If you are thinking of installing solar panels in your home, the first thing you can ask is how many solar panels do I need? Will I have enough space on my roof?

HOW MANY SOLAR PANELS DO I NEED or Watt Pico (Wp) do I need?



The amount of solar panels you need depends on the number of kilowatt-hours you use per year and the amount you want to generate through the solar panels. 

Experts recommend that you generate 75% of your maximum energy consumption with solar panels. This has to do with the network scheme and after a certain point (the limit of the network) it is less attractive to generate solar energy. See also the page:  balancing  . 

Therefore, the consumption of your home is determined by the number of panels, but remember that the space available on your roof also has a significant influence on this number.

In fact, you do not have to know at all how much solar panels you need, but  how much your solar panel installation generates in Wp  (kWh).

Look at the power of the panels 

The capacity of the solar panels is a good indicator to determine the number (in part). The power is expressed in Watt peak (Wp). This is the maximum capacity that solar panels can generate under ideal conditions (for example, in a laboratory or testing centers of solar panel manufacturers). 

These circumstances are rarely present in practice. That is why the Wp capacity is corrected by a factor of 0.85. A system of solar panels with a capacity of 2000 Wp, therefore, supplies 1700 kilowatt hours (kWh) of usable electricity.

Step by step plan to calculate the number of solar panels

The formula: average electricity consumption (kWh) / 0.85 = Wp / 260

1. How much kWh of electricity do I use?

In Belgium you will not receive a refund if you produce more electricity than you need. Therefore, it is important that you do not buy the installation too much unless you expect much greater energy consumption in the future.

Some also claim that your solar panel system should achieve greater efficiency (Wp) than what you use, but this contradicts. Unless you  achieve poor performance on your solar panels  according  to your installation plan (poor slope or orientation), it is better that you calculate 10% less than your standard consumption.

People usually charge  1,000 kWh per person  in a  house,  but as the light shines for everyone in the house, you can count a little less per extra person.

In this way, you get  3,500 kWh per year  for an  average family  in Belgium

If  we deduct the 10%  margin here, we reach the  3,150 kWh  of consumption that we want to obtain from our solar panels. Click here  for the average electricity consumption per year.

2. Convert Wp to kWh or vice versa

Solar panels work with Wp (Watt peak) and their energy consumption is calculated in kWh, so you need to convert it to know what you need. This so-called performance factor is 0.85.

If you have a solar panel that supplies 250 Wp, this solar panel will deliver approximately 212 kWh.

However, if you want to know how many solar panels you will need, perform the calculation in the opposite direction.

Let’s go back to our standard family that uses 3,500 kWh per year and want to get 3,150 kWh of solar panels.

Performance factor of 3,150 kWh / 0.85 = 3,705 Wp

So we need a solar panel system that generates around 3,705 Wp.

3. Total watt peak of your solar panel system converted by solar panel

Now, how to know how much a solar panel yields to Wp and you know exactly how many solar panels you will need. You can let this number play a role in your decision on which brand you will buy.

ATTENTION: most solar panels have a size of 99 cm by 165 cm, so you can immediately know how much surface you need on your roof.

Our system requires 3,705 Wp / 260 Wp per solar panel = +/- 14 solar panels

The same exercise can be done for a consumption of 1000 kwh, 1500 kwh, 2000 kwh, 2500 kwh, 3000 kwh, 3500 kwh, 4000 kwh, 4500 kwh, 5000 kwh, 5500 kwh, 6000 kwh, 6500 kwh, 7000 kwh, 7500 kwh, 8000 kwh, 8500 kwh, 9000 kwh, 9500 kwh, 10000 kwh, 20000 kwh, etc.

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