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Ron342 05-30-19 10:09 AM

Advice for small system grid tied inverter type??
Hello solar folks!
I'm new at solar systems and have 3 - 60w panels and 1 70w panel, all 12v, and would like to put together a small grid tied system with them feeding one or maybe 2 120v circuits in my home which have enough fairly constant loads to use most of the solar power but which would be blocked from backfeeding my smartmeter (and any other house circuits) and of course blocked when the mains were off in an outage etc.
Would like to add this inverter to a larger system maybe in the future so thats a consideration.
Are there any inverters that accomplish this?. My first thought was to simply put a blocking diode ahead of the breaker on each of the circuits fed by the inverter but somehow that seemed too simple to work!
And the grid tied inverters i've found seem to require a min input voltage of 22v so do i gang my 4 panels with 2 panels in series and 2 pairs of these in parrallel to the inverter?
Thanks guys!

CrankyDoug 05-30-19 04:57 PM

Well you were right about the blocking diode being too simple. Blocking diodes don't block alternating current.

I don't know how things are up in MD but about a year ago several people here in GA got caught using non-permitted grid tie inverters. Grid tie inverters are easy to detect and line workers are understandably short fused about things that might kill them. Depending on the codes in your area you could get fined.

If you want to use solar to reduce your electric bill you can get a device off ebay that ties the solar panels to an electric water heater. I haven't hooked mine up yet so I can't recommend it other than to say you should check out some youtube videos from people who are happy with theirs. Since DHW is the second largest energy requirement in a home you will be getting maximum use of your panels.

I have no connections with the seller.

Ron342 05-31-19 07:36 AM

Thanks Doug
Itís the same here, anything grid tied has to be permitted and I think the smart meters report back flow - the linemen have enough problems without dealing with that during an outage. I understand they ground out any dead line theyíre working on in case a connected generator etc comes online. I think some of the inverters wonít go if they loose their input from the grid. Iíll have to see how the code requirements prevent back flow.
Iíll ck out the dwh option too but I did see some inverters on ebay with what they called limiters which supposedly prevented feeding anything beyond your needs.

CrankyDoug 05-31-19 12:38 PM

As far as I know all grid tie inverters sync to the grid and therefore shut down when the grid goes down. However, the safe-off methodology used in ebay (inexpensive) inverters may not meet NFPA70 requirements. And the cost of certification precludes any chance that these devices have even been submitted for certification.

The last machine I built for a customer required EN-954 class 4 safety controls. Two safety rated contactors had to be wired in series and monitored in such a way that if any of one 6 contacts failed (open or closed) the entire machine lost motive power. The contactors alone were $280 each. A code-compliant grid tie inverter will have similar requirements, though the topology may be implemented in solid state. Either way it is going to be expensive.

Keep in mind that the people selling "NEC/UL/CE certified" inverters on ebay are the same people who sell Gibson Les Paul guitars for $200.

The DHW device I mentioned powers the lower element in the tank and is completely separated from the grid. DHW storage is the closest you can get to batteries without the cost and trouble of batteries. Heat is the lowest form of energy but that is what makes it so easy to store. And most of us use a lot of it.

stevehull 06-01-19 07:27 AM


The new Enphase IQ8 microinverter may solve some of your concerns. It has the ability to isolate the PV system when grid power is down.

That, in itself is not new, but what is unique is that this inverter can power loads when the sun is shining up to the rated output of the panels.

For example. Assume a set of panels that can put out 10 kW peak. The output peaks at solar noon in a parabolic manner (0 kW at night). The IQ 8 allows you to load the system up to 10 kW with lesser power supplied earlier and later than solar noon.

This is exciting as you can run your HVAC system in the heat of the day so long as it does not draw more than the panels can produce.

With future battery storage (prices going down rapidly) or as Doug has done with recycled Chevy Volt batteries, means you can store power.

Hope this information helps.


CrankyDoug 06-01-19 01:17 PM

I just looked at the IQ8 description on the Enphase site. They seem headed in the right direction. It requires a switch if you want to use your solar when the grid goes down, though Ron didn't mention that as a requirement.

Renvu requires sign-up to get access to Enphase prices. I recall their inverters are reasonable. Nothing I could find on the Enphase website mentions the cost of software or additional required components.

where2 06-01-19 05:25 PM

A 12V PV panel seems to typically be around 36 cells. If you parallel them, that's 72 cells. There are 72 cell grid tie microinverters out there, such as the Enphase M-250. With a combined wattage of 250W (60+60+60+70), you could build a combination of series and parallel circuits to drive all the panels into one M250, and knock up to 250W of continuous load off the grid. Being UL1741 compliant, when the grid goes down, so does an M250.

Despite using UL compliant parts, without permits from the AHJ it remains entirely illegal to grid connect anything, whether it's a new electrical panel, or a microinverter. I've got a farm in a very rural part of northern Maine. According to the town, I only need permits for plumbing related projects. According to the power company: "Yes, we want to know if you're connecting something new to our grid, especially if it generates energy". The power company hasn't said they won't allow a grid connected PV array, they simply said show us designs for what you're installing so we can make sure our linemen are safe. (A concept I totally agree with)

stevehull 06-05-19 05:51 AM

Hey Cranky,

Read a bit more on the Enphase IQ8. From what I read, no switch is needed.


CrankyDoug 06-05-19 11:27 AM

Hi Steve;

I'm finding some conflicting reports about the switch.

As I understand it the IQ8 can run independent of the grid, but to do that in accordance with the 2017 NEC it still requires a microgrid interconnect device, now known as a MID.

Enphase calls their MID Enpower. It consists of a transfer switch, neutral forming transformer, and controller.

If I understand it correctly the anti-islanding feature is adequate for a grid tie that shuts down with the grid. A system that produces power when the grid goes down still requires NEC mandated physical isolation, similar to the requirements for a generator. The Enphase MID looks like it is going to be expensive because there are no cheap workarounds for a safety rated transfer switch and neutral forming transformer.

It might be possible to eliminate the transformer if only 230V loads are applied. One would still end up with a system where licensed contractors, permits, inspections, and PE-stamped engineering drawings could double the overall cost of a small array. There would be some economy of scale, but not in my area where grid tie PV systems are limited to 10KW.

Battery prices are still going up. Now that PV panels are the least expensive part of a DIY system I think the next thing to do is eliminate the batteries. Using the IQ8 to power loads that can be switched in and out based on what the PV's are producing seems the best way to accomplish this task. DHW can be heated and stored. Hydronic heating water can also be heated and stored. Window shakers or a small minisplit can run continuously during daylight hours to take some load off the primary HVAC. None of these loads have to be connected to the grid. Take out the cost of switches, batteries, and permits and there would be plenty of money left over to buy more panels and IQ8's.


gasstingy 06-12-19 04:08 PM

If I were of the notion to add solar to power loads in my house without getting permission from the utility, or to power critical loads in a power outage (which very much intrigues me), I'd put those loads on a sub-panel with a generator transfer switch.

I believe this would absolutely protect the grid from my off-grid generation. I'd make an off-grid solar array, solar > charge controller > batteries > inverter > sub-panel. This is on my list of things to do before this year is gone, since the 30% credit starts slowly tapering away as of 1/1/2020.

dguzzi 06-15-19 10:46 AM

On our system (at work) grid tied, no batteries, if the grid goes down the solar goes down. It is quite effective but not as a backup type system.

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