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6 min Reading time If you have a lot of roof space available, it’s best to match the total capacity of your solar panels as closely as possible to your energy consumption, so your production and usage stay in balance. In practice, however, the amount of shade-free roof space is often the limiting factor.
Let’s take a look at what you need to consider when deciding how many solar panels would be best for your situation.
When calculating how many solar panels you need, it’s wise to aim for a balance between how much electricity you consume and how much you feed back into the grid. If, over the course of a year, you feed more electricity into the grid than you use, you become a net producer of energy. In this case, your energy provider will pay you less for each kWh (kilowatt hour) you return. The net metering scheme (salderingsregeling) does not apply to energy you overproduce.
That’s why it’s important to first determine how many solar panels you can place without becoming a net producer. To do this, your annual electricity usage is key. You’ll find this number on your energy supplier’s yearly statement. For reference, an average Dutch household uses around 3,500 kWh of electricity per year.
Some people use less electricity, others much more. It depends on many factors, such as cooking electrically (e.g., using an induction stove), heating your home electrically (e.g., with a heat pump), or driving an electric vehicle. If you’re planning to buy an EV in the near future, you can already take that into account when determining how many panels you’ll need.
Once you know your expected yearly electricity consumption, you can match the number of solar panels accordingly. Here’s how:
First, calculate the specific yield for your type of roof (in kWh/kWp/year).
Then, determine the peak capacity you’ll need (in kWp).
Finally, calculate how many panels are needed to reach that peak capacity.
Specific yield is a measure of how suitable your roof is for generating solar energy. This helps determine how many panels you’ll need. A south-facing roof with an ideal tilt angle of 30° and no shading can expect a specific yield of about 875 kWh/kWp/year. The more your setup deviates from the ideal angle and orientation, the lower this yield becomes.
In the chart (referred to in the original article), you can see how your specific yield compares to the ideal. White circles indicate the tilt angles of the panels, from 10° (almost flat) to 90° (vertical). The numbers around the edge show orientation. For example, if your roof is tilted 30° and oriented 45° from south (southwest), you’d hit a white dot corresponding to 95% of max irradiation. That results in a specific yield of 0.95 x 875 = 831 kWh/kWp/year.
Flat roofs typically have panels installed at a tilt angle of around 15°.
To calculate how much peak power (in kWp) you need, divide your annual electricity usage (in kWh) by the specific yield:
Example:
A typical household uses 3,500 kWh/year, with a southwest-facing roof at a 30° tilt, giving a specific yield of 831 kWh/kWp/year.
Then: 3,500 / 831 = 4.2 kWp, or 4,200 Wp (watt-peak).
Next, calculate how many solar panels are needed to reach this peak power. This depends on the watt-peak rating of the panels.
Standard solar panels typically range from 265 to 275 Wp, while high-efficiency panels like those from SunPower range from 327 to 360 Wp.
Let’s say the household chooses SunPower panels of 327 Wp:
4,200 / 327 = 12.8 panels, which you round up to 13 solar panels.
Now that you know how many panels you need, the next step is to check if they actually fit. The size of the panels and the available roof space determine what’s possible. If space is limited, you can consider higher-efficiency panels to maximize output.
Solar panels are typically about 1 meter wide by 1.65 meters long. Roofs often have obstacles like pipes or chimneys that cast shadows and affect panel placement. Fortunately, there are ways to reduce the impact of partial shading, allowing you to install panels even in less ideal spots.
So in the end, the number of panels you can install depends not only on your energy usage and roof orientation, but also on how much usable roof space you have.
