Basic principles and concepts

Power and energy

These basic principles that are not always understood. In essence power is the means of producing energy (measured in Watts: W), with a simplistic comparison being the flow rate of water through a pipe i.e. water pressure. The maximum power of an electrical system can be compared to the pressure in a water-pipe and thereby the “peak-power” of a PV-module (see explanation below).

If the tap is turned off, no water flows, irrespective of the pipe size, and similarly if there is no sun, the PV-module produces no energy. Therefore, energy can be calculated (as with water) by the time it flows and the pressure.

In real terms energy is neither generated nor lost, it is converted. If the sun is shining the energy input (solar radiation/rays,) received by the PV-module is converted in to electrical energy (measured in kilowatt-hours: kWh). By calculation, this means the power (resultant from solar radiation) is multiplied by the timescale of measurement expressed in days, months or years.

Solar radiation

The over-riding factor for a PV installation is the overall duration of the solar rays upon the module surface. The actual radiation level is measured in W/m2. This value changes according to the weather i.e. the level of direct sunlight and the angle of the sun during the day.

Therefore, the norm for energy output calculations of a PV installation is for an average daily solar radiation to be provided by selected meteorological institutions for specific geographic locations. This solar radiation value is given in kWh/m2*day (or equivalent) and based on historical data available from these sites.

Photovoltaic (PV) power

Power calculation for a PV module results relies upon calculated solar radiation levels (see above) and to a lesser extent the temperature of the PV module. The power produced is virtually proportional to the solar radiation strength at the time of calculation.

An internationally agreed standard method is established to assess and compare the performance of PV modules in terms of power generation. According to this agreement, the power of a PV module is indicated in “Wp” (Watt peak) which means the power delivered under Standard Test Conditions (STC) that are:Solar radiation of 1000 W/m2Module temperature of 25ºC Solar spectrum AM 1,5.

These conditions are optimum and seldom experienced other than for a few hours at midday in very sunny regions, especially at higher altitudes. Hence it is also called “peak power”.

Total power of a PV installation

The “peak power” of a PV installation is also given in “Wp” as the total power of the linked PV modules. A PV installation that is connected to the public grid (in principle all BIPV installations) must also have an inverter for conversion from DC to AC power and adequate fused protection devices.

Energy output

The energy output of a PV installation (in kWh/year) with a specific peak power depends primarily upon the level of solar radiation throughout the year as well as:

Module temperature (the module output decreases as it gets warmer)

Shadows on the module’s surface (influencing the direct solar radiation on the surface of the cells)

Efficiency of the electronical components for the grid connection (inverter primarily)

Chapter 6 explains in more detail how energy output varies according to the orientation and angle of mounting of the PV surface and in fig. 1 the percentages of energy output for different orientation and inclination is provided.

A precise calculation of the energy output should be sought from a competent engineer, but for an approximate estimate of the annual PV energy output in the UK and Ireland (under ideal conditions) the following data can be taken:

Site Yearly energy generation KWh / KWp* Cladding (0-10% transparency) KWh / m2** Curtain wall (30-40% transparency) KWh / m2**
London 770 100 69
Birmingham 750 98 68
Edinburgh 730 95 66
Dublin 780 101 70
Plymouth 800 104 72

* These values are the annual energy produced in kWh for every kWp of installed PV power in an ideal orientation (facing south, about 40º of inclination).

** These values are the annual energy produced in kWh per square meter of module surface under ideal orientation (as above). For cladding we assume a maximum opacity in order to optimise energy generation per surface as transparency is not of importance. So, about 130Wp/m2 of PV power can be achieved. In the case of curtain walling semi-transparency is important, so a medium transparency has been assumed resulting in about 90Wp/m2 of PV power. For comparison, the output of a conventional standard PV module can be up to 10% higher due to its higher cell density and thinner cover glass that includes additionally an antireflective treatment.

From this data an output estimation can be prepared taking account the orientation and inclination of the PV’s on the specific project as indicated in the table above.