Nominal vs actual power produced by a solar PV installation

Nominal vs actual power produced by a solar PV installation

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By Jack Ward, CEO of Soltra Energy

Home and business owners thinking of installing solar photovoltaic (PV) systems often request details relating to what they can expect in terms of energy production from a proposed system. For example, they ask if, on a sunny day, full production can be expected for all the daylight hours – say around 10 hours.

For owners who require a better idea of their proposed site’s solar potential – and perhaps a verification of information provided by vendors vying for the business – there are ways to calculate energy yields.

Firstly, some background information: Solar PV panels manufactured by member organisations of the photovoltaic industry usually have a ‘nameplate capacity rating’ that measures DC power in terms of Wp (Watts-peak). This value specifies the output power achieved by a solar PV module under full solar radiation and set standard test conditions (STCs) which are specified in standards such as IEC 61215, IEC 61646 and UL 1703.

Typically, as part of the final quality check at the factory, each solar panel is subjected to a ‘flash test’. This exposes the panel to a high intensity calibrated light source of 1,000 watts per square metre which allows the power output of the panel to be determined.

Technically, the power is measured while varying the resistive load on the panel or module between an open and closed circuit (between maximum and minimum resistance). Each panel is then marked with its nominal capacity rating. A solar PV panel labelled as a ‘250Wp module’ must have generated at least the equivalent of 250 Watts of energy during its flash test.

However, since this figure is based on a measurement taken under optimum conditions, the peak power in terms of Wp is not the same as the peak power delivered under actual radiation conditions.

In reality, the incident sunlight varies greatly over a 10- to 12-hour period. Generally, power peaks will be approximately 15 to 20% lower, mainly due to the considerable heating of the solar cells.

Perhaps surprisingly, around 85% of the sunlight that hits a solar PV panel’s surface is not converted into electricity. For example, if the panel reaches a temperature of 65 degrees Celsius – a common occurrence on a sunny summer day – the output of a 250 Watt panel will be reduced to around 200 W during peak sun hours.

The total irradiation on an array of solar PV panels during the course of an average day is equivalent to running the flash test for around 4.8 hours. This number can be referred to as the ‘daily specific energy yield ’ which will vary throughout the year.

Taking the calculation a step further, the more commonly used measurement of a solar PV panel is its annual specific energy yield  – typically around 1800 kiloWatthours/ kiloWattpeak (kWh/kWp).

What’s more, the orientation of installed solar PV panels will also have a significant bearing on their output. Under ideal conditions in Gauteng (for example) the panels should face due north at an inclination of around 28 degrees from the horizontal. If this ideal placement is not achieved (or cannot be achieved for any reason) output will be reduced accordingly.

For owners looking for an actual power calculation a good rule of thumb is that solar PV panels will run at the equivalent of their peak power rating for between 4.5 and 4.8 hours per day.

There is the possibility that panels from different manufacturers might be labelled as giving exactly the same Watt-peak reading under test conditions, but their output could be significantly different under operating conditions. This difference can be caused by a number of factors, but most likely by varying degradation rates at higher temperatures.

Let’s consider two solar PV panel brands – Brand X and Brand Y – with similar outputs in under standard test conditions. Brand X is the cheaper of the two. It is also the least productive under operating conditions. Brand Y is the more expensive and it is also the better generator of energy in real terms.

Although Brand X is the less productive than Brand Y under operating conditions, because it is cheaper it may well have the potential to become a better financial proposition for the owner.

On the other hand, Brand Y – the more expensive option – may produce so much more power that it could well financially outperform the Brand X offering in the long run.

It is therefore advisable to consult with a specialist who is able to consider all marketplace options and prepare an analysis of long-term performance versus cost. Both initial and on-going costs must be addressed in order to determine which panel will be the best – bearing in mind that a solar PV installation should be seen as a 20- to 25-year investment.