DIY grid-tie system upgradable to hybrid

[RShackleford]

Newbie here. I formed a preliminary plan with a helpful fellow on stackexchange, but I want to relieve him of the burden of teaching me everything, as well as seeking additional perspective. I am a EE, but I still find the issues involved in a hybrid system most confusing ...

My goal is to DIY build a 4-5kw system (I have room for only 15 or so panels). I've decided to back off on making a hybrid system from the get-go, as a generator (or EV) probably makes more sense for our infrequent and fairly short outages. But I'd like to make it reasonably easy to upgrade to a hybrid system in the future.

Shading is a significant issue on my wooded lot, and in fact I probably can expect a reasonable payoff period only because I'm going DIY including the electrical work (I have considerable experience doing home wiring, and our jurisdiction allows DIY, subject to inspection of course). Because of the shading, I feel it's essential to go with either power optimizers or micro-inverters. I'm leaning towards the latter. My understanding is that SolarEdge's optimizer system is superior to competitors, but if I go with them I'm more or less forced to use their StorEdge technology and expensive batteries such as Tesla PowerWall (whose capacity I do not need). Using micro-inverters forces me to AC couple, of course, but it leaves me a lot of freedom in components and battery capacity. I'm leaning towards Chilicon micro-inverters, mainly because my stackexchange pal seems to like them more than Enphase; but I see no discussion of Chilicon at this site.

So I'll start with a 15-panel array with micro-inverters. Probably arranged as a 5x3 array, roughly 16ft-square, forming a carport roof. I'll build the carport with four 6x6 or 8x8 posts on concrete piers, adding some diagonal bracing and rafter-like beams to support the panels; I hope to mount the panels directly to this wooden framing, avoiding metal racks/brackets/etc. I gather it's impossible to seal between panels well enough for a dwelling, but this seems reasonable for a carport, where the main function (besides the solar installation itself) is to shade two compact vehicles.

I'll add some sort of simple shutoff (perhaps fused) on a box on one of the piers, and run the 10awg underground (UF-B at 24" or THHN in conduit at 18") to the nearby utility room where the house's main load center lives. This should satisfy the rapid-shutdown requirement (we are subject to 2017 NEC).

If I were building s straight grid-tie system, I'd tie the 240vac from the micro-inverters straight into the feed from my electrical meter to the main breaker in my load center and call it good. So this is where I add facility for generator backup and the hooks for a future hybrid inverter and batteries. I'll add a critical-loads sub-panel, as that is an essential element in diagrams of AC coupling I've seen. The PV will feed directly into this subpanel, as will the generator, and this will connect to a breaker in the main panel; I'm still unclear as to how these connections and interlocks are configured though. For hybrid upgrade, I'd simply (?) splice the hybrid inverter into the path between main panel and subpanel.

One final thing: in lieu of a generator, I will probably use the 40kwh battery in our Nissan Leaf, connected to a 1kw/120vac inverter, to handle grid outages for now (the DC-to-DC converter from the traction battery to the 12v system can easily handle this load). I'll connect the 120vac to both poles of the subpanel (interlocked, of course). This should power our refrigerator, as well as a couple of circuits for lighting (LED) and the cable modem and VOIP phone, for a couple of days. I'll probably have only two 240vac loads on the subpanel: a minisplit and a dual-fuel range. With the 120vac inverter driving both poles, these 240vac appliances will see 0vac, except hopefully the 120vac circuits in the range (in has a neutral connection) will function and allow gas flow to the burners.

[ButchDeal]

The SolarEdge system is the only system that is itself upgradeable to bimodal (backup).

You are mistaken in thinking that the StorEdge system uses the powerwall. It does not. The powerwall has a built in inverter and would AC couple to all inverter systems.
The StorEdge system uses currently the LG RESU10H battery system but SolarEdge has announced that they are coming out with their own battery system as well.

Other systems are "upgradable" to back up only in the since that you can add a more expensive bimodal inverter and battery.

For a bimodal AC coupled, most of them have multiple AC connections. OutBack Radian has 3 AC connections.
You would tie one to the MSP (Main service panel)
One to the emergency panel
one to the grid tie inverters (micros in your case).

If there is a generator it would be on the MSP or MSP AC tap to bimodal inverter.

Tying a 120V inverter to both legs on a 240V sub is a bad idea ..

[RShackleford]

You are mistaken in thinking that the StorEdge system uses the powerwall. It does not. The powerwall has a built in inverter and would AC couple to all inverter systems.
The StorEdge system uses currently the LG RESU10H battery system but SolarEdge has announced that they are coming out with their own battery system as well.

Other systems are "upgradable" to back up only in the since that you can add a more expensive bimodal inverter and battery.

Thanks for the clarification; but I believe it's still true that StorEdge requires the use of expensive battery systems. What I had in mind was the ability for future addition of a hybrid inverter and a relatively cheap battery such as a few golf cart batteries. During grid outages, I'd be happy to have most of my power available when the sun is shining, and rely on the battery just for surge loads (well pump startup) and minimal overnight loads (refrigerator, a lighting circuit or two).

I guess I could summarize my perhaps over-long OP as:
1. I want to install a grid-tie system, with capacity for future addition or multi-mode capability.
2. Due to shading issues I require very good MPPT, which means either SolarEdge of micro-inverters.
3. SolarEdge requires expensive battery systems, so I want to go with micro-inverters.
4. So I have to AC couple my backup system.

Tying a 120V inverter to both legs on a 240V sub is a bad idea.

Not sure why. Yeah, at first blush it sounds a bit sketchy, but 240vac loads are the only issue, I believe. For ones without neutrals, they'll just see 0vac. Ones with neutral presumably have internal circuits powered off 120vac, and these should work fine (good for a dual-fuel range that needs power to let the gas flow). MWBCs could be a real issue, because of currents from both legs flowing through neutral instead of differentially, but I have none of those. Similarly, one could imagine that the 240vac appliance with a neutral has 120vac internals running off both legs, and the combined neutral current from those overloads the neutral conductor; but that seems exceedingly unlikely.

In any event, I'm only going to do this with a 1kw inverter running of a Leaf's 40kwh battery via its DC-to-DC converter. So I'll have to keep power consumption very low. Thus, of the 240vac loads that I put on my critical-loads subpanel (probably dual-fuel range, minisplit in study, well pump), they'll all be turned off when in grid-outage mode, except the range. And if we try to turn on the oven, nothing will happen; the 120vac circuits, just the control logic including the gas valve, can't possibly overload the 8awg neutral conductor.

But yes, I'd agree that as a general rule, in a home that might have MWBCs and/or inhabited by a tech-naive person, it's a bad idea.

[ButchDeal]

Thanks for the clarification; but I believe it's still true that StorEdge requires the use of expensive battery systems. What I had in mind was the ability for future addition of a hybrid inverter and a relatively cheap battery such as a few golf cart batteries. During grid outages, I'd be happy to have most of my power available when the sun is shining, and rely on the battery just for surge loads (well pump startup) and minimal overnight loads (refrigerator, a lighting circuit or two).

The StorEdge does NOT require a battery. It can be added latter but further the StorEdge interface is coming to the regular inverters, allowing much simpler future expansion to bimodal.

I guess I could summarize my perhaps over-long OP as:
1. I want to install a grid-tie system, with capacity for future addition or multi-mode capability.
2. Due to shading issues I require very good MPPT, which means either SolarEdge of micro-inverters.
3. SolarEdge requires expensive battery systems, so I want to go with micro-inverters.
4. So I have to AC couple my backup system.

AC coupling method will be more expensive as the inverters that can handle this are costly, and cheap batteries have shorter lives (meaning more costly in the long run).
An AC coupled system requires the bimodal inverter and battery to be large enough to handle the full charge from the grid tie inverter system as well.

Not sure why. Yeah, at first blush it sounds a bit sketchy, but 240vac loads are the only issue, I believe. For ones without neutrals, they'll just see 0vac. Ones with neutral presumably have internal circuits powered off 120vac, and these should work fine (good for a dual-fuel range that needs power to let the gas flow). MWBCs could be a real issue, because of currents from both legs flowing through neutral instead of differentially, but I have none of those. Similarly, one could imagine that the 240vac appliance with a neutral has 120vac internals running off both legs, and the combined neutral current from those overloads the neutral conductor; but that seems exceedingly unlikely.

In any event, I'm only going to do this with a 1kw inverter running of a Leaf's 40kwh battery via its DC-to-DC converter. So I'll have to keep power consumption very low. Thus, of the 240vac loads that I put on my critical-loads subpanel (probably dual-fuel range, minisplit in study, well pump), they'll all be turned off when in grid-outage mode, except the range. And if we try to turn on the oven, nothing will happen; the 120vac circuits, just the control logic including the gas valve, can't possibly overload the 8awg neutral conductor.

But yes, I'd agree that as a general rule, in a home that might have MWBCs and/or inhabited by a tech-naive person, it's a bad idea.

You need to consider the switch over and functionality of the micros connected to the AC coupled system as well. Micros are 240V like most grid tie systems.
This system will not pass code either.

[RShackleford]

The StorEdge does NOT require a battery. It can be added latter ...

No foolin' - meaning one could take wattage from the solar/optimizer array when the grid is down ? Can you give me a link for this ? Or do you just mean that it wouldn't require a battery to function as a grid-tie inverter in my preliminary system, only requiring one when I wish to add grid-down functionality ?

AC coupling method will be more expensive as the inverters that can handle this are costly, and cheap batteries have shorter lives (meaning more costly in the long run).
An AC coupled system requires the bimodal inverter and battery to be large enough to handle the full charge from the grid tie inverter system as well.

So those expensive hybrid inverter/chargers will tend to offset the more expensive batteries I might want to use with StorEdge.

You need to consider the switch over and functionality of the micros connected to the AC coupled system as well. Micros are 240V like most grid tie systems.
This system will not pass code either.

The 120vac inverter would probably be a short-term solution until/if I implement the hybrid system; so I wouldn't be trying to use it at the same time. As far as code, nothing would be hard-wired, I'd just install a standard 240vac generator inlet jack, and the tying together of the two hots would occur in the temporary cord for connecting the 120vac inverter to the inlet. But maybe it'd be better to just run a couple of extension cords (two GFCI jacks on the inverter), one for the fridge, one for some lights, modem, etc.

[ButchDeal]

No foolin' - meaning one could take wattage from the solar/optimizer array when the grid is down ? Can you give me a link for this ? Or do you just mean that it wouldn't require a battery to function as a grid-tie inverter in my preliminary system, only requiring one when I wish to add grid-down functionality ?

the latter, no battery no backup but still works as grid tie.

[RShackleford]

the latter, no battery no backup but still works as grid tie.

Ah, what I figured. Those big Li-ion batteries are pretty pricey, but I imagine their prices will be dropping.