Playing around with microinverters

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Minister of Fire
Jul 11, 2008
Northern NH
My original PV system now about 18 years old lost its inverter recently. Back when it was installed grid tied systems were a lot smaller and definitely had marginal payback since there was no 30% federal tax rebate or state incentives. I got a real good deal on my polycrystalline panels at $6.60 per watt ;) (these days they are $0.50 a watt). There were only a few companies that made grid tied inverters and they were not inexpensive. Nevertheless I wanted to get hands on practice with solar design and installation so I installed my 660 Watt system. It was free labor so my installed cost was about $7 a watt. The one unusual thing was I decided to run all the individual panel outputs to a junction box in my basement since I had "free wire". I then installed jumpers in the junction box to configure the panels to the inverter. This meant that at some point in the future could more easily adapt it to a newer inverter.The system has been running since 2002 until early December last year when the original inverter died of unknown causes.

Buying a new string inverter was not going to happen due to price so I decided to install microinverters. I decided to go with Enphase M215s, they a have better but not stellar record, compared to the first generation 190s. They have been superseded by newer generation Enphase's that need different trunk cables and accessories so it was time to go on Ebay. There are quite few showing up for sale along with accessories. I wanted enough parts to make a legal potentially code compliant install so I also needed a section of trunk cable used to tie inverters to the PV subpanel. I could speculate that since leasing firms are using microinverters that at least some of the equipment may be ending getting sold by folks who are removing equipment they do not own.

Microinverters do have the reputation of getting "cooked" due to their normal location under panels on a roof mount. They aren't that hard to test so I just bought used units on Ebay that were represented to be working. Worse case if I got a dud I would just return to the seller as Ebay almost always backs a buyer if the equipment was claimed to be operational and turns out not to be. I only needed a short length of trunk cable with 4 terminal blocks. Someone had listed various short lengths of trunk cable with a "make an offer" option so I got a 5 terminal trunk cable for cheap with an offer and got two inverters for a good price. The only other equipment I needed was a crimper for MC4 connectors that came with bunch of MC4s connectors.

Since its wintertime and I do not have any interest in working under my wall mounted panels, I decided to in the short term install them in the basement. I had space on the wall where the old inverter went so I installed a couple pieces of scrap unistrut and drilled and tapped it to accept mounting bolts for four inverters. Once the inverters were mounted I had to rewire the power feed as the old inverter was 120 VAC while Enphase's are 240 VAC. I installed a "gutter" system several years ago for my solar wiring between my three inverters, a PV subpanel and my main panel so the rewiring didn't take long. Then I used some spare USE2 PV wire to make up jumpers between the MC4 connectors on the inverters and my PV terminal box. I flipped the breaker on and in a few minutes I had two of the 4 panels running. A third inverter showed up yesterday evening so I wired the third panel in last night and its up and running this AM and #4 should show up by the end of the week. This is arguably a questionable legality install as it would be stretch for an AHJ to call it a replacement in kind for a string inverter. It is no more or less safe than the prior system but NH and lot of other states now have a Rapid Shutdown requirement for new PV systems. I could have saved a few bucks by skipping the trunk cable and MC4 connectors but that definitely would not be legit as the entire Enphase system is "listed" The plan is at some point when is warms up to move the microinverters to the back of the PV panels and run one home run wire into the house panel. This would meet Rapid Shutdown requirements as microinverters inherently do not put out power when they are not connected to AC. I am not worried about cooking the microinverters as my wall mount panels pivot off the wall of the house and are adjustable for angle so unlike conventional roof mounts I have lot of open space for them to get cooling. I didn't go with any monitoring but do have an analog ammeter on the output cable to the PV subpanel. My total PV output from all three systems go through one production meter for SREC sales so not worth the extra hassle of monitoring these panels.

I do see the temptation for using microinverters for bootleg solar. Snip a few wires and add some wire nuts and tie them to a 240 VAC circuit and they work. Definitely illegal for lots of reasons.

The one thing this does point out is that solar systems are still maturing. Inverters have traditionally been the weak point as the reputation is they are good for ten years. Many folks buy extended warranties but they have to be optimists that someone will be on the other end of the line when they need to use it. I have no doubt that folks with other than owned systems will at some point in the future end up with dead arrays as the firms that installed them are long gone and the successor organization probably bought the remaining life of the system cash flow at a major discount. I think they will decide the cost to fix the systems exceed the potential future revenue. The also will not remove them so the owner is in Catch 22 situation where if they need to sell the house they will have to buy the non functioning system and most likely pay to have it removed as its liability to the next owner. Even with the Enphase system, they now have a new trunk cable system incompatible with systems installed just a few years ago.
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Minister of Fire
Sep 18, 2019
Grande Prairie, Alberta, Canada
Microinverters are definitely beneficial for a lot of reasons, ease of scaling is definitely one of the best reasons, a retailer only needs to keep one or maybe two different sizes of inverters on the shelf and can cover almost any install.

I have 5 APSystems YC600 inverters on my roof, each inverter runs 2 300 watt mono panels. They also have a QS1 inverter that supports a total of 1200 watts on 4 channels. All their products still treat each panel independently, mine are basically just 2 inverters in 1 housing.

I think the worry about failure is a real one, my installer has said they have replaced a few dozen inverters by now, of course all on warranty, but the potential for failure exists. The beauty of it is you don't loose the entire system, if one of mine fail I loose only loose 20% of my production versus 100% for a string inverter. A conventional string inverter isn't feasible on my house either, I have arrays on east, south and west roofs, meaning I'd need a dedicated string inverter for each, if there was one that would support only 4 panels.

Safety is another big factor. Microinverters don't require large high voltage DC cables to be run through the house, and unless there is some kind of shutoff at the panels that line is always live when the panels see light. Could you imagine a firefighter try to open up a roof vent on a roof, or perform a building search with a 600volt live DC cable on it? Microvinverters when mounted on the roof only have an always live cable with about 40 volts and only run for a couple feet. The rapid shutdown is also nice, mine physically can't produce electricity without a grid connection.

I think microinverters will dominate the market for house scale installs, leaving string inverters for commercial scale operations.