Sizing a solar inverter - Oversizing?

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

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

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.

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.

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

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

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

Any thoughts on whether I should insist on smaller inverter?

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

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.