Sizing a solar inverter - Oversizing?

Solar Panels
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Hi,
I have a 3kw SolarEdge inverter with approximately 4.2 kw maximum production from my current panels. Because of this, actual usable production tops out at 3 kw. I am adding 4 more 300 watt panels to my array in the next few weeks. That would bring my maximum production to about 5.4 kw.

I am going to replace/upgrade my inverter. My question is . . . what is the right size? and . . . what are the advantages and disadvantages of getting an oversized inverter (say 10kw)?
Thanks,
Fran

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Hi Fran,

The right size inverter is not a simple question as it is based on your consumption and usage patterns and how efficient your house is. What is your daily or monthly usage in kWh?

Try using this tool to help - https://solar.cleanenergyreviews.info/savings-calculator

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Hi, I am not sure I understand what consumption and usage patterns have to do with the size of the inverter. I mean I understand how that would help you figure out how many solar panels when designing a system . . . and . . . therefore, that would influence which inverter and what size.

I guess my question is . . . based on the maximum output of one’s solar array . . . what are the considerations in sizing the inverter? Should it be sized right around that maximum output? What are the advantages or disadvantages of sizing it bigger than that? For example, if my maximum solar output is 5.6 kw . . . should I get 6kw or 10 kw inverter? Thanks.

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Ok, here’s a quick summary. The inverter size is almost always smaller than the solar array size. This is due to number of loss factors including:

  • Weather (clouds, fog etc)
  • Dirt and dust
  • Power temperature derating (higher temperatures reduce panel power)
  • Cable losses
  • Conversion losses
  • panel orientation and time of year
  • Light-induced degradation - LID and PID

For example, if you have a 7kw solar array, on average, the solar output would be closer to 5kW. Therefore, it does not benefit you in any way to have a larger solar inverter. Unlike battery inverters, solar inverters are designed to operate at the maximum output and are typically 96 to 97% efficient at full power. A larger size solar inverter will just cost more and add not real benefit (unless you plan on adding more panels in the near future)

However, this is very different to sizing an off-grid or hybrid inverter, which is entirely dependent on the household loads, battery capacity, and backup power requirements.

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Hi Jason,
Thank you this is very helpful.

In my situation, are these correct statements?

  • There is no value to having an oversized solar inverter unless plans for expand are imminent.
  • If my max consistent output (output that is consistently reached on sunny summer days) with all the considerations you note below is around 7kw then my solar inverter should be “around” 7kw. A little higher or lower will have no significant input.
  • A 10 kw solar inverter (if I have no plans for further expansion) is overkill. I am paying extra for capacity that I am never going to use.
  • A 7kw solar inverter will also be highly efficient.

My only remaining questions are:

  • What is the practical efficiency downside if I have a 10kw dollar inverter? For example, does my efficiency at peak output get reduced to 90%? 80%? 70%?
  • What are the efficiency measures at lower output? For example, in winter in Maine, days are short and sun is scarce. Does a larger solar inverter mean I am less likely to take full advantage at lower output? This is important because winter is when I am most likely to lose power from the grid.

I really appreciate your patient help. The contractor that is adding to my array ordered a 10kw solar inverter and I want to make sure I understand any negative implications before I consider installing a solar inverter this big.
Thanks
Fran

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Ok, so the answer is YES to all your first 4 questions.

If you have a larger 10kW inverter and only 7kW of solar, it will not operate at reduced efficiency. It will work perfectly fine. It’s strange that your contractor is offering a larger inverter. However, sometimes they are a very similar price to the smaller inverters. What brand and model inverter is it?

1 Like
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Hi Jason,
Thanks for verifying.

My current installation has SolarEdge SE3000 solar inverter with 14 panels (10 at 265w [original/2016], 4 at 370w [expansion/2020]) for a maximum output of 4.13kw. The SE3000 was fine for the original installation and ok for the expanded one but it clips out around 3kw so, in the summer, I am leaving energy “on the table” that could be sold back to the utility.

I am going to have a second expansion in the next few weeks (same installer as the first expansion). We had a few rounds of discussion as to what made sense. I settled on adding 4 more 370w panels to raise my proposed maximum output to 6.61 kw. Unfortunately, the installer used the wrong estimate (an earlier iteration that had a total of 8 more 370w panels added for a maximum output of 9.07 kw) to make the order, That order included the SolarEdge 10000H-US solar inverter (to replace the SE3000). He has proposed using the SE1000H that is already in my garage waiting to be installed instead of returning and ordering the 8kw version.

I do not know the price difference but if it is around $200 I would be ok going with the 10kw version . . . . I just want to make sure that . . . from a technical standpoint, I am not making a mistake.

It is highly unlikely (though not inconceivable) that I will be adding more panels after this expansion as the optimal spots on the roof will be used.

Thanks again for helping me understand.
Fran

1 Like
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Any thoughts on whether I should insist on smaller inverter?

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Only if they reduce the price significantly with a smaller inverter, otherwise I really wouldn’t worry about it.

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Thanks. They are ordering the 6.3 kw inverter. Their reasoning was two-fold. One, if they go to the 8 kw inverter there needs to be another agreement made with my utility. Two, they say that a bigger inverter takes more power to run and to “wake up” at the beginning and end of the day’s “bell curve”. On the second point, if true, it makes sense as I will never exceed the 6.3 kw (size of the inverter they just ordered instead of the 8 kw one) and it would mean more usable power generated.

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The right size of an inverter depends on the power requirements of the appliances or devices you want to power. You will need to determine the peak power and continuous power requirements of the devices you intend to use with the inverter and choose an inverter with a continuous power rating more excellent than all your devices’ total continuous power requirements.

Getting a large inverter means choosing an inverter with a higher capacity than you need. The advantage of a large inverter is that it can provide more power than you need. It can also help reduce the overload risk or overheat the inverter. There are several disadvantages to getting a large inverter:

  1. Cost: Oversized inverters are generally more expensive than appropriately sized inverters, so you may pay more than you need to.
  2. Efficiency: Oversized inverters may not operate as efficiently as appropriately sized inverters, mainly when operating at low power levels. This can result in lower energy efficiency and higher operating costs.
  3. Battery life: Oversized inverters may drain your battery more quickly than appropriately sized inverters, reducing the lifespan of your battery.
  4. Size: Oversized inverters are often physically larger and heavier than appropriately sized inverters, making them more challenging to install and transport.
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I’ve been using AIMS Power 6000W Source: https://www.propaneva.com/best-solar-inverter plus the 48DCV inverter pretty much powers my house appliances and wood shop tools via ac output. it even runs my 240 AC dryer that my son requires. I like how much quieter it is compared to other inverters in its class; so easy to set up. In general, I am completely satisfied with its size and power, so I can recommend it.

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The right size of an inverter for your solar panel system depends on several factors, including the size of your panel array. It also depends on the expected energy production, and your specific energy consumption needs. Generally, it’s recommended to match the inverter size to the capacity of your solar panels to ensure optimal performance and efficiency.

Advantages of an oversized inverter:

  1. Cost savings: An oversized inverter can save you money upfront because larger inverters often have a lower cost per watt than smaller ones. If you plan to expand your solar array in the future, an oversized inverter can accommodate the additional capacity without replacing the inverter.
  2. Flexibility: With an oversized inverter, you have more flexibility in adding additional solar panels or expanding your system without upgrading the inverter. This can be beneficial if you anticipate increasing energy consumptio.

Disadvantages of an oversized inverter:

  1. Lower efficiency at low power output: Inverters are typically most efficient when operating at or near their rated capacity. If your solar array consistently produces power below the minimum operating threshold of an oversized inverter, it may operate less efficiently. This may result in decreased energy conversion efficiency.
  2. Reduced lifespan: Oversized inverters may experience more wear and tear when operating at lower power outputs. This can potentially lead to a shorter lifespan than inverters operating within their optimal capacity range.
  3. Lost energy production: An oversized inverter may not utilize the full potential of your solar panel system. During periods of low solar irradiance or partial shading, the oversized inverter might not extract the maximum power output from your panels. This might result in energy production loss.

It’s imperative to consult with a professional wind turbine installer or system designer to determine the appropriate inverter size for your specific solar array and energy consumption needs. They can perform a detailed analysis considering factors like system efficiency, cost, and future expansion plans. This will help you select the optimal inverter size for your solar panel system.

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Hi Amit, I’m thinking there might be an advantage to “oversizing” an inexpensive inverter that’s convection-cooled, rather than spending more on one which is fan-cooled (or has a better thermal design in some other way). I say this because my best-guess is that my five year old Goodwe NS-3000 will derate itself (perhaps by limiting its output current) when it overheats… and I think some modest level of overheating happens after a few hours above 40% or 50% of its maximum-rated power input.

I believe it’s routine to derate power electronics when they get above 45 degrees C, to avoid premature failure.

The spec sheet on the Goodwe NS-3000 is pretty minimal with respect to its operating conditions, saying only “-25~60°C (>45°C derating)”.

This inverter is mounted indoor, pretty high up on a wall in a hallway that’s pretty well ventilated. It rarely gets above 30 degrees C at my location in Auckland NZ. The inverter runs pretty hot under even this modest loading, for example I’m looking at a day last week when its temperature crawled above 48 degrees C at 2pm – when the panels were producing about 1200W. Peak that day was 12.50pm – panels producing 1440W. My rooftop array is small: 6x Peimar 300/305WP Mono SG. The maximum they have ever produced is about 1900W, i.e. they wouldn’t overrun a Goodwe NS-2000 – unless it is derated for high-temperature operation. But I’m thinking a Goodwe NS-2000 would run even hotter when powered above 1200W than my Goodwe NS-3000 – which seems ok (albeit a bit marginal) in my installation.

All to say that I’m thinking that a convection-cooled inverter similar to the Goodwe NS line might appropriately be derated – perhaps by 30% or even 50% – even when it is run indoor in a mild climate near sea level (e.g. Auckland NZ at 300m). The idea is to avoid it ever running hot (i.e. degrading its life-expectancy by being under high load at a temperature much above 40 degrees C), and also to avoid it ever throttling itself (to avoid overheating) which will result in some of the available PV power “going to waste”.

I don’t yet know – but plan to do some analysis on 5 years of data from this installation – whether either of these “bad things” is occurring at all frequently on my Goodwe NS-3000 with an array on the roof that never produces more than 2kW.

I don’t expect to find any detailed specs on Goodwe NS-3000 thermal performance under load (as that’d depend on operating conditions e.g. ambient temperature and altitude)… but I have found a failure-rate plot for a power thyristor at https://www.darrahelectric.com/uploads/techLib/Dynex-AN4870-Effects-of-Temp-on-Thyristor-Performance__633821529348719168.pdf, which I’d guess would more-or-less applicable to the failure-prone power electronics inside the Goodwe NS-3000.

Amit, can you perchance point me at anyone who has analysed the operating performance of the Goodwe NS-3000 at all carefully, or point out some mistake or misunderstanding in what I wrote above?

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