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.

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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.

  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) ;-/

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