To determine how many solar panels you need to produce 4000 kWh of electricity per year, you need to take several factors into account.

1. Power rating of the solar panels: The power of a solar panel is expressed in Watt-peak (Wp). This indicates how much power the panel can generate under ideal conditions. Standard solar panels generally have a power rating between 250 Wp and 400 Wp per panel, but this can vary depending on the type and brand.
   
   
   
   

2. Solar irradiance: The amount of sunlight falling on the panels varies depending on location, orientation, and tilt angle. In sunny regions, solar irradiance can be higher than in areas with fewer sun hours.
   
   
   
   

3. Efficiency of the solar panels: The efficiency of the panels also plays a role. The higher the efficiency, the more electricity they can generate per square meter.
   
   
   
   

4. Consumption pattern: Your current electricity consumption is an important factor in determining the number of panels you need. The more electricity you use, the more panels are required to meet that demand.

To make a rough estimate of the number of solar panels you need, you can use the following formula:

Number of solar panels = Annual electricity demand (in kWh) / (Output of one solar panel (in kWh) x Total system efficiency)

Total system efficiency includes the efficiency of the solar panels, cabling, inverter, and any other system losses. A realistic estimate for total system efficiency is usually between 70% and 90%.

Keep in mind that this formula provides a rough estimate, and it is advisable to consult a professional solar panel installer to perform a more detailed analysis based on your specific situation and needs.