Inverter / Array sizing

The inverter power sizing is a delicate and debated problem.

PVsyst provides a graphical tool (button Show sizing) for the study and understanding of the sub-array sizing, concerning either the array voltage (number of modules in series), and the array power (number of strings).

In this tool, the upper graph concerns the Array voltage sizing. The histogram is a specific tool for the evaluation of the PNom Ratio, as explained below.

Preliminary observations about Pnom sizing

1. The Pnom of the inverter is defined as the output AC power. The corresponding input power is PnomDC = PnomAC / Efficiency, i.e. about 3 to 5% over. For example a 10 kW inverter will need PNomDC=10.5 kW input for operating at 10 kWac.
2. The Pnom of the array is defined for the STC. But in real conditions, this value is very rarely or never attained. The power under 1000 W/m² and 25°C is equivalent to that produced under 1120 W/m² and 55°C with μPmpp = -0.4%/°C. Such an irradiance is only reached episodically in most sites.
3. The power distribution is dependent on the plane orientation: a façade will never receive more than 700-800 W/m², depending on the latitude.
4. The maximum instantaneous powers are not very dependent on the latitude: by clear day and perpendicular to the sun rays, the irradiance is quite comparable, only dependent on the air mass. But it significantly depends on the altitude.
5. Many inverters accept a part of overload over their nominal power, specified by a Pmax parameter, according to the temperature of the device. This is not taken into account in the pre-evaluation, and may reduce the overload loss calculated by the detailed hourly simulation.
6. When over-sized, the inverter will operate more often in its low power range, where the efficiency is decreasing.

Sizing principle

In PVsyst, the PNom ratio sizing is based on an acceptable overload loss during operation, and therefore involves estimations or simulations of the energy yield in the real conditions (weather data, orientation, losses) during the whole year. On our tool, this pre-evaluation is based on an energy distribution histogram, established using a rough pre-simulation. However the accurate determination of the overload loss will only be possible with the detailed hourly simulation. See below for details.

Taking the following into account:

A. Overload behaviour: With all modern inverters, when the Pmpp of the array overcomes its PnomDC limit, the inverter will stay at its safe nominal power by displacing the operating point in the I/V curve of the PV array (towards higher voltages). Therefore it will not undertake any overpower; simply the potential power of the array is not produced. There is no power to dissipate, no overheating and therefore no supplementary ageing.

B. Loss evaluation: In this mode the only energy loss is the difference between the Pmpp "potential" power and the PnomDC limit effectively drawn. We can see on the power distribution diagrams, that even when the inverter's power is a little bit under the maximum powers attained by the array in real operation, this results in very little power losses (violet steps with respect to the green ones, quasi-invisibles here). The simulation - and the analysis of the overload loss - is therefore a very good mean for assessing the size of an inverter.

On the powers distribution histogram, the green curve is the potentially produced energy by the PV array (EArrPmpp), and the violet one is the power limited at the Pnom of the inverter. The overload loss is the area between both lines, with respect to the total area below the green curve.

The PVsyst criteria for an acceptable sizing are specified in the project's definition (menu "Project's settings"): by default:

In practice these criteria lead to Pnom Ratio of the order of 1.25 to 1.3 for most well-oriented systems (Pnom ratio > 2 for façades!). This ratio may become much higher when the inverter's nominal power can be increased up to a value PMax at low temperatures.

For increasing the PNom ratio above the default value, you should modify the overload limit in the project's settings.

For an economical optimization, the final overpower losses are to be put in balance with the price difference with an inverter of higher power. These considerations often lead to significantly undersized inverters with respect to the manufacturer's recommendations.

Nowadays many big installations are subject to an Injection Power Limitation required by the grid manager. This leads many PV designers to define highly oversized PV arrays, which means a much higher energy yield at the price of a resonable energy loss due to the power limitation.

Difference with the houlry simulation

Please note that the detailed simulation results (Overload loss) may be significantly different from this sizing value (they show usually lower losses). This is because the elaboration of the histogram for this rough sizing tool doesn't take all the system losses into account. And the pre-simulation cannot account for the enhanced inverter nominal power depending on the temperature.

The reference result will of course be the result of the detailed hourly simulation.