9 abril, 2019 Off By admin

Many people want a solar panel on their roof. But how to do that? How many solar panels do I need?

The amount of solar panels depends mainly on your ambition. Do you want to generate half of the electricity you use, do you want to generate a total energy consumption or do you even want to become an energy producer?


Also, it depends on how much space you have on your roof and how adequate your roof is.

If you are interested in solar energy for your home and want to be completely self-sufficient or achieve significant savings on your energy bills.


Continue reading the article and you will be able to discover how many solar panels I need for a house .


What do we need to calculate how many solar panels I need for a house?

When do you get the most money from solar panels financially? Optimum is, at least until 2020, if you have exactly enough solar panels to generate your total annual electricity consumption.

To determine how many solar panels are optimal for you, you need:

  • An estimate of the performance (power) of the solar panels that will be purchased.
  • Your own energy bill to see how much energy you use per year.
  • Dimensions, inclination angle and orientation of your roof.

Make the calculation of the number of solar panels by yourself

A good installer can determine a fairly accurate performance expectation for your roof, but for an initial estimate of the number of panels you need, use the formula:
(annual energy consumption x 1.1) divided by (the capacity of 1 panel in watts) peak) .

Example  :

  • Its annual energy consumption is 3000 kWh.
  • Your provider offers 290 Wp solar panels.
  • Of that you need 3000 x 1.1 = 3300 divided by 290 = 11.4 solar modules

Because the performance of solar panels varies from year to year and decreases gradually over the years, you could, for example, opt for 12 panels if they fit on your roof.

Number of solar panels you will need for your home


Desired annual yield (kWh)Number of panels required

The required number of panels depends on the capacity of the solar panels in peak watts (Wp, see below). The formula is based on 300 Wp panels.

With the formula:  (desired performance in kWh x 1.1) divided by (the power per panel in peak watts),  the number of panels with higher or lower power is calculated.

The power of solar panels in watt peak

The power of the solar panels is expressed in watts peak (Wp). That is the maximum (maximum) power generated by the solar panel under standard laboratory conditions (the so-called STC conditions: optimal irradiation, temperature of 25 degrees).

The peak power is necessary to calculate the annual performance, but also to determine the capacity of the   inverter   for a certain number of panels. It is often smart to choose the investor’s capacity slightly (for example, 10%) lower than the capacity of the solar system.

Then you need a ‘smaller’ inverter and can operate more efficiently. The loss of not being able to reach the maximum capacity (STC) is very small, because this situation almost never happens in the Netherlands.

Now there are standard size solar panels (1 m by 1.6 m) with a maximum power of up to 360 Wp, but it is common between 270-300 Wp. Panels with extra high power are more expensive and are often only paid when the ceiling space is limited; After all, they offer more income per square meter, but not always for investment in euros.

Calculate the annual yield of solar panels.

We consider as an example, the following golden rule applies in the Netherlands, for other countries the situation may be different

The average annual performance of solar panels in kilowatt hours (kWh) is 90% of the system capacity in peak watts.

Example: 10 panels of 300 Wp produce approximately 0.9 x 10 x 300 = 2700 kWh.

In coastal areas where annual radiation is higher, this will be a bit more. Cold temperatures are also favorable, because solar panels work better with the same amount of sunlight than when they are colder.

The difference in performance between extremely sunny years, such as 2018, and somber years above average (this has not happened since 2000!) Can reach 25%.

If the panels are not located in the south, but in the southeast or the southwest, you can safely collect 10% of the expected annual yield.

Solar panels east-west

Also oriented towards the west and the east, the solar panels are worth it. It is not without reason that many large-scale solar parks are already being placed in a so-called “east-west arrangement”, with half of the panels oriented to the west and the other half oriented to the east.

In this way, the performance of the system is distributed much more evenly throughout the day, rather than mainly around a considerable peak in the afternoon.

After all, the sun rises in the east, stands when it is higher (and, therefore, shines brighter) in the south and sets again in the west.

Solar panels thus charge less power grid. In the future, with dynamic energy rates, the electricity that comes back in the morning or night may be worth more than the electricity that it puts in the network in the afternoon.

You may also have less frequency at home in the middle of the day, and the use of electricity mainly at the time of generation will also be increasingly favorable.

Check your annual electricity consumption.

As shown above, new solar panels average 260 kWh of electricity per year.

An average household consumes around 3000 kWh of electricity per year. For this energy consumption you need 12 average solar panels (12 x 260 = 3120).

But this is an average of large and small homes. The number of devices, how efficient they are and if you drive electrically also plays an important role.

Therefore, it is smart to first take your energy bill to see how many panels are optimal for you. As much as possible, consider future changes in your home or lifestyle.

Keep in mind that  in the coming years many people will switch to electric cooking (induction), electric conduction and perhaps electric heating (with heat pump). You need a lot of solar panels for that scenario.

The subsequent installation is financially less favorable than the installation of the roof due to installation costs.

Ceiling surface

Be careful with simply calculating the area of ​​your roof to determine the possible  number   of solar panels. If some parts of the roof (sometimes) are in the shade, it may be smarter not to place the panels there. However, there are several options to optimize performance with a partially shaded roof.

From a purely financial point of view, it was always advisable not to install more solar panels than necessary to cover your annual electricity consumption, so that you can compensate as much as possible.

But starting in 2021, the compensation scheme will probably be replaced by a premium per kWh supplied. In that case, it may be worth installing as many panels as it can fit on your roof. The subsequent installation of panels entails more installation costs.

Read more about The best solar panels

First of all, it is important that you know your own energy consumption. Electricity is charged per kilowatt hour (kWh). Check your last electricity bill to find out what your consumption is.

In 2009, an average family consumed around 3500 kWh of electricity. Solar panels indicate the production capacity they have, this is shown in Watt Peak (Wp).

Therefore, if the solar panels are in good condition, they will generate 80 kWh of electricity per 100Wp of power.

On this basis, you can calculate the amount of Wp of solar panels you need. A solar panel has an average production capacity of 150 Wp per square meter, so you can also calculate the required surface area you need.

Example 1:

A family uses 3000 kWh per year in electricity. The family decides that they want to generate 70% of their electricity with solar panels, the remaining 30% that they want to recover with savings in use.

So they want to generate 2100 kWh of electricity a year with solar panels. To achieve this, they need 2625 Wp of energy from the solar panels. (2100 kWh / 0.8 (80kWh / 100Wp) For this capacity they need 17.5 square meters of solar panels. (2625Wp / 150Wp / m2)

Calculation example 2:

A large family has a ceiling area of ​​30 square meters that is suitable for solar panels and wants to know how much electricity they can generate with it.

The capacity of your system is 30 m  2  x 150 Wp / m  2  = 4500 Wp. With these 4500 can generate 3600 kWh of electricity per year. (x80 kWh / 100Wp)

They themselves use 4000 kWh of electricity. Therefore, they can generate 90% of their current annual consumption with solar energy.