Should I oversize my solar system?

Fast read

Oversizing a solar panel array can be beneficial. In real-world conditions, a solar panel often generates less than its rated output, hence an 8 kW solar panel system can function efficiently with a 6 kW inverter.

Australia's solar rebate scheme promotes installing larger arrays, and a slightly oversized system can be cheaper per kW. Oversized arrays can generate more energy, especially during off-peak hours, compensating for energy "clipping" and offering 28% increase in yield for only a 10% increase in installation cost.

Net metering, aesthetics, future-proofing, and inverter efficiency are factors to consider before oversizing. Professional consultation is advised for a tailored solution.

Oversizing solar systems – does it make sense? 

Have you ever heard of oversizing your solar system? What does it mean, and is this a good idea? A solar array is a group of solar panels installed together to generate electricity from the sun. The size of the array is determined by the number of panels installed and the overall capacity of the panels together. So, for example, 20 x 400 Watt panels mean you have an 8 kW array or, say, an 8 kW solar power system.

Usually, many people think that if I have 8 kW of panels, I also need an 8 kW inverter, but this is not the case. In this case, a 6 kW inverter is actually allowed under the Australian solar PV design rules. Why is that, I hear you say. Will my inverter not overheat or even “explode” if I overload it with 2 kW of extra panel capacity?

Can oversizing your solar system damage the inverter? 

Let me explain why this seemingly nonsensical move – actually makes sense. A 400-watt panel, before it gets the power class rating, is measured in the factory with 1000 Watts of light hitting each square meter of the panel to generate the 400-watt total panel output.

But in literally all parts of Australia, the sun never reaches an irradiation of 1000 Watts per square meter. 700 to 850 Watts is the norm on many sunny days. This means a 400-watt panel rarely generates 400 Watts in our sunlight conditions. Maybe 300 Watts on a sunny day in the middle of the day is a good result.

In Europe, such as Germany or Poland, it’s even worse, where a 400-watt panel might only generate 200 Watts on a sunny summer’s day.

Higher temperatures over 25 degrees also reduce production

When manufactured, PV systems are tested in Standard Test Conditions (STC). These conditions are insolation of 1000W/m2, air mass of 1.5 and 25 degrees. But the issue is, in the real world, how likely is your system to be in these exact conditions? As these temperatures increase over 25 degrees, the array will decrease power. This is common in Australia, especially concerning temperature. For this reason, having a more extensive array will counteract this decrease.

So if we translate this knowledge to an 8 kW system, it means the 20 panels will generate on a sunny day only about 300 watts – therefore, 300 W x 20 = 6ooo watts. So as you can see, a 6kW inverter will suit 8 kW of panels. The inverter will not fry; in any case, they have protections built in case more than 6000 watts would be generated and fed into a 6 kW inverter.

The solar rebate encourages oversizing

The solar rebate is linked to the panels in Australia, meaning the more panels one installs, the more rebate one gets. So often, the installer and homeowners aim to get the highest rebate, meaning they seek to fit as many panels as possible. This is because the rebate, in many cases, pays a considerable part towards the cost of the panels.

While there will initially be additional costs associated with oversizing your solar system due to a little more labour (remember the panels are to a great extent paid for via the rebate), a 6 kW inverter with 6 kW of panels will produce less solar than a 6 kW inverter with 8 kW of panels attachedç. An 8 kW panel system with a 6 kW inverter and an 8 kW panel system with an 8 kW inverter will produce very similar output.

Therefore oversizing your solar system will mean you will buy a slightly cheaper system per kW, as usually, a 6 kW inverter is cheaper than an 8 kW inverter. 

According to Sunny Design, oversizing your solar system by 5kW can have a return of increasing the energy yield by about 28% for only an additional installation cost of 10%.

tea being poured in a cup
Do you use energy more early in the morning, then oversizing the array makes a lot of sense

Maximise daytime energy

Depending on different consumption patterns, having early morning daytime energy may be your priority. If this is the case, oversizing your solar panel array is smart. Oversizing your system will allow the inverter to reach its full AC capacity earlier in the day and continue to perform at that maximum capacity level until later in the afternoon.

This is especially great for commercial solar systems, as offices and firms would use almost all of their electricity throughout the day and then fall off in the late afternoon or at night.

Net metering

If your energy company offers net metering, oversizing your solar system can be a financially smart move. Net metering allows you to sell excess electricity back to the energy retailer. The more you sell back, the more it offsets the installation cost.

inverter installed on wall

A better match

Oversizing your solar system allows the rated AC power of your inverter to better match the DC energy output of your solar system. This means the market of different inverters you can purchase increases, as you can now purchase one with a lower AC rating. This lower rating makes the inverter cheaper. This makes the initial purchase price lower, and most likely all other inverter-related costs such as maintenance as well.

Factors to consider before oversizing your solar system

Oversizing your solar panel array can have some benefits. There are also a few other factors to consider before deciding to do so.

Cost

One crucial factor to consider is the cost of oversizing your solar panel array. Oversizing your solar system may increase the upfront cost of the installation. It may also increase the maintenance and repair costs over the system’s lifetime, as now you have more panels to look after.

Space

It is also important to consider space availability for solar panel installation. If you have limited space, consider installing a smaller panel array to use the available space efficiently.

Aesthetics

The aesthetics of the installation should also be considered. Oversizing your solar system may make the installation more visible. This can be a positive or negative factor, depending on the homeowner’s preferences and the property’s design.

Future-proofing your home

A large solar system can be advantageous if you plan to add an electric vehicle or battery storage to your system in the future. This gives you greater freedom and guarantees that you can meet rising energy demands without changing your system.

Inverters efficiency

Modern solar inverters are built to withstand a certain amount of oversizing without breaking or performing poorly. They can control the input power up to a point known as “clipping.” At the same time, some energy is “clipped” or lost during high production. The increased output during off-peak hours typically more than makes up for this loss.

two inverters on wall for an oversize solar system

Determining the maximum panel capacity for your inverter

The maximum panel capacity that an inverter can handle is determined by its maximum power point tracking (MPPT) rating. The MPPT rating is the maximum power the inverter can handle from the solar panels and is expressed in watts. To determine the maximum panel capacity for your inverter, you will need to know the MPPT rating of your inverter and the wattage of the solar panels you are considering.

The Clean Energy Council generally allows 1/3 bigger panel capacity, compared to inverter capacity – as the golden solar panel system design rule. So a 5 kW inverter can get 6.6 kW of panels, and a 9 kW inverter can have 12 kW of panels attached.

In summary

In summary, oversizing your solar system is fine. Doing so can save you money in the long run, as it will increase the efficiency of your inverter. Overall, the decision to oversize your solar panel array depends on your specific circumstances and energy needs. Consult a solar installation professional to determine the suitable size panel array for your needs. By considering the installation’s cost, space, and aesthetics, you can make an informed decision about whether oversizing your solar system is the right choice for you.

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