Why is the max power of my inverter less than the peak power of the panels?

In the Netherlands it is recommended to choose an inverter with a capacity that is smaller than the peak capacity of the connected solar panels, because the solar panels rarely or never reach their peak capacity and because a smaller inverter has a better yield. At times, the inverter may reach its maximum, though this usually doesn't happen very often. You can recognize when this happens by a flattening of the yield graph as in the image below.

Example of a flattened graph: the inverter produces no more than 3 kW. During sunny moments, part of the potential power of the panels cannot be used

The figure above shows an example of an installation with 15 solar panels of 250 Wp and an SMA Sunny Boy 3000TL inverter. The peak power of the solar panels is 15 x 250 = 3750 Wp or 3.75 kiloWatt peak (kWp) while the maximum output power of the inverter is 3 kiloWatt (kW). The inverter is therefore 20% ‘lighter’ than the solar panels. On this sunny day in April, the production of the panels was limited by the upper limit of the inverter. So it makes sense to say this customer is losing energy. Or does it?

Peak power

The peak power (also called nominal power) of solar panels is the power they can produce under standard test conditions (STC), i.e. an irradiation of 1000 Watt per m² and a panel temperature of 25 ^oC. Whenever they receive less sunlight or when they become warmer than 25 ^oC, the panel power output quickly diminishes. In the Netherlands, a light incidence of 1000 W/m² is not that common, certainly not when the sun falls on the panels at an angle, which reduces the amount of irradiation. In addition, solar panels in the sun quickly become a lot warmer than the ambient temperature, which reduces production. On a beautiful sunny day, they can become 20 to 30 degrees hotter than the outside temperature.

Due to these two factors, solar panels in the Netherlands rarely if ever produce their peak power. It is therefore a waste to choose an inverter with the same capacity as the peak capacity of the panels, because you end up paying extra for capacity that you will (almost) never actually need.

Efficiency

And there's another important reason for choosing a smaller inverter: efficiency. Inverters work more efficiently when they are working under a heavier load.

Efficiency curve of a Sunny Boy 5000TL. The X-axis indicates how heavily the device is being used, the Y-axis shows the corresponding efficiency

The above graph shows the efficiency of an SMA Sunny Boy 5000TL inverter. The Y-axis shows the efficiency of the inverter while the X-axis shows the load on the device (ratio between produced power and peak power). You can see that the efficiency is higher when the inverter is heavily loaded: for example, when the inverter produces 40% of its peak power (40% of 5kWh = 2kW), it delivers an efficiency of 97%, while the efficiency at a load of 10% (500 watts) is only 94.5%.

At a high load, an inverter therefore delivers a higher efficiency. At a low load, on the other hand, the efficiency is lower and therefore the relative loss is greater. There are many times during the year when the inverter is loaded less, for example when the sun is low or when it is cloudy, and at the beginning of the morning or at the end of the evening. It's important to choose the inverter in such a way that it performs optimally in less sunny weather and at the same time does not reach its maximum capacity too often during sunny moments.

Zonnefabriek uses special software to find the right balance, taking into account the peak capacity of the panels, but also the location, ambient temperature, azimuth (orientation relative to the south) and angle of inclination. This way we can pick the most suitable inverter for each installation, with the best yield at the most favorable price.