Oversizing solar inverters

In May 2017 Power and Water Corporation, the body responsible for electricity transmission and distribution for the Northern Territory, altered their protocols for limitation of Solar PV installations.

The limiting factors for pre-approved Solar PV installations on single phase supplies  was changed from a maximum of 4.5kW of solar PV panels to a 5 kVA inverter and three phase from 6 kW of solar PV panels to a 7kVA inverter.

This opened the consideration of oversizing the inverters by including additional panels to create more energy from the solar PV.

Some questions have been raised as to the limits of oversizing the solar Inverters and the clarification of data sheets.

Using the SMA  SB5000TL-20 as an example, NT WorkSafe contacted SMA Technical Services and subsequently the SMA Solar Academy for clarification. This is a summary of those details. If you require further details or information  on other inverters you will need to contact your inverter support group.

The Information received:

Extract from the data sheet is below.

Image show an extract from the SMA data sheet.

From the data sheet above:

The Max. DC power (@ unity power factor) 5250 W

Clarification: - This value is the wattage required at the input of the inverter in order to generate an output of 5000 Watts taking into consideration the inherent losses in the inverter.

  1. Determining the maximum over-sizing of PV panels:
    1. Statement from SMA “providing the voltage and current limits for the inverter are not exceeded, solar panels can be installed to the maximum rating for each of the inputs.”
    2. The Inverter can be oversized by solar panels, however there are detrimental effects which are directly related to the amount of oversizing.
  2. Rules:
    1. Maximum input voltage is 750 Volts (600v Domestic. AS5033:2014)
    2. Maximum Power Point Tracking is effective between 175 and 500 volts.
      1. The input voltage is the maximum calculated using the minimum daily temperature (AS5033:2014 - 4.2)
      2. Maximum input current is 15 amps for input A and for input B.
  3. Warnings:
    1. Do not exceed these maximum voltages or current values.
    2. Depending on the amount of oversizing, the inverter and possibly the panels  can be effected detrimentally as excess current generated may flow back into the panels, heating and deteriorating the panels.
    3. The life of the inverter may be reduced due to the additional time in operation, the workload and the heat generated.
    4. Excessive energy generated by the oversizing the panels may cause the inverter to heat and reduce its performance.
    5. Excessive oversizing may affect the claims for ongoing warranty after the first 5 years.
  4. Recommendations:
    1. It is advisable to check the effects of oversizing with the inverter manufacturer as a part of the design.
    2. The designs as supplied by SMA Sunny Design provide the optimal configurations for both output and long term performance.