How to select and calculate the power of a photovoltaic system?

In recent years, photovoltaic energy has become increasingly popular. Today, the sun’s energy is obtained by homes, farms and businesses. When planning a home power plant, attention should be paid, among other things, to the power of the photovoltaic installation.

How to choose the power of a photovoltaic installation for the building?

Knowing what power of the photovoltaic cells for your home will be optimal, you can properly select the power of the system  so that the energy produced does not exceed the annual consumption of the farm. 

Adequate designs help, but if it is a complete professional photovoltaic, the appropriate power will be calculated by the specialists of the company executing the project. However, it is worth knowing what determines the size adjustment of a home solar power plant.

It is essential to determine the amount of electricity consumed by the farm during the year. This is the most important parameter for the size of the photovoltaic installation and the return on investment. As a general rule of thumb, and provided the correct calculations are made, the larger the installation, the shorter the payback period.

With buildings already built, the easiest way is to use last year’s electricity bills. Possible increased demand (eg installation of a heat pump, new energy consuming equipment) must also be taken into account. Electricity consumption for new buildings is more difficult to estimate. In this case, the planned consumption must be estimated taking into account the number of household members and the number and type of devices that consume a lot of energy.

Although the photovoltaic installation must cover the needs of the farm, it must be remembered that it is always possible to increase its power. While the savings on electricity bills are lower, the payback period will remain at a similar level. Of the two bad things, it is a much better solution than a power plant with too much energy. So higher financial outlays go hand in hand with a longer payback period.

Meanwhile, oversizing the installation and expanding it with new modules often does not even require the replacement of the inverter. As a general rule, there is no such obligation when adding one or two panels to a photovoltaic installation. The use of microinverters, on the other hand, is recommended for further expansion. In this case, the new panels will work independently of the original system.

How to choose a photovoltaic installation based on the location and size of the roof?

In addition to calculating the power of the photovoltaic system, it is important to identify the installation options before starting the installation work:

• ceiling or floor surface,
• exposure of the surface to the sun,
• the angle of inclination of the slope or roof frames,
• possible shadows.

Specialists must also determine the condition of the roof truss and covering. If the conditions for the installation of the photovoltaic system are not favorable, it may be necessary to increase the power of the installation to fully cover the demand of the farm.

The problem can also be an insufficient ceiling surface. For example, a 4 kW installation requires 18 m² of ceiling area. If the ceiling is too small, then installation on the ground, using special frames, should be considered.

How to calculate the power of a photovoltaic installation – formula

The optimally selected power of the photovoltaic panels has an impact on savings and determines the return time of the investment. Therefore, before starting the investment, it is necessary to consider what size photovoltaic installation is necessary for the needs and requirements of the farm.

First of all, the calculations must be carried out by specialists. There are also some simple formulas on how to calculate the power of solar panels.  The choice of formula depends on the data available (including electricity bills) and the period of the year when energy consumption is highest.

The formula for the power of a photovoltaic installation

The amount of the monthly electricity bill (in PLN) × 12 × 2 / annual production in kW = power of the photovoltaic installation, where:

• 12 – number of months of the year,
• 2 – coefficient taking into account the method of accounting for photovoltaic energy by the energy company,
• annual production in kW – approx. 800 kWh (for installations facing east or west) or approx. 1000 kWh (for installations facing south).

Example of a PLN 200 monthly bill and a south-facing facility:

• 200 × 12 × 2/1000 kWh = 4.8 kW

The formula for the power of a photovoltaic installation with electricity consumption mainly in winter

The amount of the monthly electricity bill (in PLN) × 12 × 2.2 / annual production in kW = power of the photovoltaic installation

Example of a PLN 200 monthly bill and a south-facing facility:

• 200 × 12 × 2.2 / 1000 kWh = approximately 5.3 kW

The formula for the power of a photovoltaic installation with a known annual energy consumption

If we know the annual energy consumption indicated by the energy bill, we can use the formula:

Annual consumption in kWh × 1.2 / Annual production in kW = Photovoltaic power

If electricity is mainly consumed in winter, replace factor 1.2 with 1.25.

Example with a power consumption of 4000 kWh and a photovoltaic production of 1000 kWh per 1 kW:

• 4000 kWh × 1.2 / 1000 kWh = 4.8 kW

The power of the photovoltaic plant is therefore a key issue for saving on bills and the payback period.  Taking into account the average cost of a domestic photovoltaic installation, which amounts to approximately PLN 30,000, specialists should calculate its size. They will take into account the expected energy consumption, the parameters of the photovoltaic panels, the mounting possibilities and the technical design.