Grid-Tie and Microinverters

[Dagoth Ur Does Solar]

I've been searching both the forum and the internet for the past few days trying to wrap my head around grid-tied systems with microinverters and battery backup.

I tried to break down my questions:

• How exactly do grid-forming microinverters + a battery backup operate? As in, what determines the system's output when the grid is down?
• Do they have a second traditional inverter that pulls from the battery?
• On a cloudy day with the grid down, is energy being pulled from the battery rather than PV and inverted at the microinverter on each panel, then back down to the control hub?

I am asking this because I want to wrap my head around the technology and understand if the system's inverted AC power output when pulling from battery would scale with the number of panels, or if there is another bottleneck like a secondary inverter.

Any answers or suggested reading on this would be awesome.

 

[pvgirl]

The battery in a micro inverter system, has it's own inverter/charger, when the grid is down, and there is no solar power output will be limited to the output capacity of the battery inverter.

 

[Hedges]

Here's a perspective drawing for Enphase showing IQ System Controller as disconnect from grid.
I think Enphase makes a box with multiple microinverters each fed by a battery in the box. So basically a second traditional inverter with battery, but I think transfer switch in separate box.

Enphase Storage for IQ™-series microinverters | Enphase Storage

Enphase Solar systems with IQ7 and IQ6 series microinverters are fully compatible with Enphase Storage, making retrofit upgrades as simple as new installations.

enphase.com

You can put microinverters downstream of an inverter (that supports frequency-watts) and has internal transfer switch. People here do that with Schenider inverters, maybe others.
We hear that IQ8 doesn't play nice, maybe too aggressive anti-islanding, but IQ7 works. (for IQ8, better use Enphase battery solution.)

 

[Dagoth Ur Does Solar]

The battery in a micro inverter system, has it's own inverter/charger, when the grid is down, and there is no solar power output will be limited to the output capacity of the battery inverter.

That's sort of what I expected. My main concern is in the future, if I wanted to add a second battery for extra capacity, how complex/costly this becomes (i.e. could I, or am I required to also add a second dedicated inverter).

Here's a perspective drawing for Enphase showing IQ System Controller as disconnect from grid.
I think Enphase makes a box with multiple microinverters each fed by a battery in the box. So basically a second traditional inverter with battery, but I think transfer switch in separate box.

You can put microinverters downstream of an inverter (that supports frequency-watts) and has internal transfer switch. People here do that with Schenider inverters, maybe others.
We hear that IQ8 doesn't play nice, maybe too aggressive anti-islanding, but IQ7 works. (for IQ8, better use Enphase battery solution.)

Ah, thanks for the link. In this case, the modules he is recommending are the Enphase IQ8+, and my understanding is they'd be configured in what Enphase calls multimode, which allows them to switch between being grid-interactive during a grid outage into microgrid mode to keep providing power to the home via solar and battery backup. At least, that's my understanding. I see a lot of confusion on the internet in regards to the modules, and it seems like it probably depends on what mode the installer has configured them in and what supporting equipment is there.

I think if they actually come in close to the amount quoted, between my income level and the federal and state tax breaks, it would be economical for me to opt for a grid-tied system. The big questions come up obviously surrounding if the government and utility companies further screw with net metering to make it a poor economical choice.

Ultimately I'm leaning heavily towards just going the Will Prowse route and getting grid-tied solar for the home, and then build out a totally off-grid system for the workshop that I could lean on in addition (even if it's running extension cables).

Spoiler: Seething

Really though, screw mandatory utility connection charges. If I'm churning out way more power than I produce, at peak hours, which is presumably subsidizing the utility company at the times when they have to use their most expensive generation sources, I shouldn't be also charged monthly just for the privilege of doing so

 

[Hedges]

Enphase probably uses proprietary signaling to turn of anti-islanding when using battery backed-up grid.

Most AC coupling between microinverter or string inverter and separate battery inverter uses Rule-21 frequency-watts, and anti-islanding remains active (battery inverter has to be stiff enough to look like the grid.0

SMA previously used RS-485 for Sunny Island to signal Sunny Boy when anti-islanding not needed. Newer Sunny Boy use Rule-21 (or if completely offgrid, "Island" setting.)

Hybrids use DC coupling between PV MPPT, HF inverter, and DC/DC bidirectional battery charger. Some like SolArk also support AC coupling to additional GT PV inverters. Sunny Boy Smart Energy is presently on-grid only, expect backup/offgrid later with external transfer switch.
Many of these hybrids support PV wattage higher than AC wattage, can use the extra for battery charging. SolArk one model 15kW PV, 12kW AC. Sunny Boy Smart Energy (available January) 15kW PV and 7.7kW AC (11.5kW model due end of 2024.)

Modest utility connection charge like $10 to $15/month is OK, cheaper than a big battery and lets you store power for off-season.
"Net metering" is moving to where credits are wholesale except or a couple hours peak times, so backfeed from battery could still be a good deal.
When no retail credit for backfeed, then you'll want to just use PV an battery locally, no backfeed, and consider utility connection as a cheap reliable generator.
(California utilities have requested connection fees as high as $95 to $135/month, but some say final decision might be $40. That comes to $500/year, at which point you might prefer generator backup fed by natural gas. Depending on how much you have to use it. Consider implementing CHP for domestic heating.)

 

[Dagoth Ur Does Solar]

Modest utility connection charge like $10 to $15/month is OK, cheaper than a big battery and lets you store power for off-season.
"Net metering" is moving to where credits are wholesale except or a couple hours peak times, so backfeed from battery could still be a good deal.
When no retail credit for backfeed, then you'll want to just use PV an battery locally, no backfeed, and consider utility connection as a cheap reliable generator.
(California utilities have requested connection fees as high as $95 to $135/month, but some say final decision might be $40. That comes to $500/year, at which point you might prefer generator backup fed by natural gas. Depending on how much you have to use it. Consider implementing CHP for domestic heating.)

As best I can tell, my connection charge would be something like $50 monthly, which to me is absurd- since this is only likely to increase over the years; I seriously doubt any utility company would just decide to lower it for whatever reason.

 

[Hedges]

Do you have natural gas?

Have an idea of how many kWh you'll be needing that aren't available from PV & batteries?

Do you have microinverters already? If not, I recommend looking at other options. Microinverters are optimum for zero-thought grid-tie PV install by vendors.

As people here say, "Power audit". Determine your peak wattage, starting surge wattage, kWh/day, kWh/night (when no sun), and seasonal variations.

Wide range of prices and capabilities, from HF hybrids to multiple LF battery inverters stacked. 3 kW to 25kW ~ 50kW.
HV or LV batteries. Lead-acid and lithium.

 

[Scrugs]

What is the common thought on a microinverter system as opposed to a centralized inverter system ? for residential power mainly for HVAC and applicances