Inverters/Micro-inverters/DC-DC optimizers/maximizers etc

 What they are

Inverters convert your power from DC to AC for use in your home or on the Grid.  Along with solar modules, they represent the most important aspect of your solar array.  When considering an inverter for your PV system one would look at the following: efficiency of the system, warranty (product life)  and safety.

What do they cost

For system from 2 Kw to 10 Kw the inverter should cost from $1200 to $7,000.  The lower the price the lower the Kw that the inverter can handle.  Some inverters require additional accessories in order to facilitate their web communication interface.    If you purchase inverters and want to handle the MPPT (maximum power point tracking) separately then you would add Solaredge Optimizers or Tigo Maximizers and have to add the cost of the small unit that would be required for each solar panel you purchase, although I believe Solaredge has come out with a device where 1 optimizer connects to 2 panels now.  Prices range from $50 to $125 each.  For the microinverter (DC/AC inverters) such as Enphase or Enecsys, Powerone, SMA and Sparq  you would pay $150 to $200 each multiplied by the number of solar panels you have as well although in this case you would not need an inverter.

How long do they last

Inverters should last about 10-15 years and you can purchase extended warranties for them.   Solaredge for example is warrantied for 12 years although the company believes they will operate for at least 18 years without failure.  DC-DC power  and  microinverters optimizers/maximizers are another matter as they are relatively new.  Since they are new there is no long term history as to their ultimate reliability as opposed to the MTBF info provided.   The MTBF (meantime between failure) for these devices usually looks great when looking at the spec sheets but in the real world –it’s a guess and we not have realworld figures for awhile yet.  Remember with Enphase for example (its MTBF is 331 years), if there is a failure the panel will not produce power and you will have to have someone go up onto the roof to change the microinverter.  With Solaredge however, if the Optimizer fails it just fails and should be replaced but the panel will still produce electricity, just not managed.  One argument you hear, is the fact that Enphase uses electrolytic capacitors which some say dry out over time, while other companies such as Tigo and Solaredge use another type of capacitor that they say will not dry out and become defective.  In the end, these arguments could just be fake arguments since it just depends upon the quality of the capacitor being used and I am sure Enphase would like to be in business for a while and would not purchase capacitors that would fail in their devices.   But I’m not being a capacitor specialist, I don’t really know for sure.

Here is a discussion about this:

http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/2012/036407.html

Where should your inverter be installed

Inverters that are properly certified can be installed and mounted outdoors or indoors without any problems.  But it would be a good idea to make sure that the sun is not shining directly on I, if mounted outdoors as it will reduce your energy harvest.  If you do place it indoors, there may be a humming sound that is distracting if you are working near it for any length of time.  Keep in mind as well, that they are quite large and especially the transformer version – very heavy.  Microinverters do not have this issue as they are mounted behind the module and there is no inverter at the panel side.

Should I get an inverter, a microinverter or a DC/DC optimizer/maximize?

This is the big question.  The simple answer would be to consider consider microinverters or DC/DC devices if there are shading issues or if you have panels facing east or west, have different panels or need more than 2 strings. There is also the electrical safety and fire safety argument.  Microinverters shut off at the module if they fail.   The Solaredge Optimizer will only generate 1 volt when the inverter is shutdown.  The Tigo is a little bit more problematic as it must communicate with each inverter through its wireless gateway to shutdown its Maximizers.  I don’t particularly like this approach.  Communication with the Solaredge Optimizers are through the electrical lines and therefore much more efficient. 

Solaredge Optimizer attached to rack

Inverters are the solution for a south facing, no shade issue installation.  They are cheaper and easier to install.

Update
Here's an update from May 6, 2013 (9.5 months operational) I now have 2 modules that are dark (show no power generation) based upon my web portal.  They are also not registering with the Inverter.  It could be that the problem is not the Optimizer, but the module or communications.  However the only way to find out is to bring in the company that did the install to check it out.  Knowing that micro inverters and optimizers could have problems my guarantee with the company is with installation only.  So although I have a 25 year warranty on the equipment, I still need a crew to come and determine what the problem is and potentially come back a second time to put in the replacement parts.  This is not a $50 fix so I did a cost benefit analysis of the impact of the loss of power over the 20 year period and it is worth it for  me to bring in the crew.  Remember with micro inverters and Optimizers or DC-DC equipment you have many more potential points of failure.  Now, do I regret using the Optimizer...no.  I still think from a fire and safety point of view, the web portal reporting aspect and having individual management of each of my panels that I am ahead financially.  However, If I was in an unshaded south facing situation I would definitely go with an Inverter only solution.

Note the dead modules-

Transformer or Transformerless

Transformerless inverters are relatively new.  The benefits of going TL are as follows:

•  less weight

• less expensive

• less noise

• firmware upgradeable

• safer

Many new inverters are transformerless. While these inverters offer several advantages, neither DC current-carrying conductor is grounded. Ungrounded systems must still “equipment ground,” i.e., connect all metallic enclosures, module frames, and racking to a common grounding conductor. Ungrounded systems have additional requirements per National Electrical Code Article 690.35.


This is an explanation by Kaco, the first inverter manufacturer to come out with a TL transformer.

http://www.kaco-newenergy.com/downloads/KACO_articles/110809_TLinverters.pdf


The following is a “buyers guide” to inverters which you may find useful.

https://homepower.com/articles/2012-grid-tied-inverter-buyers-guide

And this is an informative pdf on how inverters work:

http://solar.gwu.edu/index_files/Resources_files/How-Solar-Inverters-Work-With-Solar-Panels.pdf

Maximum Recommended PV Power. 

Systems are often sized so their power output is larger than the inverter output.  This may seem odd, but due to a number of factors such as climate( module temperature affects power production), soiling, production tolerances etc the modules would in fact produce less power the specified.  In order to maximize power production and potential financial returns balancing the ratio of modules to inverter can be contentious.

Inverter manufacturers specify the maximum pv  power that it’s inverter can handle and it is usually in a range of 115%-125%.

 Here is some terminology that will come up as you learn about inverters

Maximum Open-Circuit Voltage (Voc). 

Voltage is electrical pressure, and all electronic components, including inverters, have a maximum voltage they can withstand. This specification is the maximum input voltage that the array may reach under any condition.

Maximum Power Point Tracking (MPPT) Window

The MPPT window is a voltage range that the inverter can work within to find the maximum “knee” of the array’s IV curve to maximize array output as conditions vary.

Some Inverter Manufacturers

http://www.schneider-electric.com/site/home/index.cfm/ca/




http://www.power-one.com/renewable-energy




http://www.solaredge.com/



http://www.kaco-newenergy.com/index



http://enphase.com/



http://www.sma-canada.ca/




http://www.sparqsys.com/aboutmicro.html