I have installed both micro-inverter Solar systems and central inverter series-string Solar systems, both as a volunteer for SunWork in Palo Alto.
The series-string system uses the wires already attached to the solar panels to create a single string (up to 10-12 panels) or two independent (not connected) strings that each feed independently into the inverter. This is especially significant if the two strings are oriented towards different parts of the sky or experience different shading. The only added wiring is typically from the start and the end of the string of panels to the central inverter. Typical string is 10 panels, about 2400Wp of solar at minimum of approx 6A, 400VDC. The micro-inverter system has the same panel wires, but not plugged into each other to create a string, rather they are plugged into the micro inverter which is mounted to the rails below each panel and the 240V plug from each micro-inverter (they typically come with an AC pigtail and plug) goes to a special cable that you order with the micro inverters and which has the mating connectors where the micro inverters plug into. This special cable strings along all panels, connecting all micro inverters in parallel and the load to the cable for a 240W panel is 1A at 240V. So a typical 10 panel small installation means a 10A 240V power running through that cable which is spliced into standard conduit and wiring at one end of the string and from there to the service panel. Since a typical central inverter is also mounted within a few feet from the service panel, the differences in losses between either solution are typically very minimal. The only real big thing, why I would always opt for central inverter, is that maintenance on an inverter while standing on solid ground is so much easier than maintenance on an inverter attached to rails on the roof under a solar panel. Secondary, the price of micro inverters is not competitive with central inverter and also the location of a central inverter can be chosen so it is in a cool spot and has an easy life with plenty of air ventilation while micro inverters by their nature are hermetically sealed and live on a hot roof. I hope the designers don't use any Electrolytic caps in them. I also hope for people with micro inverters that the designers made sure that the failure rate is more than 10 times lower than central inverters, even in hot environments or you will need a lot more times someone on the roof than someone working on the central inverter. Another concern (and I work in wireless communication) is that micro inverters report their status via a broadcast of information that you need to capture using a dedicated receiver in the home, attached to your computer or sometimes to the home router. This wireless communication from the inverters on the roof to the receiver in the home might not always be reliable, whereas a central inverter can easily be hard-wired to your home router and be accessed from your computer reliably. If you want, it can even be hard-wired directly to a computer via RS-485 and a converter from USB. Statistics from the solar system may or may not be uploaded to a server. If not, then you can't typically retrieve the info once the sun is gone since modern inverters shut down completely without solar input to minimize nighttime parasitic draw. Unless you add a dedicated DC power supply that you can manually engage to start up the inverter, you can't read at night what it did during the day. I hope that micro inverters efficiency and parasitic draw are much better than the central inverter, otherwise you end up with a less efficient system overall. One last and not insignificant concern with micro inverters: These electronic devices are out there on the outside of the home, high-frequent switching power all day long and creating interference. Even though they should be EMC compliant, which means that under certain circumstances they radiate an amount under a certain threshold, they will still add noise to the radio environment all day long, which can be a hindrance to anyone interested in low signal, long distance radio communication anywhere in the neighborhood of your home. In addition, each one has a radio beacon that is broadcasting their status all day long which adds to the emissions, even though that should be confined to a specific free radio band. Of course you are free to go with any system you like, I can just share that I have a central inverter sitting in my garage. Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info http://www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of Lee Hart via EV Sent: Wednesday, June 08, 2016 11:03 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Off-grid solar house and electric car charging Robert Bruninga via EV wrote: > Still not true. The output of microinverters is at 240 VAC and the average > current in the wires will be double as the same number of panels at 480 VDC. But each microinverter has its own wire. The total current may be twice as much in a 240v system as in a 480v system; but there are twice the number of wires, each half the size. You wind up back where you started. > Further, AC peak currents are 1.4 times higher than DC so the peak currents > (where the losses are) are 2.8 times greater No; AC voltages and currents are normally expressed in RMS (Root-Mean-Square). RMS voltages and currents have the *same* effective values, voltage drops, and losses as DC. 120vac and 120vdc are completely equivalent. > The difference in buying #6 wire instead of #12 is only $180 versus $30. Or > you can ignore the extra 10% or so losses and use #10 wire. But over the > life of the system (20 years) the losses in your solar system can add up to > many thousands of dollars. Sure; it's basically an economic decision. How much can you afford up-front, to reduce long-term losses? It's further complicated because when you eventually scrap the system, much of the cost of the copper is recoverable. Note that this is the EV discussion list. Besides these tradeoffs, weight is also an issue. You may be ahead by deliberately undersizing the wire to save weight. The benefit from weight reduction can exceed the efficiency loss. Racers know this well! > Maybe I am just being nitpicky, but my solar arrays are all over my yard and > house. Some runs are over 300 feet! (shortest is maybe 60'). Ah; no wonder you are so concerned with wire lengths. I definitely consider 60-300 feet *long* runs! The PV panels on my house are only 20' from my circuit breaker panel. -- "IC chip performance doubles every 18 months." -- Moore's law "The speed of software halves every 18 months." -- Gates' law -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA) _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
