Some households and businesses, when considering whether to install a solar power system, are tempted to oversize their inverter with the idea of later expanding their solar panel array to fill it out. Generally, this is not an approach that we recommend, but
Q: Should you undersize your inverter? Maybe a little bit.
Previously on the Solar Choice website, we published some general guidelines regarding inverter sizing for the optimisation of efficiency of solar photovoltaic systems. One of the points made in this article was that the power output capacity of a solar array is limited by inverter size–i.e. a solar PV system will not produce much more power than the nameplate capacity of the inverter. This means that, when designing your system, keep in mind that the size of your solar panel array should not greatly exceed what your inverter is rated to be able to convert from DC electricity to AC electricity. However, thanks to the fact that solar panels actually only rarely operate at exactly their nameplate capacity, it’s ok to have an inverter that is slightly oversized. For example, a 2kW solar panel array may work fine with a 1.7kW inverter. Check the spec sheets for both the panels and the inverter!
So undersizing your inverter is not usually a good idea, unless it’s only slightly undersized and you understand how this may compromise your system’s power output. How about oversizing your inverter?
Q: Should you oversize the inverter for your solar power system? Maybe a little bit.
As a general rule, it is advisable to try to make your solar panel array capacity roughly the same capacity as the inverter. Although it may be tempting to get an ‘extra-large’ inverter with the intention of later adding more solar panels, putting the system together properly from the get-go will eliminate unnecessary headaches in the future. When a solar panel array is expanded, it is highly advisable to use the same make and model of panels as the existing ones in order to avoid efficiency losses. If you install your panels in more than one instalment, the same stock may not be available at the later date, may be harder to find or may be more expensive. Additionally, you’ll have to pay an accredited installer to come out and install the panels properly, which adds to the overall cost of your system as compared to having it all done in one go.
If you find you must (or really want to) oversize your inverter…
So in summary, if you do find it necessary or in your best interest for whatever reason to add more panels to your existing system, 1) make sure you understand what your inverter’s window of operation is so that you know what sort of performance to expect from it (see chart above and read more here), 2) make sure that you know exactly what you will need to expand the system (you should add the same brand and capacity of panels), and 3) don’t wait more than 3 months to install the additional panels.
Some more info on how inverters operate
Solar panels do not always produce the same amount of power consistently at all times, due to fluctuations in sunlight throughout the day and the effects of heating and shading; inverters are designed with this fact in mind. They therefore have what is called an optimal voltage operational window, which specifies the range of input voltages in which an inverter will function optimally. Although the efficiency may still vary slightly within that window, it does not vary radically. (See the graph above). Generally, if the power produced by your solar power array exceeds or does not reach this window, your system will suffer significant efficiency losses, or you may end up burning out your inverter. If you want to oversize the inverter for your system before you expand the number of panels, you’ll want to make sure that the voltage output of your array falls within the window.
When oversizing your inverter, the efficiency of the system (see graph at top) depends on what your average point of operation across the year is. This in turn will be based on the configuration and operating conditions of your solar array. During the design process, inverter manufacturers predicted that people may want to increase their solar PV array size over time, and designed them with this in mind. This is why modern inverters have a fairly wide operational window to accommodate fluctuations in power generation with minimal efficiency loss–or possible system expansion. In the case of the graph above, for example, efficiency for an inverter operating within its the optimal voltage operational window is between 85-95%.
Some inverters (transformerless and high-frequency transformer inverters) are optimised for efficiency toward the high end of the efficiency window, while other inverters (those that have conventional transformers) reach their peak efficiency toward the low end of the window. Generally, if you are planning to add panels to your system in the future, transformerless or high-frequency transformer inverters are the way to go. It is important to match the output of your solar PV array with the operational window of the inverter, not only for efficiency, but also for the well-being of your system: if your array voltage exceeds the rated voltage of the inverter, you may void your warranty!
Sources and Links:
Personal communication with Prateek Choudria–MEngSc “ Photovoltaics and Solar Energy, UNSW
Previous related Solar Choice blog entries: Sunny boy visits Solar Choice : Feed-in tariffs state by state : Can I add a second solar system to another building on my property? : Optimising solar system efficiency through inverter sizing : Troubleshooting your grid-connected solar power system : Home energy consumption vs PV system production : AC vs DC explained : Transformerless vs Conventional Inverters
He is now the communications manager for energy technology startup SwitchDin, but remains an occasional contributor to the Solar Choice blog.
James lives in Newcastle in a house with a weird solar system.
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I have a 6kw array, but have an inverter rated for 5000 max output Watts. Is this installation OK?
Hi Mark,
All solar PV systems are subject to some inefficiencies – a total efficiency of 85% is not uncommon (in fact, it’s the assumption we make in a lot of our modelling to be conservative). In your case, I’m imagining that 6kW represents the peak power output of the panels – before being put through the inverter. A 6kW solar array operating at 85% efficiency would produce about 5.1kW of instantaneous power output.
So in short – yes, your system is probably operating fine. Some of the inefficiency may come from the inverter itself (different models have different peak efficiency) or the tilt & orientation of the panels (e.g. do they face north? how are they tilted?). You might want to keep an eye out to see if your panels are being shaded at all during the day.
To get a more definitive answer, however, we’d recommend getting in touch with a certified solar installer.