The efficiency of solar water panels depends on several factors, including the system size, geographic location, tilt and orientation of the panels, as well as the amount of sunlight available during winter months. It is common for solar water panels to have lower efficiency during winter compared to summer months.
To heat a 500-liter water heater to 50 degrees Celsius in winter conditions, it would be necessary to have an adequate number of panels and a properly sized system that can provide sufficient hot water. The exact number of panels required can vary based on factors such as climate conditions, thermal insulation of the system, and hot water consumption.
For more precise information and an assessment specific to your case, consultation with a solar energy expert or a solar water system installer is recommended. They can analyze the characteristics of your property and provide appropriate information regarding the number of panels and system size needed to achieve the desired results.