How to choose the right capacity of a PV installation?

In the last several years, there has been a rapid increase in the popularity of in-house PV systems. Before investing, it is important to determine what size of the installation a given person actually needs.

This article analyses the factors which influence the right choice of solar panels and explains how to estimate the capacity that will cover the annual electricity demand. It is essential to remember that the capacity of a PV installation should not exceed such demand.

When considering the installation of a PV system, a potential investor is faced with several dilemmas:

• Should the installation be roof- or ground-mounted?
• Which installation company to choose?
• Which manufacturers have the best product offer (solar panels and an inverter)?
• What capacity of the installation will optimally cover the annual power demand?

The answers to these questions will facilitate putting the plan to own a mini solar farm into effect.

Surplus energy cannot be sold. As of 01 July 2016, private prosumers in Poland are required to make cashless settlements with the power distribution companies in relation to the energy produced and fed into the grid. Therefore, it is pointless to have the PV installation oversized by more than 20%, as unused excess electricity is lost after each year. Why 20%? Because with an in-house PV system with the capacity of up to 10 kW, maximum 80% of the generated power can be taken from the grid. In the case of installations with the capacity between 10-50 kW, this proportion drops to 70%.

By making a very careful choice in relation to the capacity of a PV installation, the monthly bills can be reduced to more than dozen PLN, i.e. to the value of fixed charges.

Electricity bills can be of great help when making such a decision, as they show the current electricity consumption. It is important to bear in mind that the household’s demand for energy is constantly increasing due to, for example, the transition to electric heating in a given house/flat, or the purchase of many new energy-intensive appliances. Initially, the proper capacity of the installation can be determined using simple indicators, such as the amounts on bills or the quantity of energy consumed:

• every 1,000 kWh of electricity consumed per year translates into approx. 1.25 kW of the PV installation’s capacity;
• every PLN 100 included on a monthly bill translates into the capacity of approx. 2.5 kW.

The above assumptions are, of course, generalised – a fully accurate choice of capacity also depends on the location of the installation, including the tilt angle of the panels as well as their inclination in relation to the south.

In Polish climatic conditions, an optimally positioned PV system is capable of producing a little over 1,000 kWh per year from 1 kWp of the installed capacity. If the system is properly selected, it should be able to compensate for the losses associated with the fact that it is not be possible for 100% of the produced energy to be taken from the grid.

When determining the appropriate capacity of a PV installation, a potential buyer can also use the following formula:

• Ek – energy consumed on an annual basis (final energy) [kWh]
• a – share of the current own consumption [%]
• b – share of the amount of energy fed into the grid [%]
• a+b = 100%
• net-metering – 0.8 per 1 kWh (installation with the capacity of up to 10 kW); 0.7 per 1 kWh (installation with the capacity of 10-50 kW)
• yield – annual energy production by the PV installation per 1 kWp of the installed capacity [kWh]

The ultimate aim is to ensure that the PV installation covers the building’s electricity needs. Unfortunately, there is not always enough space on the roof. Thus, each self-respecting company should carry out an audit, during which several important factors will be analysed:

• size of the roof;
• strength of the roof trusses;
• tilt angle of the panels;
• type of roof covering;
• inclination towards the south (alternatively the east and the west);
• possible sources of shade (chimneys, aerials, trees or neighbouring buildings).

Where possible, ground-mounted PV installations should face south – at an angle of 20-30^o in southern Poland and 25-35^o in the northern parts of the country. In the case of systems installed on flat roofs, an optimal tilt angle is 10-15^o. The right angle of inclination will provide protection against strong gusts of wind that could rip the panels off the roof.

If the roof angle is not optimal, suitable support structures can be used. Their price depends on the material covering the roof, e.g. tiles, trapezoidal sheets or felt. From an economic point of view, if the roof angle is between 15-60^o, with a south-facing tilt of +/-45^o, there is no need for adjustments in this respect.

If the roof is too small, the ground-mounted installation comes to the rescue. In this case, it is important to make sure that the area is not shaded by, for example, buildings or trees (and that there will be no such sources of shade in the future). This will guarantee the availability of a large surface area, allowing for the installation of additional modules.

Currently, a standard PV panel is 2 metres long and 1 metre wide. In the case of the 400W modules, the surface area needed for the installation of 1 kWp is approx. 5 sqm. However, the more powerful 445W modules require the area of 4.5 sqm per 1 kWp.

At this point, PV panels with a capacity below 400W are considered archaic technology. In fact, the best systems are those with as much capacity as possible, because:

• they require less space;
• the labour and material costs are lower (e.g. due to the fact that the installation requires fewer panels and support structures as well as a smaller amount of cable!);
• the installation takes less time;
• there are fewer components that could potentially break down in the future.

An investment in modern, high-quality PV products is an investment into the future – in terms of both the environment as well as personal energy independence.

 fotowoltaika@eneria.pl

Autro: Tomasz Chomski

Ekspert ds.Rozwiązań Fotowoltaika