Bucharest Apartment Pilot Uses Solar Windows to Heat Water Without Inverter or Battery
A two-year pilot in a Bucharest apartment uses semi-transparent photovoltaic windows to directly power an electric water heater in DC, storing solar energy as hot water. The system operates off-grid, with no solar inverter or battery, and switches to the AC grid only when solar input is insufficient. It serves a four-person household with about 845 Wp of installed capacity on a south-facing balcony.
The pilot’s design eliminates two common solar components—an inverter and a battery—by routing DC power directly from semi-transparent balcony glazing to a standard water heater’s heating element. A controller switches to the AC grid only after sunset or when solar input drops, ensuring the thermostat setpoint is met. Over two monitored October-to-September cycles, the system delivered 446 and 462 kWh annually, covering roughly 44–47% of a four-person household’s hot-water demand, despite about one month of vacancy each year. The installed capacity was later revised upward from 683 to 845 Wp based on updated manufacturer specifications, though measured energy delivery remained unchanged. This approach treats the water tank as thermal storage, prioritizing self-consumption over peak output, and operates entirely off-grid on the PV side.
This pilot could reshape how apartment dwellers adopt solar, particularly in dense urban areas where roof access is scarce. By using windows as generators and hot water as storage, it may lower upfront costs and simplify maintenance, making renewable integration more accessible to renters and multi-family buildings. If replicated, it could reduce grid strain during peak evening hours and cut household energy bills, though its impact depends on climate, balcony orientation, and hot-water usage patterns. The concept may also spur innovation in building-integrated photovoltaics, but broader adoption would require regulatory clarity and installer training.