Grid Tied Options?

Want ~14-15k, 32 to 40 400W panels, on a building distant, but fed, from the meter. Utility (net zero) said grid tied easily approved but hybrid required $$ meter changes, batteries and transfer switch 'near' the meter, not in a separate building. Noted that only two of the 30 or so nearby solar installs had battery or generator backup.

Growatt was an easy alternative to hybrid but reading other posts have me confused. I thought a grid tied inverter changed solar DC into AC and sent it to the grid. Then I read a comment about CTs managing a Growatt's output - really? I thought CTs were for hybrids and the less capable grid tied just sent max converted AC back to the meter and into the grid.

Second uncertainty is one or two inverters. I hoped to divide panels between two inverters and each would send their AC back to the meter. But another discussion here showed it was more complicated then that. As high Isc panels turn Growatts into single string MPPTs, having a pair would give me 6-8 strings, allowing for growth, rather than just 3 or 4 strings.

Can I get by without CTs, should I be happy with an 11.4 Growatt with 4 strings and lengthen strings as needed? It seemed so easy to change from hybrid to grid tied.

What you want is an on grid inverter that can export to the grid. The exporting is why you need a new meter. If you get on grid with no export, then you don't need a new meter.

The isolation (transfer switch) can be at the building, since islanding would not send power to the meter. You may need a "stop" button near the meter to shut down the system for firefighters.

Multiple inverters can export without talking to each other. If you wire in parallel, then they can easily run when the grid is down.

With that much pv, I suggest the eg4 18kpv, or solark 15k. Wire equal strings in parallel into max out an mppt (each has 3). You may be able to leave one free for expansion. If not, adding another inverter in parallel, or a cheap one as AC connected PV is not hard.

Unless you are doing something fancy, you probably don't need external CT's.

If you're set on Growatt, they've got a new AIO style inverter coming out hopefully soon, that's basically a 10kw inverter, can handle being paralleled and whatnot. SPH 10000TL-HU-US.

How much current/power are you allowed to backfeed into the grid? How much does "120% rule" let you backfeed into your breaker panels?

I will be installing some Sunny Boy 7.7 kW -41 series (recently discontinued), maybe also Sunny Boy Smart Energy (just introduced, not yet on CEC list.) Later they will have 11.4kW model.

7.7kW fits 120% rule for 40A PV breaker in 200A panel with 200A main breaker.
11.4kW for 70A PV breaker and 225A panel with 200A main breaker.

The SBSE will allow an (expensive) battery to be added, storing extra PV to export later. PV panels can be 200% of inverter rating.

Hedges said:

How much current/power are you allowed to backfeed into the grid? How much does "120% rule" let you backfeed into your breaker panels?

I will be installing some Sunny Boy 7.7 kW -41 series (recently discontinued), maybe also Sunny Boy Smart Energy (just introduced, not yet on CEC list.) Later they will have 11.4kW model.

7.7kW fits 120% rule for 40A PV breaker in 200A panel with 200A main breaker.
11.4kW for 70A PV breaker and 225A panel with 200A main breaker.

The SBSE will allow an (expensive) battery to be added, storing extra PV to export later. PV panels can be 200% of inverter rating.

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The backfeed amount would most likely depend on your local Utility.
I spoke to mine as part of getting some initial quote information when I first dipped my toe into learning about solar (so I could learn the price difference between DIY and installed for my area). My utility provider won't approve a panel array being connected to its grid that provides more than 80% of your average billed consumption.
Needless to say, I'm not going to be feeding the grid when I get this thing off the ground.

Thank you for the replies. Hopefully to clarify
Hedges - no mention of a grid feed limit until Tariffs where under 50kW get credited against grid input. Net zero on a monthly bill. Next tariff level is greater than 50kW but less than 10MW. How does a 225a panel with a 200a main breaker allow more amps, thought main breaker controlled.

Wish I had bought the now discontinued SBs. SBSE are more expensive and not do not need batteries.

Cessna - Don't have to have Growatt, just not sure of other options. The new 10K you mentioned mentioned batteries and seems more hybrid than grid tied.

DIYrich - Exactly, an inverter that sends AC to the meter and can do the sine wave two step synchronizing with the grid. No transfer switch, grid down then inverter down. Utility requires disconnect within 10ft of the meter and before any home components. When installation is approved the utility swaps to a dual read meter. Inverter doesn't have to do anything except invert all DC from panels.

Watched Will's 18kPV comparison with the 6000XP and agreed the 18kPV was worth the extra money. But Signature says the 18 requires CTs which means trenching a Cat cable 150' and deciding which of two 200a home panels gets the CTs. The the utility pre inspection said grid tied approved pending installation inspection but hybrid needs more done.

Still learning.

You just need a grid tie inverter. You don't need CTs

Growatt 11.4k should handle 15kw- I have 16.8kw on mine right now but want to drop that down to 15.6kw

Get the voltage as high as you can. There are calculators to help you. Midnite Barcelona calculator is a good one.

I have 3 strings of 14 panels in series each, or 42 panels total

Cmy said:

Hedges - no mention of a grid feed limit until Tariffs where under 50kW get credited against grid input. Net zero on a monthly bill. Next tariff level is greater than 50kW but less than 10MW. How does a 225a panel with a 200a main breaker allow more amps, thought main breaker controlled.

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Power from PV inverter goes through backfed PV inverter in breaker panel and main breaker.
120% rule says main breaker (200A) plus PV breaker (40A) = 240A can't be more than 120% of busbar (200A) = 240A.
If you have a 225A busbar, 120% = 270A, so main breaker (200A) plus PV breaker (70A) is allowed, 270A total.

If you downsize main breaker to 150A, then up to 90A PV breaker allowed for 240A total, with 200A busbar.

There are ways around that, "line side tap" or "load side tap" where PV doesn't backfeed same breaker panel, has its own. That's what I'm doing.

DIYrich said:

What you want is an on grid inverter that can export to the grid. The exporting is why you need a new meter. If you get on grid with no export, then you don't need a new meter.

The isolation (transfer switch) can be at the building, since islanding would not send power to the meter. You may need a "stop" button near the meter to shut down the system for firefighters.

Multiple inverters can export without talking to each other. If you wire in parallel, then they can easily run when the grid is down.

With that much pv, I suggest the eg4 18kpv, or solark 15k. Wire equal strings in parallel into max out an mppt (each has 3). You may be able to leave one free for expansion. If not, adding another inverter in parallel, or a cheap one as AC connected PV is not hard.

Unless you are doing something fancy, you probably don't need external CT's.

Click to expand...

Local rules vary for where this lives. Typically it is where the service entrance is or the nearest accessible place. IF you have an auto-transfer switch a RSD button can be tied to it.

Take a look at this link pages 32-37 --- it has connections for nearly every scenario, just sub in your inverter setup in place of the EG4.

Cmy said:

Watched Will's 18kPV comparison with the 6000XP and agreed the 18kPV was worth the extra money. But Signature says the 18 requires CTs which means trenching a Cat cable 150' and deciding which of two 200a home panels gets the CTs. The the utility pre inspection said grid tied approved pending installation inspection but hybrid needs more done.

Still learning.

Click to expand...

if you have load between the inverter and the meter, AND you want to limit export to the meter, THEN you need CT's to zero out power at the meter. If you can export all you can produce, the CT's do not tell you anything actionable.

Next door neighbor has what I want, he claims electric bills have only the meter charge (now $27) for the last six years and shows zero grid consumption. He doesn't remember panel watts but looks like four strings of 12 panels. His inverters are two Sunny Boy SMA 6.0s, I want today's equivalent. Panels are cheap enough these days, only penalty for too many is higher mounting cost. Maybe a plug in hybrid in the future.

Strange electrical world we have when dropping to a 150a main can increase a breaker's size. Main Breaker - is that the first panel from the meter? Originally, the meter fed a smaller disconnect, then 4 AWG ALU THWN2 to the shop's 100 amp panel. We added a mother in law addition with its own 200a panel, meter now feeds a 200a disconnect and larger wire 50' to the new panel. The shortened old shop line now connects to a pair of 100a breakers in the new 200a panel.

It appears current code lowers 4 awg to 95 anps. I have bought 2/0-2/0-1 direct burial (245a/180a conduit) and planned on a new, larger shop sub panel. Initially wondered about the existing 4awg carrying the 10k's 42a AC out or the 11.4k's 48a AC out. But the 1987 Crouse-Hinds panel is ugly and all 16 slots filled. What amp panel/main combination works best given the initial 200a path and new 2/0 wire to the shop? Shop has only intermittant loads - 240v20a bifold door, compressor, saws, etc. Inverter will have the highest amperage.

I used this as service disconnect. Then branched to multiple sub panels with Polaris.


100A fused disconnect to be backfed with PV, 100A panel for protected loads, 200A panel for large house loads.

Downstream of the disconnect I may use a 225A busbar panel (fed by the 100A fuses) just so 225A x 1.2 = 270A, 270A - 100A = 170A, can put 40A + 40A + 30A + 30A breakers in it for 2x PV inverters and 2x AC coupled battery inverters. Haven't bought that panel yet, may sharpen my pencil and reduce some breaker sizes. The panel would also feed garage tools.

I'm using all QO panels. Copper bus, and used in light industrial applications as well.

I have the Square D Q22200NRB next to and fed from the meter. Lots of confusion as the utility specifies external handle, non modular which I thought meant blade type. After several emails they sent their solar guy out to review what I had. He said the 222 met code and I just needed a second disconnect where the underground line surfaces and enters the shop. Not sure why both ends need a disconnect but it enables a grid tied connection without any meter wiring changes.



OK what is protecting what? If a 225a panel is fed by a pair of 100 amp fuses, won't the 100a break before getting close 225a? Or is this just a code game where 225a panel is needed to allow the 30 and 40 breaker pairs?

My breaker box is service disconnect and OCP.
I have a visible blade disconnect (with external handle) for PV disconnect so PG&E can shut off any possible backfeed, without yanking my meter. Used to be required, now optional, but ensures I won't get shut off. It will also trigger RSD, and I'll make that happen even if I put in battery backup.

If I overload a breaker panel with 240A or more of branch circuits, and it is fed by 100A fused from grid and backfed by 2x 40A + 2x 30A inverter breakers, there is 240A of OCP feeding it (OK, looks like 200A busbar fits 120% rule.)

Not actually protecting anything because busbar is fed from opposite ends, won't get overloaded. I read the NEC committee expressed concern that if breakers got moved to wrong end, then the 240A would be going through a 200A busbar.

But actually, I do know how to overload something this way. Put 240A of single phase loads all on one phase of the panel. The L1 or L2 busbars could handle that, but the only neutral connection is the 2 awg wire going back to disconnect. Utility transformer is where neutral current comes from, when inverters drive 240V into L1/L2.

The 120% rule only protects you from overloading more than 20% if neutral wire back to utility is rated as high as busbar.

Caveat Engineer/DIY Guy.

I just got my 200A service switched over. Since I'm not actually adding loads, PG&E didn't upgrade anything. I put in 3/0 copper wires out the riser, and PG&E crimped that to their 2 awg aluminum. The guy said it is good up to about 150A (in the air, of course.)

Previously robbob suggested another look at the 18kPV manual wiring. It still shows CTs used in all five diagrams. Signature has said the same.
It was a good idea so I pulled up the S-A 15 manual and in section 4:

Grid-Tie and No Battery Install Tips (Passthrough mode)
1. Check the “🗹 No Battery” setting: → Battery Setup → Batt → No Battery . The inverter will fault momentarily.
2. A complete Power Cycle IS REQUIRED when changing the battery mode to “No Battery” (see section 2.12 “Power cycle
Sequence” for detailed instructions).
3. Enable “🗹 Grid Sell”: → Limiter → Grid Sell. Make sure to disable all other modes.
4. Tap the battery Icon to access the “Details Screen” and verify grid parameters and power import / export.

Need to confirm with S-A that this will sent all AC out and not need the CTs.

So a Growatt 11.4 for $1.5k or a S-A 15 for $7k?

Cmy said:

So a Growatt 11.4 for $1.5k or a S-A 15 for $7k?

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Is this a serious question?

Almost. Will raves about S-A 15 and 18kPV quality and 10 year warranty enough so that others seem marginal. Little is lost trying Growatt and a lot saved. With utility saying grid tied OK, not sure they would accept a hybrid acting as grid tied. Just haven't seen as much on Growtt compared with the other two.

Hybrid is on average less stress on the grid so from an engineering and no politics/economic angle it is good for the grid.

The main issue with hybrid is gaming of 1:1 net metering rules so it is subject to heavier scrutiny during interconnection here in California.

Agree but I pretend I am a good neighbor and not take grid watts from them .

Rewiring the house for a single feed (disconnect and CTs), before feeding the two house panels. Changing the meter's shop line to a house panel was included in the initial $7K estimate and with more questions left.

So grid tied saves that and the hybrid inverter cost. Actually I wanted hybrid just in case battery backup becomes worth the expense. A GT inverter saves enough that a future hybrid shouldn't be too painful.