How does a single phase Enphase Microinverter work with a residential system with a neutral?

DavidM

I am considering installing a grid tie system based on the Enphase Microinverters. These apparently have only an L1 and L2 and no neutral for 110V power. How does this work in compatibility with a residential system with L1, L2, a neutral and a ground?

It seems that even if they two could be connected all of the house's 110V loads come from the utility company. Does the utility meter know this and you only get credit for 220V used and no credit for 110V?

Also if the grid is down and the only power comes from the microinverters, I presume it will only be 220V so the house will have no 110V power. True?

BB.

Welcome to the forum David,

Assuming you are somewhere in North America (you are somewhere around North East Coast (guessing Connecticut to New York area?), your GT inverters only need L1 & L2 (240 VAC) to work correctly and interface with the 120/240 VAC split phase 60 Hz utility meter used in USA, Canada, & Mexico.

The meter will record the amount of power (and total energy) in/out to the meter. And the pole transformer (L1, Neutral, L2) will generate the "3rd leg" (neutral) needed for your 120 VAC appliances.

A quick link:

https://samlexamerica.com/wp-content/uploads/2019/12/13010-0614_120240VACSingleSplitPhaseandMulti-WireBranchCircuits.pdf

As always, the details are more complex... Newer AC inverters may (do?) use the Neutral line as one of the "qualifiers" that your AC power is good (240 VAC 60 Hz from L1 to L2, and 120 VAC from L1 to Neutral). My first GT inverter (different type/brand) had L1 and L2 only. The replacements had L1, L2, and Neutral--Where the Neutral was just to make sure all the AC wiring back to the pole was OK (no current/power flowed through the Neutral leg).

The additional complexity... Most utilities and Local Building Departments require an engineering plan and inspection (and even homeowners insurance) to approve the installation. And you generally have to register with a GT Solar power plan. In general, installing a since "small" GT System may not even be worth the fees.... And some utilities do not allow GT Solar.

And, depending on your meter type/age... The old meter may work "transparently" (go forward if you use power, turn backwards if you generate power). Other meters may only run "forward" (no credit for generated power). Or even run "forward" if you generate or consume power (if you generate "extra power", the meter will charge you for it.

And newer electronic meters may even "tattle" back to the utility that you have GT Solar on your home (what happens next--I really have not read any stories--But in theory, the utility can pull your meter, the building department can red-tag your home, and you could be liable for any electrical fire down the road and not covered by your insurance). Has any of this happened to "real people"--I don't know one way or the other.

If you are installing only one solar panel--It is possible that your power will never flow backwards to the utility, and the meter may never detect the solar system...

That is the quick and dirty.

Questions?

-Bill

DavidM

Thank you, Bill, for your advice. I am in Litchfield, Ct, in the NW corner of the state I will have to do some more research on grid tie solar and perhaps have a conversation with our local building inspector as well as a guy I know who built a net zero home nearby with extensive solar that is tied to the grid.

Not that this is a big consideration, but when the grid is down, does the 240 V power produced by my solar inverters feed back to the pole (in my case ground) transformer and then provides a neutral for 120 V to my house? If so I might include a small LFP battery bank to keep things going when it gets dark outside and the grid is down.

BTW parts only are going to cost about $9,000 for a 4.4 kW system. I think I can handle installation myself. The NREL PVWatts calculator estimates I will produce about $1,000/yr in power from a 4 kW PV system at our current incremental rate of 20 cents per kWh. That is a return of about 11% annually, not great but maybe doable. If I contracted for it to be installed there is no way it would work economically for me.

David

BB.

The "standard" Grid Tied inverters shut down their output if the utility power goes away. For the safety of the linemen and such. And because of the way GT Inverters work--They "follow" the AC sine wave--GT inverters do not have the capability of generating 60 Hz AC power by themselves.

There are several options for "Grid Tie/Utility Interactive" power...

The first, there are a few GT inverters that can generate 120 VAC power when the AC mains fail through a separate circuit. SMA has their "secure power" model. And, I believe, Enphase is working on/has a version of secure power. These only generate electricity when the sun is up and shining on the panels. And only support as much 120 VAC output as their is sun on the array... Maybe 1,500 Watts max and no "surge" current to support starting motors and such. And if there is less sun (morning/afternoons/cloudy weather) less than 1,500 Watts (just a very light cloud cover, probably less than 50% of rated power)....

Then there are "Hybrid Inverters". These are inverters that are both GT and Off Grid capable. When utility power is up.. The Hybrid Inverter takes solar power to both charge the battery bank and feed "extra" power back to the main panel of the home or "GT mode". If utility power fails, the inverter transfer switch opens and allows the inverter to run in OG mode and power the "protected circuit" with 120 or 120/240 VAC power (fridge/freezers, lights, etc.--Or whatever loads you have on the protected panel). These require a good size battery bank to feed the inverter and accept charging current from the solar array--Basically just Off Grid solar until the utility power comes back.

And I have seen units (mostly from China?) that are Hybrid AC inverters that will operate with or without a battery bank attached. If battery bank, they operate as above as hybrid AC inverters. If no battery bank, they operate with just the solar power available and supply 120/xxx power as long as the sun is up and shining.

Batteries tend to be the expensive and "weak link" of solar power (batteries last 5-15 or so years, electronics last 10+ years typically). Depending on how often you get power failures--Your most cost effective solution may be to have plain GT Solar (cheap, reliable, the utility acts like a "giant" AC battery bank). And get a genset+transfer switch and run from natural gas, propane, diesel, gasoline with fuel preservative for gasoline and diesel systems) when power is out for a few days a year... That is what I do. GT Solar and a 2,000 Watt gasoline powered genset for the 1 time in 60+ years that we had a multi-day outage (high winds and wild fires caused by poor utility power line maintenance--for the most part). And just recycle the gasoline in our car once a year.

-Bill

DavidM

Thanks Bill, I think I have a better understanding of my options now.

Let me share a story of how I used a generator, an inverter and two (12V 75 Ah lead acid) batteries to keep our home going when the power went out last winter.

The power went off at about 3:00 PM on a pretty cold day: 40 during day and 20 at night. I immediately rigged up my Honda 2000 to power the fridge, and LR lights by plugging in with extension cords as well as ran an extension cord over to our neighbor's house so they could do the same.

I have a propane fired gas insert in our fireplace. Not great efficiency with the blower going and worse with it not. I was able to light it (it has an emergency 2 AA battery for that) but the blower wouldn't run as it needs AC. The fireplace insert is on the same circuit that our TV, cable box, wifi router, etc is hooked up to. So I went down in the basement and turned off that circuit breaker. I had previously confirmed which was it by turning it off and noting all of the services noted above being off.

Then I used a male to male 120V adapter I made up to back feed from the generator to that circuit. The blower started just fine and low and behold the TV came on and we had wifi. Obviously our cable/wifi carrier wasn't affected by the power outage.

We pulled some twin mattresses from our basement up to the LR and got ready for bed. At about 10:00 I shut down the generator, but I used a 150W inverter hooked up to the batteries to continue to supply power to the blower circuit. I shut off all of the cable/wifi boxes and measured 4 amps DC coming from the batteries. That was easily covered by the batteries overnight.

Sleeping wasn't great but at least we were warm .and cozy in front of the fireplace

The next morning I started the generator and let it recharge the batteries using an old RV converter's charger. The power came on about 10:00 AM and I returned everything to normal. 

I had about 50 gallons of propane in a 100 gallon tank and could have gone on for about a week that way.

Some will chastise me for back feeding the AC but I am a pretty competent guy (retired engineer) and I think it was reasonably safe for me and the power company guys.

If this power outage was a routine thing, I certainly would install a 4-6 position transfer switch and run a 10/2 cable from an outside inlet down to the basement transfer switch. But this is the first time the power has been out for more than an hour in about 8 years.

BB.

Yea--You can wire stuff this way (for example use a "suicide cord" with male blades on both ends--Turn of the Circuit Breaker (branch circuit, or main house breaker). The whole issue is if "somebody" messes up... Lineman gets electrocuted, genset gets hit with 120/240 VAC when power is restored and starts a fire, if somebody flips the breaker back on, etc.

It sounds like you unwired your Living Room branch circuit from the main panel and wired to to a plug for the genset. If you need more circuits (fridge/freezer/etc.) then going with a transfer switch is going to be easier the next time you need power.

Lots of ways of making things work. And if you do it right so that nobody makes a mistake (use circuit breaker lockouts, etc.)--It can work.

Longer term, better to wire in a transfer switch to power your critical loads with the genset and never have the issue of cross wiring main with genset power.

We always try to help folks do things the "right way" and "safe way"... But there are always workarounds.

Understand the "rare" power outages... First time in >60 years for me to have an outage that lasted more than a couple of hours. Spending $$$ on a natural gas fired genst + automatic transfer switch and such... Difficult to justify doing that (or solar+battery+inverter+etc.) for my home.

-Bill

bnes`

The "old way" of doing this was the SMA "Secure Power Supply" : basically a "Grid Down Sun Up" solution that worked great, prior to the 2017 NEC that mandates rooftop shutdown for solar panels and made things complicated.


The system sort of still exists, and would be far safer than the DIY methods mentioned above.
You can certainly get this as used equipment:

https://www.sma-sunny.com/us/how-to-explain-secure-power-supply-to-homeowners/
https://www.youtube.com/watch?v=Z9r8XfmErNs

For me the SPS solution makes far more sense than the cost, embedded energy, risk and complication of batteries or a fossil fuel backup generator.  The SPS just powers the freezers.