The monthly savings you can achieve with solar panels depend on various factors, including the size of your solar panel system, the amount of sunlight your location receives, the orientation of the solar panels, shading, and other environmental factors. Here are some guidelines:

1. Size of the solar panel system: The larger the solar panel system, the more electricity it can generate and the greater the potential savings. The capacity of solar panels is usually expressed in kilowatts (kW) or kilowatt-hours (kWh).

2. Sun hours: The number of sun hours per day and the annual variation in sunshine affect the output of solar panels. Locations with more sun hours will generally have a higher monthly output.

3. Orientation and tilt: The ideal orientation of solar panels is facing south, with a tilt that matches the geographical latitude of the location. Deviations from this ideal position can affect the output.

4. Shading and obstructing objects: Shade from buildings, trees, or other obstacles can reduce the efficiency of solar panels. It is important to ensure that the panels receive as little shade as possible.

5. Energy consumption: Savings also depend on your own energy consumption. If you consume a lot of electricity and the solar panel system covers a large part of this need, the monthly savings will be higher.

On average, solar panels in the Netherlands can have an annual yield of approximately 850 to 950 kilowatt-hours per kilowatt of installed capacity. For example, if you have a 4-kilowatt solar panel system, you could theoretically produce between 3,400 and 3,800 kilowatt-hours per year.

To calculate the monthly savings, divide this annual yield by 12. Keep in mind that savings can be variable and depend on the factors mentioned above. It is advisable to consult a professional solar installer for a more accurate estimate based on the specific circumstances of your situation.