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By Adrian Băisan | PhotoVoltaic Windows SRL
Edited by Paul Gerke
At PhotoVoltaic Windows, we have been working with semi-transparent photovoltaic glass and exploring how it can be integrated into buildings — windows, facades, balustrades, terraces, greenhouses, and other glazed surfaces.
The photovoltaic glass we are working with can have different degrees of transparency and different colors, dimensions, and shapes, and the generated power depends on the transparency: the lower the transparency, the higher the power. For example, photovoltaic glass with 40–30% transparency typically has a nominal power of approximately 8.2–10.2 W/ft² (88–110 W/m²).
We found an interesting application for photovoltaic glass in apartments: heating domestic water.
The idea came with a leaking electric water heater. The tank had to be replaced, but the heating element was still good. We took the heating element out and connected it directly to one photovoltaic glass element.
It got warm immediately. That was a eureka moment.
From there, we started experimenting with several photovoltaic glass elements and eventually built the system that has now been operating in the apartment for almost four years. The pilot has been operating since December 2022. Systematic measurements began in October 2023.
The pilot system consists of eleven semi-transparent photovoltaic glass elements, each measuring 47.2 × 23.6 × 0.28 in. (1200 × 600 × 7 mm): nine with 30% transparency and two with 10% transparency.
They are installed on the south-facing glazed balcony of an apartment in Bucharest and supply an ordinary 21-gallon (80-liter), 3 kW electric water heater directly in DC.
Although this pilot is installed on a glazed balcony, the application is not limited to balconies. Photovoltaic glass can also be used in apartment windows, depending on the available glazed area and its orientation. Depending on the project, the photovoltaic glass can be installed directly into the window frame or fixed onto the frame as an additional glazed element. Because the photovoltaic surface is itself glass rather than an opaque solar module added to the building, it can preserve the appearance of the facade and look much like conventional tinted glazing.
During the day, electricity from the glass goes straight to the heating element.
No inverter.
Electrical energy simply becomes heat and is stored in the water.
If the solar contribution during the day is not enough, the same water heater is automatically switched to the normal AC grid around sunset and continues heating until its own thermostat reaches the set temperature.
The basic principle is simple: use the solar electricity directly when it is available, and switch to the grid only when needed.
The apartment is on the eighth floor of a residential building in Bucharest. The balcony faces south.
The eleven photovoltaic glass elements, each measuring 47.2 × 23.6 in. (1200 × 600 mm), have a combined nominal installed power of approximately 845 Wp.
Four people live in the apartment and use hot water every day, including roughly one shower per person per day.
Figure 1: Interior view through the south-facing 30% photovoltaic glazing in the Bucharest apartment.
We monitored the energy delivered by the photovoltaic glazing to the water heater over three consecutive October-to-September periods.
During 2023–2024, the photovoltaic windows supplied 446 kWh directly to the water heater, while 508.4 kWh came from the grid. Solar therefore covered 46.7% of the household’s domestic hot-water energy demand.
During 2024–2025, they supplied 462 kWh, while 594 kWh came from the grid, giving a solar coverage of 43.7% of the hot-water demand.
During 2025–2026, the photovoltaic windows supplied approximately 360 kWh directly to the water heater, while 586.2 kWh came from the grid, giving a solar coverage of 38.0% of the hot-water demand.
The lower solar contribution in the last year should be viewed in the context of unusually wet weather in Bucharest. According to Romania’s National Meteorological Administration (ANM), October 2025 and February 2026 were the second-wettest since 1901, while November 2025 ranked as the ninth-wettest November over the same historical record.
Each year the family was away for approximately one month, so each October-to-September period represents about eleven months of actual household use.
We have published monthly results on our Facebook page since October 2023 for traceability.
One challenge common to photovoltaic systems is that electricity production and electricity consumption do not always happen at the same time. Plug-in balcony PV systems provide a useful comparison.
Germany’s Federal Environment Agency — Umweltbundesamt, or UBA, the country’s central environmental authority — gives the example of an 800 W south-facing, vertically mounted plug-in balcony PV system producing approximately 532 kWh per year. Without battery storage, UBA assumes that around 45%, or approximately 240 kWh per year, is consumed directly in the apartment.
That 45% figure is not a guaranteed self-consumption rate. In a broader UBA study, market participants estimated self-consumption for plug-in PV systems without battery storage at between 18% and 60%, with 45% as the median.
UBA also points out that adding battery storage to small plug-in balcony PV systems is generally difficult to justify economically. It notes that the amount of surplus electricity available to charge a battery may be limited, especially outside summer, while storage losses, purchase cost, and limited lifetime reduce the economic benefit.
For us, the obvious storage medium was already there: the water heater. The hot-water tank effectively acts as a low-cost thermal battery: instead of storing electricity in an electrochemical battery, the system stores the solar energy as heat in water. Domestic hot water is needed throughout the year, so the water heater provides a useful load in every season.
If a conventional balcony PV system is producing 500 W while the apartment happens to be consuming only 150 W, only those 150 W are immediately useful inside the apartment. In the plug-in systems considered by UBA, surplus electricity is generally fed into the public grid without compensation.
Our system works differently.
The photovoltaic glass is not connected to the apartment electricity network, and it is not a plug-in system. During daylight hours, the electricity goes directly from the photovoltaic glazing to the water-heater resistance. The water heater therefore provides a load that can accept solar energy for several hours and store it for later use.
A resistive water heater does not need a minimum voltage level before the energy becomes useful. Whether the photovoltaic glass is delivering 100 W, 300 W, or 500 W, that energy is converted into heat and accumulated in the water over time. This is very different from most other household appliances, which require an appropriate supply voltage before they can operate at all.
The electricity produced during the day becomes hot water that can be used in the evening.
For an apartment, this changes the problem completely: instead of trying to match household consumption minute by minute, we can store the solar energy as hot water.
Once we started testing the system, it became clear that direct connection to a water heater is not only a question of photovoltaic wattage. Voltage matters.
In our case, we have an ordinary 3 kW, 230 V electric water heater. Its heating element is a fixed resistance. If a photovoltaic array is connected directly to that resistance, without an inverter, the operating point depends on the current-voltage characteristics of the PV source and on the resistance of the heater.
For comparison, we looked at two opaque, conventional modern photovoltaic modules with a combined nominal capacity of around 800 Wp. Using the module characteristics in our comparison, two such modules connected directly to the resistance of a 3 kW, 230 V water heater can deliver a maximum of only around 300 W.
There is nothing wrong with the modules. The problem is simply that their voltage is not a very good match for that particular resistance. Our photovoltaic glazing operates at a different voltage, much better suited to the water-heater resistance, and our maximum recorded power is 499 W delivered directly to the heating element.
Figure 2: Measured direct-DC operating point: 94.6 V × 5.28 A = 499 W delivered to the water-heater circuit.
We also learned that there is no single electrical configuration that works for every apartment. Each system has to be designed according to the particular installation: the orientation of the balcony or windows, the number of people using domestic hot water, the available glazing area, and the degree of transparency desired by the customer.
Photovoltaic glass can be produced with virtually any degree of transparency and in different colors. Higher transparency means lower photovoltaic power, so the final configuration is a balance between daylight, appearance, available surface, and the amount of solar energy required for hot-water production.
Figure 3: An earlier version of the Bucharest pilot used three photovoltaic glass elements of different colors and transparencies to supply a 4-gallon (15-liter), 1.2 kW water heater, before the system expanded to the present eleven-glass configuration.
The electrical configuration and the water-heater rating can then be selected according to these conditions. The objective is not necessarily to operate the photovoltaic glass exactly at its maximum power point, but to obtain a simple and effective system that supplies a useful amount of solar energy to the water heater throughout the year.
We tested several configurations and found that the same direct-DC principle can work with different common household water-heater ratings.
Photovoltaic glazing also has a useful by-product: shading.
Tinted glass is already common in buildings because it reduces glare and summer solar heat gain. Photovoltaic glazing can provide the same effect while also producing electricity. In our approach, the photovoltaic glass is installed only on fixed sections of the glazing, while opening sections remain conventional. There is a trade-off in winter, but in many homes it matters less in practice because winter days are short and people often return from work or school after dark.
Figure 4: Tinted-glass facades in Doha, Qatar. Modern buildings widely use tinted glazing to reduce glare and solar heat gain.
The south-facing glazed facade of the apartment has a total area of approximately 139 ft² (12.9 m²), including 85 ft² (7.92 m²) of semi-transparent photovoltaic glass.
Figure 5: Exterior view of the photovoltaic glazing integrated into the apartment facade and balcony enclosure.
Under the assumptions used in our thermal analysis, the PV glazing reduces solar heat gain through the complete glazed facade by approximately 40% compared with conventional clear glazing.
For this apartment and the air-conditioning operating assumptions used in the calculation, the estimated reduction in cooling electricity consumption is around 433 to 481 kWh per summer.
We have not measured the air-conditioning savings directly, so these figures should be treated as estimates. Even so, for apartments with large south-, east- or west-facing glazed areas that become very hot in summer, the effect can be significant.
Before installing the local boiler, the apartment used centrally supplied domestic hot water, and we had to let the water run while waiting for usable hot water to reach the shower.
Our original case-study estimate was based on a measured shower flow of 1.77 gallons per minute (6.7 L/min) and a late-September waiting time of about five minutes, with seasonal waiting times of roughly five to eight minutes extrapolated across the year. That calculation indicated about 16,400 gallons (62.1 m³) of water per year flowing during the waiting periods for a four-person household taking one shower per person per day. We subsequently carried out monthly spot measurements over a full 12-month period. Using those measurements to annualize the waiting-water volume gives approximately 17,000 gallons (64 m³) per year, very close to the original estimate. Under the billing arrangement in the apartment building studied, this water is charged as hot water even while it is still cold or lukewarm and before usable hot water reaches the shower. With a local water heater only a few yards (a few meters) from the point of use, this waiting loss is eliminated.
The water loss also represents an energy loss. Using seasonal cold-water temperatures ranging from approximately 50°F (10°C) in winter to 81°F (27°C) in summer and a target shower temperature of 106°F (41°C), the estimated 17,000 gallons (64 m³) of annual waiting water corresponds to approximately 5.7 MMBtu (1,675 kWh thermal, 1.44 Gcal) of hot-water energy per year. This energy is effectively paid for as part of the centrally supplied hot-water service even though it is discharged before reaching a useful shower temperature.
It also has a broader economic impact. In Bucharest, households pay only about one third of the cost of centrally supplied district heating, while the Municipality of Bucharest covers about two thirds through public subsidies. Therefore, reducing centrally supplied hot-water consumption not only reduces the household bill; it also reduces the cost borne by the municipality and, ultimately, by public funds.
The estimated water and energy savings are specific to this apartment. Another apartment may have much shorter pipes, a better circulation system or very different waiting times.
A local water heater also provides another practical benefit: backup hot water if the centralized system fails.
The methodology used to assess the hot-water and air-conditioning savings was also reviewed by the Romanian Association of Energy Auditors for Buildings (AAECR).
The system we ended up with is quite simple. During the day, photovoltaic glazing supplies the water-heater heating element directly in DC, with the system sized for the specific apartment according to the orientation, the number of people, the available balcony or window area, the desired glass transparency, and the water-heater capacity.
Around sunset, if the water is not hot enough, the controller automatically switches the same water heater to the normal AC grid. The water heater’s own thermostat controls the final temperature.
Figure 6: The local water heater and the DC/AC control and monitoring equipment used in the Bucharest pilot.
The automatic switching concept between photovoltaic DC and the AC grid has also been included in a Romanian patent application published by the Romanian State Office for Inventions and Trademarks.
But the switching equipment is not the most interesting part. For us, the interesting part is how all these things work together.
Like most apartments, we simply do not have much surface available for solar generation. Photovoltaic glass can use that surface while still functioning as a window or balcony enclosure. Unlike conventional opaque PV modules mounted onto a building, it can be seamlessly integrated into the glazing, resulting in a much smaller visual impact on the facade.
Several photovoltaic glass elements can provide very good power transfer to a water heater, which does not require alternating current. The amount of energy produced is comparable to what can be obtained from a small balcony PV installation using microinverters, but in our system most of the photovoltaic energy is used in the apartment and stored as hot water.
The water heater itself is already an inexpensive storage system. Besides this, the water waste associated with waiting for hot water from the centralized network is practically eliminated, while the photovoltaic glazing is also considerably reducing the summer cooling load.
The initial investment in photovoltaic glazing is higher than for a plug-in balcony PV system, but the system itself remains relatively simple and inexpensive because it requires no inverter or battery. With few components and no moving parts, maintenance requirements are low, and the system is inherently robust. The annual savings are also higher. As a result, the payback period can be similar, and after the initial investment has been recovered, those annual savings continue.
For apartments using individual gas boilers, the electric water heater can be installed in series upstream of the gas boiler, so that water is first preheated using photovoltaic energy. The gas boiler then provides only the additional heat required, if any. In this way, every kilowatt-hour supplied by the photovoltaic glass reduces the amount of gas the boiler burns and, therefore, the combustion-related emissions released into the air around the building. This is another reason why using balcony photovoltaic generation for water heating makes sense.
After almost four years of operation, our main lesson is simple: the real value comes from the synergy between photovoltaic glass and the water heater. In an apartment, the photovoltaic glass can be seamlessly integrated into windows or balcony enclosures, supply a useful DC load directly, store energy as hot water, reduce summer solar heat gain and, when replacing centrally supplied hot water, practically eliminate the water and energy losses associated with waiting for hot water to reach the apartment. Our experience shows the value of considering solar generation, energy use and storage together when designing a system for an apartment.
This approach also complements plug-in balcony PV systems. Our photovoltaic glass forms a fixed installation for windows and other glazed surfaces, with an off-grid PV circuit supplying the water heater directly. Plug-in systems use grid-connected modules that are generally easier to relocate and are suited to opaque areas, such as solid balcony parapets. Both can be used in the same apartment: photovoltaic glass can supply hot water, while plug-in modules help power household appliances. Together, they can make fuller use of the limited surfaces available for solar generation.
Adrian Băisan is the founder and managing director of PhotoVoltaic Windows SRL, a Romanian company that develops applications for semi-transparent photovoltaic glass for buildings. An instrumentation and control engineer with more than 30 years of international experience in the energy sector, he has worked on projects in nuclear power, conventional power generation, oil and gas, wind and solar energy. Since 2022, he has been developing and testing a photovoltaic-glass system for domestic hot-water production in a Bucharest apartment.

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