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Risks associated with the under-dimensioning of the inverter of PV plants

An important point of attention when purchasing a PV plant, is the dimensioning of the inverter in relation to the solar panels. For example, one can opt for an over- or under-dimensioning of the inverter.

In this article, we will focus on the advantages and disadvantages of both approaches.

Over-dimensioning inverter

One of the motives to over-dimension an inverter in relation to the solar panels (eg inverter 3.3 kW and PV plant 3 kWp (*) ) is often the idea to add additional solar panels to the same inverter at a later stage. In this case, it is important to take note in advance of the legislation about the extension of a PV plant. For example, in the Flanders area it is not allowed to simply add solar panels to an existing inverter in an installation that is entitled to green current certificates

Another disadvantage of over-dimensioning is that an inverter with a higher yield is usually more expensive than a smaller inverter.

Moreover, the efficiency of an oversized inverter is low in periods of low production, such as winter. Ideally, the production of the solar panels is as close as possible to the power of the inverter.

Futhermore, solar panels in Belgium almost never achieve their peak efficiency because of non-optimal weather conditions. In most cases, the under-dimensioning of the inverter will therefore be chosen.

(*) The Watt peak capacity of a solar panel installation, expressed in Wp, can be read on the plate of your installation. This concerns the maximum capacity that solar panels can generate at a temperature of 25°C, a light intensity of 1,000 W/m² and a perpendicular incidence of the sun.

Under-dimensioning inverter

When an inverter is under-dimensioned (eg 2.7 kW inverter and 3 kWp PV plant), the inverter and its capacity are used more efficiently. At peak times in production, an under-dimensioned inverter misses some solar energy productionfor the simple reason that the inverter can not process all energy produced.

The under-dimensioning of the inverter in Flanders, however, results in savings on the prosumers rate. This levy from the Flemish government depends on the maximum capacity of the inverter of the PV plant. The smaller this inverter capacity, the less one pays for using the distribution network.

In other words, the under-dimensioning of the inverter requires a trade-off between the savings on the prosumers rate and the loss of yield at times of peak production by the solar panels.

According to inverter manufacturer SMA, the standard inverter has been developed to convert energy at maximum power for 8 hours per day. In addition, inverter guarantees are often only compromised with an under-dimensioning of more than 200%.

Many PV-installers make specific adjustments to the inverters, allowing for a far-reaching under-dimensioning. Some installers request the neccesary certificates from the inverter manufacturer, but must of them don’t.

It is therefore hardly surprising that our employees are regularly contacted by customers of PV-installers who no longer exist. They are confronted with defective inverters as a result of too far-reaching under-dimensioning of the inverter. In some cases, the defect was not even covered by the inverter warranty! Of course iLumen has the necessary kwowledge and experience to help this PV-owners in an appropriate way.


A few weeks ago, iLumen launched home battery iLubat. This battery storage system enables the user, in combination with a smaller inverter, to save considerably on his prosumers rate and to optimally use the energy produced by the solar panels. For example, the under-dimensioned inverter will be able to perfectly capture the peaks in solar production in the iLubat, while it would have been lost without a battery. Thanks to the iLubat, iLumen can go even further in under-dimensioning of the inverter, regardless the type of inverter.

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