Oversizing Solar Panels on an Inverter

“Ha, check this out, Honey! They have quoted us for 13.2kW of panels but only a 10kW inverter! Surely the oversizing solar panels won’t be good for the inverter! Is that even legal?”
This post will first address the question of the inverter’s capability of handling excess power. Next, we will see what Australian regulations have to say about the practice of oversizing, and what happens when the regulations are broken. Finally, we will see if it a good idea in your situation to supersize your solar.

Oversizing solar: Inverter Capability

Inverter manufacturers publish maximum input capacity on their specification. This is often stated in volts and amps rather than watts. SMA, for example, document an extremely conservative maximum input, but on calling their tech support team, this limit is increased, and they can send supporting documentation. In short, all inverters worth their salt can handle input power much greater than an electrician would ever attempt to connect. The reason for the electrician’s conservative approach is Australian regulations.

Oversizing solar: Regulation limitations

The success of solar in Australia is due to the federal government’s Renewable Energy Credits program. The credits are paid proportionally to the amount of renewable energy a system will produce, or more importantly, the amount of carbon it will offset in a 15 year period. A 13kW solar system will attract a financial incentive of about $5000 under this program. Strict regulations mandate the way systems are to be installed to ensure this program is successful.

The Clean Energy Council are the body that is responsible for developing standard and regulations that promote renewable energy. The clear thinking folk at the Clean Energy Council realize that a 10kW of solar panels will only operate at 10,000W when the sun shines at a 90-degree angle to the panels, the temperature is 25 degrees, and there is 1000W of irradiation: or hardly ever. So to get reasonable use out of your inverter, it’s wise to oversize. By by how much? The CEC surmised that the panel capacity should be no more than 33 percent of the nominal output of the inverter before your panel’s production starts getting wasted, and the Renewable Energy Credit payout is misused.

Oversizing solar: Energex limitations

So why not just get a 13kW or 15kW inverter? If your home (like the majority of households) runs on single phase, Energex will only approve inverters up to 10kW.

If you have 3 phase at home, we can submit an application to Energex for a large system. It takes a few months to process and they may or may not approve it. Additionally, because 5kW and 10kW inverters are the most common size used, inverters within this range a un-proportionally expensive.

Nominal output limitation

You may notice I have mentioned nominal output and maximum output. What’s the difference?

• Maximum output is what your inverter will output on a perfect day (within tolerance). Most 10kW inverters have a maximum output of 10,000W. If you oversize your solar panel array, it’s not unusual that a 8000w inverter would reach 10,100W or even 10,200W.
• Nominal output – for all intents and purposes –  is a joke. It’s just what the inverter manufacturer decides to label the inverter to meet particular power authority thresholds. A common nominal output thanks to the solar giant SMA, is Germany’s regulated 4.6kW, even though the inverter will easily run at 5000W.

This is important because the CEC only allows a system to be oversized by 33 percent of the nominal output. This means the German made SMA 5kW inverter, and all who followed in its footsteps, are limited to 1.33 x 4.6kw = 6.118kW.

The Danger

It’s this annoying little “nominal output” figure that has left many companies like Appolo Solar red-faced. Recently they were advertising 6.5kW of panels on a Sungrow 5kW inverter with a 4.6kw nominal output. Technically they cannot claim the $2000 renewable energy credits on these jobs – but many companies do! The Clean Energy Regulator and Clean Energy Council are aware of this neglect by what is commonly uninformed solar sales companies. The CER and CEC are currently working on methods to catch and enforce this regulation. They will catch up with the many companies that are “fraudulently” claiming STC’s. It will come back to bite the company who sells the system and customer. At very least, the extra panels will have to be removed, and the STC’s that were claimed under the customer’s name would have to be paid back. It could get messy.

Oversizing solar and Your Investment

So the Inverter manufacturer is ok with you oversizing your panel array, and if done to their guidelines, the Clean Energy Council consider it to be financially responsible. The inverter runs better when at capacity, you know you are maxing out your single phase limit, and you can rest easy knowing you’ve done it right the first time.

Mark Cavanagh