old house with high heat consumption (over 25,000 kWh/year) – a heat pump may be more efficient here
The electrical connection is weak and the PV system would require costly upgrades

The graph shows a fundamental problem that isn't discussed much in advertising: PV production and heating consumption are not synchronized in time. In winter, PV production is minimal and heating consumption is at its highest – and vice versa. This doesn't mean the combination doesn't make sense, but it's important to have realistic expectations. An electric boiler powered by PV will cover a large part of summer and transitional-period domestic hot water consumption, and part of the transitional heating period. The heating months of January–February will remain largely dependent on the grid.

ELÍZ EQH 802: a specific look at a model suitable for PV integration

When talking about ELÍZ electric boilers in the context of photovoltaics, it's worth taking a closer look at a specific model. The ELÍZ EQH 802 is a compact electric boiler suitable for smaller to medium applications – apartments, smaller family houses, cottages. For PV integration, it's important that it is a straightforward electric heating device without complex electronics, which can be controlled by an external thermostat or switch – this is the basis for any smart control.

In general, ELÍZ boilers are designed for closed heating systems with the option of connecting a storage tank. This is exactly what you need for PV integration. The tank is a key buffer between instantaneous PV production and actual heat demand – without a tank, the system works less efficiently, because the boiler must respond immediately to heat demand rather than to the availability of solar energy.

Further details on choosing the right model and output can be found in the topic What output of ELÍZ electric boiler do I need for my house, which also includes specific calculation methods.

Subsidies and legislative framework: what you can draw on in 2024–2025

One of the most important conditions for the economics of the whole project is subsidies. Several schemes operate in Slovakia:

Green Households II

This program by the Slovak Innovation and Energy Agency (SIEA) provides vouchers for PV systems. In 2024, the voucher amount for PV systems up to 10 kWp was up to €1,500 per kWp of installed capacity, up to a maximum of €15,000. An electric boiler as a supplementary heat source can be part of a combined project. The condition is that the house must be used for residential purposes and the installation must be carried out by an authorized supplier.

Recovery Plan

For comprehensive building renovation including PV and heating systems, there are calls under the Recovery Plan that also cover the combination of insulation + PV + new heat source. The level of support can reach 30–50% of total eligible costs.

Net-metering vs. net-billing

Since 2024, Slovakia has switched to a net-billing system, where surplus energy fed into the grid is compensated at the market price of electricity for that hour, not the consumer price. This significantly increases the incentive to consume as much of your own electricity as possible – and an electric boiler is one of the simplest ways to achieve this.

Practical procedure: how to start with the ELÍZ + PV combination

If you're deciding on this combination, I recommend the following procedure:

Energy audit of the house – determine the actual heat and electricity consumption. Without this figure, nothing can be properly sized.
Boiler sizing – choose the right output based on the house's heat loss. The topic What output of ELÍZ electric boiler do I need for my house will help you with this.
Selection and sizing of the PV system – don't maximize panel output, but optimize the ratio of production to self-consumption. For most family houses, this means 6–12 kWp.
Decision on heat storage – sizing the tank according to daily heat demand and expected solar production.
Choice of energy manager – the device that will control the boiler according to current PV production. This is a key element for efficiency.
Installation and setup – professional installation of the boiler (more in the topic Installing an ELÍZ electric boiler: procedure and installation requirements) and of the PV system, setting up the controls (see also Setting up and programming the controls of an ELÍZ electric boiler).
Monitoring and optimization – after the first heating season, evaluate the real results and adjust the manager's settings if necessary.

Comparison with a heat pump: when an electric boiler is the better choice

A heat pump (HP) combined with PV is a popular solution that achieves a higher coefficient of performance (COP 3–5), meaning that from 1 kW of electrical energy it produces 3–5 kW of heat. At first glance it looks like a clearly better choice. So why even consider an electric boiler?

From practical experience, I know the situation is more complex:

Price: An air-to-water heat pump, including installation and adaptation of the heating system, costs €10,000–25,000. The ELÍZ electric boiler is only a few percent of this amount.
Suitability for older systems: Heat pumps work most efficiently with low-temperature heating (35–45 °C). If you have an old system with temperature gradients of 70/55 °C and cast-iron radiators, the HP either won't work efficiently or will require a complete redesign of the radiators – thousands of euros more.
Reliability and maintenance: An electric boiler has no moving parts, compressor, or refrigerant. It requires virtually no servicing (more in the topic Maintenance and servicing of an ELÍZ electric boiler: how to extend its lifespan). A HP requires regular servicing.
Winter efficiency: An air-to-water HP at -15 °C works with a COP of 1.5–2.0 – still better than an electric boiler with a COP of 1.0, but the difference is not as dramatic as it might seem at first glance.

In practice, the ELÍZ electric boiler combined with PV is suitable for well-insulated houses, where heat consumption is low and where the entire heating system doesn't need to be changed. For higher consumption or new builds with underfloor heating, a HP may be the better choice – but that is a topic for a separate analysis.

Safety and electrical installation requirements

The ELÍZ electric boiler is a powerful device that must be professionally installed. When integrating with a PV system, additional requirements arise:

The PV system must be installed by an authorized person and the grid connection approved by the distributor (VSD, ZSD, SSE).
The energy manager must be connected after the main meter, but before the boiler's distribution board.
The boiler's fuse protection must match its input power – for more powerful models (6–12 kW, three-phase connection), a 3×16 A or 3×20 A circuit breaker is required, depending on the specific model.
The boiler must never be connected directly to the DC output of the PV panels – it must always operate from the AC grid, downstream of the inverter.
When using SSR or PWM control, compatibility with the specific boiler model must be verified.

More on electrical installation can be found in the topic Installing an ELÍZ electric boiler: procedure and installation requirements.

Real-world examples: what experience from dozens of projects shows

Case 1 – customer from Trnava, new build 2022: 140 m² house, underfloor heating, 9 kW electric boiler, 600 l tank, 7.5 kWp PV on a gable roof, SW orientation. Fronius Ohmpilot energy manager (specifically designed for controlling ohmic loads including boilers). Result after two years: in the months April–September, almost the entire domestic hot water consumption and part of the heating is covered by self-generation. Annual savings of approx. €1,200. The customer is satisfied, the system runs without any fault.

Case 2 – customer from Žilina, older house after insulation: 180 m² house from 1988, fully insulated in 2021, old radiators replaced with larger ones, 12 kW electric boiler replaced a gas boiler (more on this process in the topic Converting from gas heating to an ELÍZ electric boiler: what you need to know). 10 kWp PV on the garage, south orientation. 1,000 l tank. Shelly Pro 3EM + Shelly Plus 1 PM energy manager for switching the boiler. Result: winter months still mostly from the grid, transitional period and summer almost exclusively from PV. Annual savings of €1,600 compared to previous gas + electricity costs.

Case 3 – customer from Košice, cottage: Smaller 60 m² cottage, used year-round, small-output electric boiler (3 kW), 3 kWp PV. No tank. Result: in summer the boiler practically draws nothing from the grid (domestic hot water and occasional reheating), in winter full dependence on the grid. The economic benefit is smaller, but the customer values above all the energy independence and zero faults in the system.

Most common mistakes when planning the combination

From practical experience, I know people make several typical mistakes when planning this combination:

Oversizing PV without a storage tank – a large PV system without heat accumulation won't fully use its potential, because the boiler can only take as much power as it currently needs.
Underestimating the winter balance – calculating based on summer PV production. Realistically, you need to expect that December/January will be covered by PV for only 10–20% of heat consumption.
Missing energy manager – connecting the boiler directly without intelligent control means you'll have to manually switch the boiler on/off, which no one does long-term. The investment in a manager (€200–800 depending on the solution) pays for itself quickly.
Poor placement of the tank – the tank must be in a utility room with sufficient dimensions and access. Retrofitting a 1000-liter tank in an already-built house can be complicated.
Ignoring tariffs – some electricity tariffs have favorable night-time prices, which may be suboptimal in combination with PV. It's advisable to consult the system setup with an electrician or energy advisor.

Frequently Asked Questions (FAQ)

Do I need to replace my ELÍZ electric boiler with a special "PV-compatible" model when using PV?

No. ELÍZ electric boilers are standard ohmic appliances that work with regular mains electricity from the PV system's inverter. They don't require any special modification or special model. The only condition is that the boiler must be powered from the AC side of the system – downstream of the inverter, not directly from the panels' DC output. Managing surplus energy is handled by an external energy manager.

What PV system output do I need if I want to cover most of the electric boiler's consumption?

Rule of thumb: if your electric boiler is rated at 6–9 kW and you want to cover domestic hot water consumption (approx. 3,000–4,000 kWh/year) and as much of the transitional heating as possible, you need a PV system with an output of at least 6–8 kWp. Without a storage tank, efficiency is lower. With good accumulation of 600–800 l, even a 6 kWp PV system can cover 60–70% of annual domestic hot water consumption. A more precise calculation depends on location, orientation, roof pitch and actual consumption.

Can I connect the ELÍZ EQH 802 to a common energy manager from a PV inverter (e.g. Fronius Ohmpilot, SolarEdge Home Hub)?

Yes, the ELÍZ boiler can be connected to most energy managers. The Fronius Ohmpilot is specifically designed for smooth regulation of ohmic loads (including electric boilers) and works very well with ELÍZ boilers. For on/off control, any smart relay (Shelly, Sonoff, Aqara) with energy flow measurement is sufficient. For smooth regulation, an SSR module and a manager with a PWM or 0–10 V output are required.

Is it worth investing in battery storage alongside an electric boiler and PV?

Battery storage and an electric boiler with a storage tank partly serve a similar function – both "store" surplus energy. A battery is more flexible (you can use it for any consumption including lighting, appliances, EV charging), but it's significantly more expensive (the price of a 10–30 kWh battery system is €5,000–20,000). A heat storage tank is a cheaper "accumulator" of thermal energy, but it only works for heating and domestic hot water. The optimal solution for most households is a combination of PV + heat storage tank (cheap, simple), possibly supplemented with a smaller battery for evening consumption. A large battery system without a storage tank is usually economically less advantageous for primarily electrically heated houses.

How long does it take for the investment in a PV system combined with an ELÍZ electric boiler to pay back?

At current electricity prices, PV system prices, and without a subsidy: 8–12 years for a typical family house. With a subsidy (Green Households II): 5–8 years. This depends on many factors – location, orientation, system size, actual self-consumption and electricity price trends. As electricity prices rise, the payback period shortens. The ELÍZ boiler itself has a lifespan of 20–30 years with proper maintenance, so from an overall balance perspective, it's an advantageous combination.

Is official approval or notification required for connecting an ELÍZ boiler to a PV system?

The PV system itself requires a connection approved by the distributor, and for outputs above 10 kWp, also a building notification/permit. The ELÍZ electric boiler, as an appliance within a home installation, does not require special approval, but must be installed in accordance with the applicable electrical installation standard (STN 33 2000) by an authorized person. For the overall installation, it's advisable to prepare an electrical installation inspection report.

Conclusion: realistic expectations and the right decision

The combination of an ELÍZ electric boiler and photovoltaics is not a trouble-free "miracle" investment, but with proper sizing, the presence of a storage tank and an intelligent energy manager, it is an economically sensible and technically reliable solution. The strength of ELÍZ electric boilers in this context is their simplicity, resistance to frequent switching, and zero dependence on external servicing.

Key lessons from practice: don't expect PV to cover heating in winter – that's simply not physically possible with today's typical family PV system sizes. But in the transitional period and in summer, you can significantly reduce or eliminate the costs of domestic hot water and space heating.

Do you have a question about this topic?

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