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micro vs central inverter  RSS feed

 
Annie Gibbons
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Location: NE Ohio
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Apologies if this topic has already been addressed - did a quick search but didn't find the answer - feel free to point me to it if it's out there.

My husband and I are working on contacting local solar companies about our options for a grid-tied system. So far we have had one company quote on a micro-inverter system, and one on a central inverter system. They've both told me the reasons they do each particular type. But I'd like to hear from folks who are objective, and who have experience with one or both re: pros/cons/things to consider.

Happy to provide any info about our site/needs that would be helpful.

Thanks!
 
Tom OHern
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Location: Seattle, WA
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I just had my solar system installed and we got microinverters on ours. The main reason being is that I have modules on both the south and west slopes of my house, and due to the fact that the two sides have vastly different power outputs at different times of day, a central inverter would not properly maximize the power from each. And now that I've been watching my power output for a few weeks, I see that occasionally one or two panels drop around 20% but all the other panels are still maintaining a steady power output. My understanding is that if they were all strung together to a centralized inverter, those low output panels would drag down the output of the whole string. The other benifit is that if we ever have a failure on one of the microinverters, we only have to replace that one which is far cheaper than buying a whole new central inverter. Granted, since I have seven inverters rather than just once, that is 6 more chances for a device failure, but I still think I like those odds.
 
Annie Gibbons
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Location: NE Ohio
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Thanks - that's what I like about the micro inverter system, from what I know. But also a concern about more parts to have trouble with. I've been told that central inverter systems are more efficient - anyone know if this is true? We might also have a split between south and west, and I hadn't thought about how that might be a good reason to go with micro, based on the different faces.

I also just learned in conversations with two different companies today about optimizers - a hybrid, it sounds like, between micro & central? Anyone know more about this? Any idea if optimizers would address the issue with the split between south & west facing arrays?
 
Terry Ruth
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We have a new expert in the house, a PV Electrical Engineer named Dacian. Ask him on this thread: http://www.permies.com/t/45425/passive-solar/Passive-house-PV-electricity-heating

I would but he may be tired on me picking his brain I watch from the sidelines and see his response. Also, we been going through some PVWatts calculations to look at return on investments if you have not done so already. It considers the inverter efficiency for one.
 
Sean Kelley
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Greetings Annie,
Congrats on going down the solar road. Hopefully I can provide you with some information that will help you with your decision.

Let's start with the DC optimizers that you mentioned. You are correct they are are a bit of a hybrid product each module would have its own optimizer that finds that panels maximum power point, the optimizers are then connected to a central inverter that converts the DC into AC. They work very well for installations where multiple orientations are used or there are some shading issues. SolarEdge is the main DC optimizer company and they have a nice online monitoring platform where you are able to see individual panel production. The optimizers have a 25 yr warranty and the inverter has a 12 yr warranty with the option to purchase an extension. The upfront cost is slightly more than just a central inverter but the systems I have installed with these have been producing really well.

If you are looking at microinverters they offer the same benefit of the optimizers where each panel will produce its maximum power and shading on one panel will not bring down the rest of the modules. Except now the conversion from DC-AC takes place on the roof at each panel. This is nice if you do not have the space to mount a large central inverter. If you are looking at Enphase microinverters they also offer a nice online monitoring platform but if you want to see panel level details it costs an extra $299. Their warranty on the microinverters is also 25 yrs. Most likely this would be your most expensive option, but should be pretty close to an optimizer system.

As far as maintenance if you did ever have to replace an optimizer or micro it usually involves removing a few modules to access the problem child, which is not a big issue as long as you have that detailed in the contract that you sign will your installer. I have yet to have a micro fail and have had one optimizer that was bad out of the box.

If you look at central inverters a lot of them come with something called a dual MPPT input which would allow you to have two strings of modules with different orientations. The biggest knock is going to be if you have any shading. When you have modules connected in series to form a "string" of modules it is possible for shading on one or a few modules to bring the power of that string down even if some of the are not shaded. Most central inverters have a 10-12 yr warranty but also offer the option to purchase an extension. This inverter setup will be the least expensive option.

When you start to look at efficiency numbers all of the inverters will be pretty close. You really want to look at overall system efficiency and this is where a tool like PV watts is very handy. SolarEdge and Enphase both have white papers detailing their overall system efficiency compared to central inverters. Hopefully this helps a little, if you would more detailed information on any of these systems I would be glad to help.
 
Annie Gibbons
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Location: NE Ohio
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Thanks - that's really helpful. I'm definitely leaning toward optimizers as the way to go, as we will have multiple orientations and possibly a small amount of shading on a portion.

Are there any downsides to optimizers?

Sean Kelley wrote:As far as maintenance if you did ever have to replace an optimizer or micro it usually involves removing a few modules to access the problem child, which is not a big issue as long as you have that detailed in the contract that you sign will your installer.


Can you explain what it would look like to have that detailed in the contract?
 
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