This is probably the question that we are most frequently asked, hence the decision to write an article to explain. Surely it would be better if the inverter is rated at, or even above the total installed capacity of the solar panels? Surely a smaller inverter would be damaged by a larger array of solar panels?
Wrong. It is quite normal and good practice to undersize an inverter to the solar array. There are good reasons for this.
Standard Test Conditions
The rating of a solar panel as quoted on its manufacturer’s data sheet is determined using Standard Test Conditions (STC). This means that the test was performed with a cell temperature of 25°C, an irradiance of 1000 W/m² and an air mass 1.5 (AM1.5) spectrum. Standard Test Conditions allow us to compare solar panels on a level playing field (i.e. conditions in a laboratory), but those conditions do not represent what happens in an installed system.
By the time a solar panel is exposed to a solar irradiance of 1000 W/m² “in the wild” it will easily be well over STC temperature. When exposed to solar irradiance the cells on the solar panels will begin to heat up and will usually get to a temperature at or exceeding 45°C. Solar cells have a temperature coefficient which is usually around -0.45%/°C. This means that it loses 0.45% of its output with every degree warmer it gets, so at 45°C a solar panel will be producing around 9% less than at 25°C. This means that a 250W panel will be producing 225W, or a 300W panel will be down to 273W. Your 4kW system will be producing 3.64kW.
Inverter manufacturers quote voltage and current ratings on their data sheets. A Solis 3.6-4G inverter, for example, has a maximum DC voltage of 600V and maximum current of 11A per input. Provided we don’t exceed these maximums we can connect any size of array to the inverter without the risk of damage. If we undersize the inverter too much then we will simply observe ‘clipping’ where the solar panels have the potential to produce more than the inverter can convert to AC, but the inverter will just limit the output to produce its rated maximum.
The orientation of the solar array is also a factor in our choice of inverter size. In an array on both sides of a house where the two halves of the system are facing in opposite directions, the peak output will be much lower than if all the panels were facing in the same direction. This would also influence our choice of inverter size.
Inverters have an efficiency curve which for most inverters is similar to the one in the graph. At our latitude in the UK, and with our weather, solar arrays spend the majority of their life producing much less than they are capable of. You can see from the graph that the peak efficiency of an inverter is achieved when it is operating at around 70% of its total capacity. If we were to specify too large of an inverter then we actually produce a lower annual yield than that of a more suitably sized model, and in fact would pay more money for the privilege.