Sizing an inverter

Not all inverters will work with all panel combinations. In most cases, it is sensible to choose the panel layout, and then find an inverter to suit. It is important to ensure that the inverter works well with the panels, otherwise the output of the system can be compromised.

If for example, it is possible to fit 15 200W monocrystalline panels on your roof, the panels would be connected in either one string of 15 panels (15 panels connected in series), or 3 strings of 5 panels ( 3 sets of 5 panels connected in series).

One 200W panel has the following electrical characteristics at standard test conditions (these values vary depending various environmental factors – the main one being temperature);

Open circuit voltage – 45.1V

Short circuit current – 5.9A

Max power voltage – 36.4V

Max power current – 5.5A

 

We now need to consider the voltage and current of the array of 15 panels. 1 string of 15 panels would have the following characteristics;

Open circuit voltage (at -10 degrees C) – 764V

Max power voltage (at -10 degrees C) – 617V

Max power voltage (at 40 degrees C) – 518V

Max power current (at 40 degrees C) – 6A

 

3 strings of 5 panels would have the following characteristics;

Open circuit voltage (at -10 degrees C) – 255V

Max power voltage (at -10 degrees C) – 206V

Max power voltage (at 40 degrees C) – 173V

Max power current (at 40 degrees C) – 17A

 

We now need to find a 3kW inverter that can handle these voltage and current inputs.

The Eltek 3100S can handle high input voltages and would work well with 1 string of 15 panels. The current would be to high if 3 strings of 5 panels we used.

An Ever Solar TL3200 inverter can handle high input currents and lower input voltages, and would work well with 3 strings of 5 - but the voltage would be too high if 1 string of 15 was used.

The Mastervolt XS3200 would not work with either combination as the voltage and current inputs don't match.

Most 3kW inverters would work well with an even number of panels (14 or 16) as they are designed to work with 2 strings of 7 or 8 panels – but as you can see, odd numbers can often limit the choice of inverter. It is important to ensure the inverter will work with your panels. Feel free to ask us for more details.