Micro-Inverters vs. Central Inverters

Central inverters have dominated the solar industry since its inception. The introduction of micro-inverters marks one of the biggest technology shifts in the PV industry to date. Manufacturers are touting 5-25% increase in power output, which in the long run can bring in a lot in savings for many homeowners.

For a list over the best performing inverters on the market today, go to Most Efficient Solar Panel Inverters 2013.

 

What is a Micro-Inverter?

Although micro-inverters have been available since 1993, Enphase Energy is regarded as the company that first built a commercially successful micro-inverter. More than one million units of the Enphase M175 have been sold since its release in 2008. [1]

Several other companies in the solar industry have followed suit and launched their own micro-inverters, validating their potential.

Since your solar panels generate DC (direct current), we need some kind of device to convert DC intto AC (alternating current), in order to power your electrical appliances (without burning down your house!). This is where the solar inverter comes in.

Inverters also enable us to switch off all electrical current in the case of a blackout or if repair is needed. This is of course also useful for maintenance, troubleshooting and system upgrade as well.

One central (string) inverter would normally cover an entire residential solar system (assuming that the central inverter is strong enough for your entire array). Micro-inverters, on the other hand, sit on the back of each and every solar panel.

Micro-inverters bring several significant benefits to the table. Do these benefits outweigh the extra costs?


 

Benefits

Individual Optimization

Micro-inverters optimizes for each solar panel alone, not for your entire solar system, as central inverts do. This enables every solar panel to perform at their maximum potential. In other words, one solar panel alone cannot drag down the performance of entire solar array, as opposed to central inverters that optimize for the weakest link.

Shading of as little as 9% of a solar system connected to a central inverter, can lead to a system-wide decline in power output with as much as 54%.[2]

 

If one solar panel in a string had abnormally high resistance due to a manufacturing defect, the performance of every solar panel connected to that same central inverter would suffer.

Likewise, coverage issues such as shading, dirt, snow and even slight orientation mismatch on one of the solar panels would not bring the entire solar system down.

Enecsys micro-inverter

 

Maximum Power Point Tracking (MPPT)

One of the tricky things about solar cells is that voltage needs to be adjusted to light level for maximum output of power. In other words, the performance of a solar panel is dependent on the voltage load that is applied from the inverter.

MPPT is a technique used to find the right voltage – the maximum power point. When MPPT is applied to each individual panel, as opposed to the solar system as a whole, performance will naturally increase.

 

Longer Warranty

Since micro-inverters are not exposed to as high power and heat loads as central inverter, they also tend to last significantly longer. Micro-inverters typically come with a warranty of 20-25 years – 10-15 years longer than central inverters.

 

Easily Expandable

Expanding your solar system with more solar panels later on is easier with micro-inverters. You don’t have to worry about restringing or getting a second central inverter installed.

Central inverters come in limited sizes – you might end up having to pay for one that is much bigger than what you actually need.

 

Performance Reports

Web-based monitoring on a panel-by-panel basis is usually available both for homeowner and installer. Continuously analyzing the health of the solar system can pave the way for additional tweaks and performance improvements. There are even mobile applications that enable you to monitor your PV system when on the road.

 

No Single Point of Failure

Unlike central inverters, if there is something wrong with either one of the solar panels or the micro-inverter that sits on the back of it, the rest of the solar system is unaffected and still up and running.

 

Improved Safety

Solar panels are connected in series before they are fed into a central inverter, typically with an effective nominal rating of 300-600 VDC (volts of direct current). This current is potentially life threatening.

Micro-inverters eliminate the need for high voltage DC wiring, which improve the safety for both solar installers and system owners.

 

Silent

Since micro-inverters dissipate significantly less heat than central inverters do, there is no need for active cooling, which enables them to operate without noise.

 

How Much Micro-Inverters Cost

Micro-inverters are flat down more expensive than central inverters.

Numbers from 2010 reveal that central inverters averaged at $0.40/Wp (watt-peak), while the price of micro-inverters significantly higher at $0.52/Wp.

Higher initial cost per watt-peak does not necessarily mean micro-inverters are ultimately going to cost more. Several other factors have to be taken into account.

Solar installations with micro-inverters are simpler and less time consuming, which typically cut 15% of the installation costs. Better durability and longer lifespan should also be considered.

 

Dual Micro-Inverters

In 2011, dual micro-inverters were introduced to the market. They essentially do exactly the same as regular micro-inverters, only on two solar panels instead of one. This lowers costs, but at the price of performance.

 

Are micro-inverters, dual micro-inverters, or a central string invert the better choice in your particular situation? It depends. In certain situations, micro-inverters should clearly be given serious consideration.

The homeowners who are more affected by shading are also those that can benefit the most from micro-inverters. Micro-inverters are also excellent for difficult roof orientations, starter systems and small applications.

 

Cost Analysis is Necessary

We evaluate the usefulness of micro-inverters by looking at two numbers:

  • Lifetime costs ($)
  • Lifetime energy production (kWh)

This is essentially what it all comes down – divide costs by energy production and you`re left with how much money you have to pay for every kWh your solar system produce. Every situation is different – there are a lot of variables to take into account in order to find those two numbers.

 

Enecsys, one of the leading micro-inverter manufacturers, sums it up nicely:

“A total cost of ownership analysis of a PV solar system can only be carried out after detailed examination of capital and maintenance costs, and an understanding of how much energy will be harvested over the life of the system.”[3]

For help to determine if micro-inverters can benefit your situation, sign up for a Free Solar Consultation.


References: [1] Enphase Energy, [2] Renewable Energy World, [3] Enecsys.

Author:

Comments

  1. Ron says

    You are correct that micro inverters are better than central inverters but the new SolarEdge technology is better than both. SolarEdge offers up to the same 25% increase in energy harvest only better because of its much higher 98.3 peak, 97.5% CEC inverter efficiency. SolarEdge also offers individual solar module monitoring via the Internet on your smart phone or computer. Unlike Enphase that charges for expensive cabling and Internet communications portal, SolarEdge’s cabling is included at no extra cost and its Internet communications portal is built in, again at no extra cost. SolarEdge also offers a much higher power rating per solar module at up to 300 watts instead of Enphase’s 215 watts so no power is wasted with larger solar modules. In addition SolarEdge does not use any electrolytic capacitors in their under module mounted power optimizers. Enphase does use electrolytic capacitors in their micro inverters which have the potential to leak and dry out when used in hot environments (like your roof)which may result in expensive non warranty reimbursed service calls as a micro inverter installation ages. Imagine having to pay a contractor $400.00 to $500.00 to remove and replace several solar panels just to gain access to a failed micro inverter. Now multiply $400.00 to $500.00 times 20 or 30 micro inverters. You are right, micro inverters don’t have a single point of failure, they have multiple points of failure and ufortunately they won’t all fail at the same which would save the consumer money on multiple service calls SolarEdge simply makes a higher performance, higher reliability, more cost effective product.

    • Bob says

      Sounds like Ron works for SolarEdge. FYI, Enphase only charges a monitoring fee if you own the system. If you lease your panels the enlighten home monitoring is free. In addition, the micro inverters are warranted for 25 years so any labor incurred to fix and or replace them is the responsibility of Enphase. So there are no shocking $500.00 charges to repair them. Ron is a great example of someone who doesn’t know what they are talking about yet feels compelled to post false information regardless. Do your homework.

      • chad says

        Well said Bob. His comments can’t be backed up. SolarEdge is so much better, higher quality, etc etc. And the Solar Edge warranty = 12 years. Yeah 12 years. Unbelievable.

      • Mike says

        Keep in mind Enphase his a public company and is yet to be profitable. I would not put much trust toward Enphase or any other warranty of companies without profits.

      • Ron says

        @Bob you need a pair of reading glasses. Where in my post did I say that Enphase charges for monitoring ??? I said they charge for the “communications portal” Maybe you’ve heard of it, it’s called Envoy. With SolarEdge the portal is built in.

        And your statement “the micro inverters are warranted for 25 years so any labor incurred to fix and or replace them is the responsibility of Enphase.” is false or a flat out lie.

        I have Enphase’s warranty in my hand and it states “The Limited Warranty covers a replacement unit to replace the Defective Product, but does not include labor costs
        related to (1) un-installing the Defective Product or (2) if applicable, re-installing a repaired or replacement
        product”

        How embarrassing for you Bob. So in reality, it looks like Bob is a “great example of someone who doesn’t know what they are talking about yet feels compelled to post false information regardless.” Do your homework fool. Enphase’s warranty can be downloaded from their website.

  2. Albert says

    Actually, as of 3/15/2013, the PowerEdge optimizer warranty is also 25 years. They make an inverter that has a 12 year warranty.

    That said, not all factors are equal. Cost is important, but safety is even more so. High voltage on the roof, and the much higher possibility of arc faults with high voltage DC current make me hesitate to use central inverters, now that we have an alternative. Any faulty connection that develops over the life of the system places the system at risk for an arc fault with a resulting fire. Customers who are given the choice almost always choose to pay a little more up front for peace of mind.

    • Albert says

      One more thing – it is true that SolarEdge Power Optimizers have built-in arc fault detection, but this does not always work well, and you’ve still got high voltage DC on the roof. The Power Optimizers shut down if they detect high heat, but again, you’re creating a basically unsafe condition (high voltage DC) and then relying on technology to shut it down if there’s a problem. If/when that shut-down technology fails, you’re back to your unsafe situation.

  3. Joe says

    Hi Just want you to know that Bob is off on his facts a little, I called enphase they only cover labor if the part fails within the first 2 years of installation, after that the cost is the responsibility of the owner. Also, what happens if a solaredge box fails does the panel go off line or do I just loose monitoring of it?

  4. Stefano says

    Why on earth would i pay extra to monitor my own installation? Surely if you’ve just dropped $20k onto a solar system you should be able to connect it to your own wifi / lan and monitor it yourself for free.

    • Lars says

      Actually Enphase does not charge anything for monitoring. I have been using their system since 2012 on my owned system and monitoring is free. Period.

  5. SolarEdge Fan says

    I’m with Ron. Solaredge all the way, when it comes to shading.
    On a non shaded roof a string inverter is just as good as as any of the rest.
    My problem with micros is to many points of falure and no one can say 10 years down the road, the micros won’t go pop one after the other. The labour bill will be masive.
    My 2 Cent’s

    • Mike says

      Massive

      But if you are leasing a system through enphase, with their 25 yr warranty, why worry about potential failures when they cover those expenses.

      Shading is a non-argument since both systems handle shading, professionally.

      I am saddened to hear about extra fees to monitor your owned system, however. I will investigate this further. Whether i get back here to update this or not is another thing altogether.

      We need an updated cost analysis of Enphase micro inverter system VS SolarEdge’s Optimizer system. I am a fan of both of them.

    • Mike says

      I can confirm Enphase does not charge monitoring fees anymore. They used to, then they raised the price of the EMUs, and stopped charging for monitoring. You can even access it from your smartphone!

  6. dave says

    Ron’s comment had some valid points, but much misinformation about the difference between Solar edge and Enphase. Many of the errors have already been pointed out, but here are some more points.
    He cites a 97.5% CEC inverter efficiency. Yes the central string inverter is rated at 97.5%, but there is also the DC to DC Optimizer 98.8%, so the combined conversion from Panel to AC is 96.3%. An Enphase M215 is 96.5% CEC weighted.
    Both the Enphase and Solar edge under the panel modules have 25 year warranties, but Solar edge system also needs the central inverter which has a 12 year warranty.
    Both systems could require roof top module replacements (under warranty) if they aren’t reliable designs.
    Solar edge has optimizers that cover a wider range of panel sizes than Enphase, though Enphase does now have the M250 that is rated for 300W modules.

  7. yewloong says

    Hi, may i ask if there is a difference between a central and string inverter?
    From what i know(through research), Centralized MPPT inverter technique uses ‘central inverter’ approach where multiple PV panels are connected like ‘string’ in series, then parallel the ‘string’ together and series with one inverter for all. i.e. one inverter for the entire design.
    As for Distributed MPPT inverter technique, it uses ‘string inverter’ approach where multiple PV panels are connected like ‘string’ in series with one inverter in each ‘string’, then parallel them together. i.e. 3 ‘string’ will have 3 inverters.
    And, does micro-inverter classified under Distributed MPPT inverter technique as well?

  8. t says

    Providing a source for a HOME OWNER to obtain dual micro inverters would be nice since this is “The Homeowners Guide to solar panels”

    I’ve email Enecsys and distribs and searched and searched with no one replying to tell me where to obtain them. thanks for any follow up links in advance

    (ps. I am a contractor not just a one time buying owner however) ;-/

  9. Bruce Whitfield says

    Micro inverters have one common problem and that relates to the amount of EMI/RFI that radiates from both the AC & DC wiring. At least one large name brand US manufacturer of micro inverters is currently struggling with radio interference complaints, especially from those customers who have an interest in good radio/amateur communications. The issue is one that can’t be solved by applying numerous ferrite filter cores to the cabling, and its interesting to note that one or two responsible Chinese manufacturers are producing external filter boxes to try and limit the interference levels. However the EMI/RFI issue is also compounded by the use of power line monitoring for data from each inverter…and the capacitive coupling involved simply guarantees high levels of interference. The up shot of all this is that String Inverters have a decided advantage when it comes to suppressing EMI /RFI ..

  10. says

    Hi,

    There is a new answer around safety, isolation, repair and maintenance for those considering solar string systems and an inverter. And, its a lot cheaper even when including the Micro Inverters. Have a look at the Remote Solar Isolator I’d be interested in your feedback. Thanks.

  11. Peter Bulanyi says

    Hi Yewloong,
    Terminology is important!

    It goes like this:
    Central inverters are the giant 3-Phase ones in solar farms the size of a car. Their power ratings are in practice 500kW to 1MW but can extend lower and higher anywhere from 100kW to 1.5MW. Often they are a single MPPT, but more recent evolutions have 2,3 or 4 MPPTs.

    String inverters can be single or 3-Phase and in practice range from 2kW to 30kW but can range from less than 1kW to 100kW. They usually have 1, 2 or 3 MPPTs.

    Microinverters can be single or 3-Phase and in practice range from 200W to 300W and have 1 MPPT

    DC Optimisers require in addition to the panel DC-DC converters, a “String” inverter to process the the String voltage.

    When Microinverters first appeared, I found myself inadvertently using the term “central” to describe what we had been calling string inverters for decades. It’s a natural response as you try to distinguish in your language the topology difference, i.e. the power is distributed with Microinverters, therefore it must be “centralized” with string inverters and next thing you know you’re calling string inverters central inverters…..oops!

    Just one of those quirky brain tricks I think, but as mentioned, the correct terms for Central, String and Microinverters are as described above.

    Hope this makes sense.
    Peter Bulanyi
    29Apr14

  12. Albert says

    I think you have to compare costs and safety. In my opinion if your roof is open to good sunlight and no shading issues central inverters have no competition (micros have multiple failure points, heat issues and difficulty with replacements). The claim of monitoring is ridiculous. If you wanted to spend all your time seeing how much solar you are producing buy a bus ticket and get a life. The safety issue is also concerning. Placing numerous AC generators onto a roof with no safety switches is another recipe for disaster. i think a central inverter with dc volts on a roof and a Remote Solar Isolator for safety is both reliable, safer than any of the above systems and the cheapest option. Maybe with this combination you could save the planet and your wallet, have the safest system and buy a first class ticket on an A 380 around the world with your savings

  13. Ron says

    Sorry to burst your bubble Chad but all of the SolarEdge inverters that we offer include a free warranty upgrade to 25 years. And it looks like my prediction concerning micro inverter failures is coming true. Just call me Solardamus.

    Type in the keywords “Enphase warranty issues” into Google and read all about all of the issues for yourself. Especially the SeekingAlpha article.

  14. Larry says

    Sorry to burst your bubble. I have had my Enphase micro inverters (43) up and working for 4 years. 2 failures. The first one was within a month of installation. second was 3 years in. Never a problem getting them replaced under warrantee and when they went out I lost only one panel, not the whole string or in the case of my neighbor the entire site. I highly recommend the micro inverters and if I had it to do over again I would do it the same way. No heat issues like the central inverters, no phase timing issues also like the central, they just work.

  15. says

    A little IT knowledge can take place here in regards to any monitoring. I have a SolarEdge system and love it. I also looked at the Enphase system and just made my choice, but I found a way to monitor in my home without using the SolarEdge monitoring option. I still use it as it is free, but wanted to find a way to monitor myself. You can use a router to reroute the packets coming from the inverter trying to go to the SolarEdge server to a local server. Create a listening software application or use something like Kepware to grab the packets and there you go. This will take IT knowledge, but it is possible.

  16. Mike says

    Ron, with all due respect you are the one that is wrong. We have had reason to replace (6) Enphase M190 micros and we were paid $150 per unit by Enphase. That does not include the replacement micro which is provided to the contractor at no additional charge and no prorata basis.
    One more suggestion, why don’t you stop your nasty and arrogant comments and participate in an exchange of ideas and opinions the way mature adults are supposed do: calmly and intelligently without inferring that all who don’t share your opinion are stupid dolts.
    Have a nice day.

  17. John Harvey says

    I’ve had the Enphase M215 inverters on a 30 panel system for about a year and a half now. They’ve worked great and there have been no failures.

    Why I like the monitoring, it allows me to tracking in real time if there are any problem panels *and* just to track the panel to panel differences. Since the panels (SolarWorld) also come with an output warranty I want to be able to see if any individual panels are under performing relative to all the others. While all seem to be up to spec, some of them seem to be over spec and it is interesting to track the differences in turn on time, ramp time, and total output panel by panel, and array by array. Obviously with a traditional string inverter I couldn’t do that. However, with the SolarEdge system mentioned above I could do this as well. It is a great innovation for the industry to consider.

    With the twice the warrantied lifetime on the inverters themselves (12 versus 25 years) the cost difference of going with an Enphase versus a traditional string inverter is more than used up with the first string inverter replacement.

    Compared to the SolarEdge hybrid solution (power optimizer panel units plus a string inverter) the cost (currently – which is less than it would have been a year and a half ago) is right around $4,900 (for my 30 panel system). My Enphase system (in today’s prices as well) would be about $3,600. Since the larger inverter is the part which likely would need to be replaced at least once every 12 years or so the system lifetime cost would much more expensive. Alternatively you can extend the warranty to 25 years on the SolarEdge, but that increases the system cost by about $750 ($5,650). Making an Enphase system about $2,050 less costly than an equivalent extended warranty SolarEdge system.

    BUT SolarEdge would allow for a higher final AC Watts rating (and presumably higher output as well). So the net present value calculation has to compare the dollars savings of going with the Enphase solution against the increased output value of the SolarEdge solution. In market with very high usage rates (California etc.) the increased value of the production probably pushes the analysis into the SolarEdge camp. In my case I live in Utah which has some of the lower residential electricity rates in the country (8 to 14 cents, inverted block rates) so the upfront savings mattered much more.

    So just for a back of the envelope calculation take the final DC difference (at most 900W, probably a fair bit less) calculate the maximum incremental expected annual production of going with the SolarEdge system for my location (about 1.5MW max), and value that at 10 cents a kWh and one gets about $150 a year. With a NPV of about $1,500 to $1,750 depending on the discount rate used). It’s somewhat close, but buying a SolarEdge with the upgraded (equivalent) warranty costs about $2,050 more than the Enphase. Combine the upfront savings with the recommendation of the engineer and installers I chose the Enphase system – and would still do so. (But again looking at a state with high incremental usage rates the SolarEdge probably gets the edge from a NPV perspective).

    In the end the engineering firm which designed my system recommended the Enphase micro inverters based on the objective of maximizing net earnings from the system over the expected life of the system, reliability, and ease of installation.

  18. Thomas Runaldue says

    Two things the conduit needs to be metal and grounded and shield the electrical wires. Also the Micro inverters need to also have metal housing. You can’t radiate much energy if all the Electromagnetic fields terminate inside the conduit. The electronics need to also minimize common mode noise. What could be a problem is the HAM equipment needs to have an isolated ground tied as far back to the load panel as possible. and maybe filter the power-supply with a true UPS.

  19. says

    Mike are you happy with your enphase micro inverters? How long have you had them installed? Are they perfectly safe under our hot summer skies?
    I am still tossing up about a micro system versus a single “string inverter”. I know the micro system costs more, but it makes sense to me to have the rest of the system working if one panel goes down, instead of the whole system going down. Trying to decide what to do.
    Cheers

  20. David Harrington says

    I am planning a system that would be installed on a hill in my backyard. I will have a run of about 80 feet to the house. Would a microinverter be more advantageous for this system?

  21. says

    The only point I ever see to use micro inverters is when you have some minor obstructions, in which a string or central inverter would not operate at full capacity if one or two of your solar panels have a shadow cast over it from a certain time of the day. String inverters are more cost effective and they generally work very well if you don’t have any obstructions on your panels. Here’s where you can get them all at great prices: http://www.solaris-shop.com

  22. Herman says

    I have 15 230watts panels and a central inverter, can I connect them in parallel so that it wont affect by shading or a panel failure?
    Thanks

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