Solar panel yield: what is efficiency and the payback period?

The yield or efficiency of solar panels is the amount of electricity produced by a solar panel system. This is usually expressed in kilowatt-hours (kWh) per year.

• The power of your solar panels: the power of solar panels is expressed in watt peak (Wp). The higher the power, the more electricity the panels can produce.
• Orientation and pitch: solar panels facing south with a pitch of 30 to 40 degrees will theoretically deliver the most energy. However, a combination of east- and west-facing panels is often better, as they capture a large part of your daily energy peaks (your east-facing panels provide electricity in the morning and the west-facing panels take over in the evening).
• Location: the more sunlight, the greater the yield. In Belgium, 1 kWp of solar panels can generate about 850 kWh per year.
• Shade: shade from trees, buildings or other obstacles can reduce yields considerably.
• Maintenance: by cleaning your solar panels from time to time, you can maximise the yield.

To calculate the yield of your solar panels, you need to know the exact power output. Power is expressed in ‘watt peak’ (Wp). That is the maximum power output, based on the optimum amount of sunlight in the most ideal conditions. The higher the watt peak, the greater the productivity of your solar panels.

To calculate the potential yield of your solar panels, you need to convert watt peak to kilowatt hours (kWh), the unit in which your electricity consumption is expressed.

To convert watt peak to kilowatt hours, we use a conversion factor of 0.9 (or 0.897 to be precise). This is the conversion factor used by the Flemish energy regulator. So a 400 Wp solar panel produces 360 kWh per year, provided the panel is facing south.

You have a household of four people and you consume 4 000 kWh of electricity per year. On average, a solar panel achieves 90% efficiency. This means you need a total output of around 4 444 Wp (4 000 / 0.9). If your solar panels have a peak output of 400 Wp, you will therefore need 11 panels (4444 / 400 = 11).

The payback period of solar panels is the period of time you need to recoup the investment by saving on your energy bills. This period varies depending on:

• The cost of your solar panel system
• Energy prices
• The amount of electricity produced by your solar panels
• Your energy consumption

You should also take the following factors into account to help you decide whether or not solar panels will be beneficial for your situation:

• The lifespan of your solar panels (around 25 to 30 years on average)
• The location (shade, pitch and region of your property)
• Availability of grants
• The cost of having your panels cleaned (higher cost if you live in a wooded area)

On average, you’ll earn back the cost of your solar panels in eight years in Flanders and Wallonia. In Brussels, you can recoup the cost of your installation in five years because you can get ‘green certificates’ in addition to the savings you make.

So how much your solar panels yield and how much you can save on your energy costs as a result depends on a lot of factors. The additional ‘Solar panels and battery’ service in KBC Brussels Mobile lets you easily work out the estimated output and cost of your solar panels.

If you are interested, you can also obtain a no-obligation quote from a reliable contractor from the network of our partner ‘Impact us today’. They will contact you within five business days to set up an appointment.

The KBC Brussels Energy Loan is an instalment loan. Lender: KBC Bank NV, Havenlaan 2, 1080 Brussels, VAT BE 0462.920.226, RLP Brussels, FSMA 026256 A. Subject to your loan application being approved by KBC Bank NV.