Step 1: Intro, PV Peak Power >>>


Is solar power a good investment?

Various countries worldwide, including UK, Canada and USA have a multitude of incentives programs or offering Feed-in Tariffs for electricity generated by solar. This makes an investment in a photovoltaic system attractive with a high yield return.

But apart from any incentive program there are some more factors you need to dertermine to answer the question if solar is a good investment for you.

The electricity production and efficiency of your solar PV system is influenced by various factors. The most important are:

  1. The PV system size you are able to install = depends on your budget, the size of your roof (for roof mounted systems) and on the type of solar modules you are using

  2. Does your Solar PV system operates at or near its rated efficiency = depends on the orientation of your roof, the incline and shading

  3. Solar irradiation = depends on the geographical region

  4. System performance = depends amongst others on the type of solar panel and solar inverter efficiency

The 4 factors above determine the efficiency of a solar PV system and therefore the annual electricity production and eventually the profit of your system.

In our PV guideline we explain the aspects above and navigate you through a PV system planing, step by step

Step 1: Calculate your roof size = determines the maximum size of your PV system

Step 2: Analyse the orientation of your roof, incline and shading = determines how much sun-light your system is able to capture and if your system operates near its rated efficiency

Step 3: Solar radiation of your geographical region

Step 4: The expected generated electricity of your future PV system

Step 5: Profit and return on investment of your future PV system

Step 1 - Calculate your roof size and determines the maximum size of your PV system

The maximum size of your photovoltaic (PV) systems is limited by your budget, on the other hand by the available space, which is in most cases the roof of your house as well as the type of solar panels you are using.

In the first step we will explain how to calculate the roof size. Also the differences of the types of solar panels available on market. With these information you are able to calculate the maximum size of the solar PV system you can install on your roof.

The maximum size is rated in peak power, so called Kilowatt peak power (KWp). The peak power is the amount of power output a PV panel produces at Standard test conditions (STC) of a panel operating temperature of 25 degrees Celsius in full sunshine (irradiance) of 1 000 watts per square meter. This is a clear summer day with sun approximately overhead and the cells faced directly towards the sun.

The bigger the size of your PV system, the more annual electricity output.

a/ The Roof Size:

Calculate your roof size:

- Measure the length of your building (roof): Length "b" in the picture below

- Measure the width of your roof (typically half width of your building for a gable roof): Width "a" in the picture below

- Estimate the incline of your roof: Angle "α" in the picture below. The value will be used as a multiplier for the roof size calculation.

Get the multiplier from angle "α" for a simplified calculation of your roof size:
- Low roof slope (slope of 1:3 or angle of ~15°): Use a multiplier of 1.2 for "a"
- A medium slope (slope of 7:12 or angle of 25°): Use a multiplier of 1.3
- A steep incline (slope 1:1 or 45°): Use a multiplier of 1.4

Sample Calculation:

Width a = 30 feet, Length b = 45 feet, Steep Incline, α = multiplier 1.4
Roof Area = 30 feet x 45 feet x 1.4 = Total of 1890 Square Feet


House PV - Estimate the roof size

Courtesy: PV*SOL® Expert   

You can also use the free software Google Earth to measure the length and width of your roof. It is quite easy. Just type in your address and get the satellite picture of your building. Use the "ruler tool" to make the needed measurement.

b/ Type of Modules and the required module space

There are three different types of modules (find more details in our PV Wiki) with different efficiency. Therefore, the required module space to install 1 KWp varies with the type:

  • Single Crystalline Modules: "Most efficient, but most expensive"
    This is the oldest and more expensive production technique, but it's also the most efficient sunlight conversion technology available. Module efficiency averages about 12% to 18%.

    An area of around 8 square meter to install 1 KWp is required on a pitched roof, around 14 sqm on a flat roof

  • Polycrystalline Modules: "Widely-used, best performance and Price ratio"
    This has a slightly lower conversion efficiency compared to single crystalline but manufacturing costs are also lower. Module efficiency averages about 10% to 11%

    An area of around 8 - 10 square meter to install1 KWp is required on a pitched roof, around 16 - 20 sqm on a flat roof.

  • Thin Film Modules: "Cheap, but less efficient, often used for west or east roofs"
    Silicon material is vaporized and deposited on glass or stainless steel. The cost is lower than any other method.

    Module efficiency averages 5% to 7%.

    An area of around 16 - 20 square meter to install1 KWp is required on a pitched roof, around 40 sqm on a flat roof.

House PV - Photovoltaic Module Sizes

Courtesy: PV*SOL® Expert

Comparison Single, Poly and Thin Film Panels: Required Module Space to Install 1 KWp


Sample Calculation:

Pitched roof, 150 sqm effective surface (see above, Step 1a), polycrystalline module (see above, Step 1b: 10 sqm per KWp):

150 / 10 = Installation of 15 KWp is possible

Remark: 1 sqm = 10.76 sq ft

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