How close to true zero export do you need to be? There are several ways this can be done.
Appreciate the extensive response.
I have a NEM agreement for 3kW peak and there are those who say it is ‘better to ask forgiveness than to ask permission’ but my current priority to to be certain I stay within my 3kW peak (meaning no export beyond what my existing grid-tied array is producing).
The other atypical constraint of my system is that I do not want to require my main panel or create a critical-loads panel.
I have 2 SUN 1kW zero-export inverters which will convert 24V battery power to up to 1kW per leg to offset self-consumption (monitored with clamp sensors).
So all I’m looking to do is get my new partially-shaded PV array to charge my 24V LiFePO4 as effectively as possible.
DC-coupled is obvious and easy but not well-suited to partial shading. Microinverters would be better for getting more output from a partially-shaded array, but need to either be throttled-back of have a sure on-premises destiny for the power produced (which is where my idea of a zero-export AC battery-charger to consume all Microinverter-generated power beyond self-vonsumption
The easiest way to be true zero export is to go with a DC coupled battery charging setup. It is less efficient as it will just throttle back the charging to hold the batteries full during the day, and you run on the batteries at night. If the batteries run low, you can buy grid power to keep you going.
Yes, that’s my current plan. And as long as the battery is oversized versus the daily generation and fully-drained from self-consumption overnight, it is more efficient than any other alternative I have found (into the battery, the losses add up when converting that DC battery energy to AC for consumption).
The biggest loss of efficiency I’m seeing with DC-coupled is how poor it will be in handling shade. MPPT-per-panel is much less practical with DC strings versus microinverters...
If you are going to do it with Microinverters, Enphase has a setup where the Envoy measures the grid current and commands the microinverters to throttle down to limit any export. But then your battery charger needs to adjust the charge current to maximize charging to use the "extra" power to keep the microinverters from throttling down. There is no add on system that I know of that has the control you are asking for.
That’s what I was afraid of. But if you agree it is a good idea, I think it will only be a matter of time.
The GTIL inverters I’m using didn’t exist a few years ago. They have completely changed the game as far as installing zero-export solar. They have a clamp sensor to tell them how much power is being consumed on their specific leg and then use that to limit the amount of energy they produce, feeding it into the same leg (literally through an outlet).
You can supply your self-consumption with literally no rewiring (and if their is a failure in the zero-export system, the grid and all rewiring remains unchanged and unaffected. Truly a game-changer as far as the cost and complexity of adding some solar production to offset a portion of self-consumption.
So on the AC-world, where conversion to AC happens in the array, what you need is the GTIL-equivalent for consumption rather than generation. Monitor generation and anytime it goes positive, increase battery charging current to offset export and bring back to zero. It would also need the ability to throttle-back microinverters once the battery is full, which can be a brute-force switch on the grid signal for once-per-day shutdown (rarely needed as long as the battery is oversized).
If this idea is as good as I think it is (which is why I value your opinion), I think it will only be a matter of time before one of the Chinese companies puts something like this on the market.
My Schneider XW-Pro has a lot of capability, but when it is tied onto the grid, some of it is not able to function. You can't frequency shift the grid. So while I am on grid, any extra production from my solar array is still being exported. I have to manually set how much charge current I want to to use. But if it goes off grid, it becomes a whole different animal. It will let the microinverters power all the loads, and any extra power will be used to charge the batteries. If the extra power exceeds the maximum charge current, or if the batteries become fully charged, then it will start to frequency shift to tell the microinverters to throttle back. If I had enough solar and battery to completely run my home, then it would work perfectly like this. But only about half of my home is on the inverter output. The rest is still back in the main panel. So I run the system on grid. This was the extra power does flow back to my main panel and helps power the rest of my home, and does export some when I am not using all of it. And I end up buying power for about 5 - 10 hours overnight once the batteries are back down to 40% where they started the day before. While on grid, I still need to manually trigger it to start the charge cycle each morning, but I am working on a simple fix for that. When off grid, this is not a problem, as it just charges from any extra power from the microinverters, but while on grid, it will never run the battery below the "Recharge volts" setting. If you have DC coupled charge controllers, that also "fixes" the issue. This is one case where I think the Skybox actually works without external intervention. I am still trying to convince Schneider that this function is needed and could help them sell more units. But they just don't seem to care.
I have not done a test since the software update, but Enphase has assured me that it should fix the issue I had with the microinverters going offline when I had a power failure. Basically, when the power failed, the Schneider XW-Pro did it's job. It disconnected from the grid, and started producing the local micro grid. But during the brief switching time, the microinverters saw it as a grid frequency error. They disconnect and should monitor the grid and if the grid is good for 5 minutes they should reconnect. But there was a logic error that was preventing them from connecting again. I hope to try a power fail test soon, but I want to shut down my PC's and such first, just in case it fails.
Since the system does work so well while off grid, I have looked into having a controller take the system off grid for part of the day. If the power fail disconnect issue is fixed and does not cause any problems, this may be the easiest fix. I can just have a simple time disconnect and reconnect the grid input to get the best of both worlds. This will likely cause a bit deeper cycling of the batteries, but that is adjustable to where I could make it use less while on grid to balance it out.
It really comes down to your power usage, you rules for power export, system cost, and solar production capability. For the last 10 days, I am very close to net zero. I have been buying some cheap overnight power, but then exporting some cheap power, and a few KWHs of peak rate power as well. Half way through a month billing cycle, SCE current owes me $3.00 as I have exported a little more than I bought. My peak export power did hit 2,000 watts when my battery topped up and the sun was still shining strong. I have delayed my charging start time a bit to reduce that. My peak power usage rate hit a maximum of 1,200 watts from the grid. 2 PC's on, watching Dish Network TV, and the furnace was running. Might have put something in the microwave as well. And of course, this was at night, no sun, and after the battery inverter had shut down as we were past the end of the peak rate time, and the grid power is cheaper.
The Conext solution is beautiful, as is Solark and even Magnum PAE, but they all require getting wired into the critical path, which means more $$$ and an additional potential point of failure.
I was headed down that path before I realized it made little sense for my use-case.
I’ve already got a 4kW grid-tied system under NEM1 that covers my annual consumption (at least before they start reducing credit by shifting peak TOU hours this summer).
2-3 years from now, we’ll have an EV, so by then, I want this battery+ GTIL system powered by a new array to offset a major portion of self-consumption (and 100% of peak TOU consumption) which will leave plenty of generation credit from the existing grid-tied array to power an EV.
My fall-back for dealing with shading issues is just to add more panels on another part of the roof, so that kind of sets my budget for how much I’m willing to spend to get back a portion of the ~40% of potential generation I lost to shading.
But it is annoying to me that there is not yet a solution to allow AC-coupled battery charging designed for zero-export (full consumption of excess generation). If there were, I’d be using microinverters in the new array rather than looking at using more MPPTs and possibly Tigo optimizers...
Oh well, I suppose I’ve got a couple years to see how things evolve...