Confused About AC vs. DC Coupling

mciholas said:

I don't think that is right.

Let's analyze PV directly to grid:

A string inverter going from PV input to AC output is way better than 87-92% efficient. EG4 18KPV (just as an example) is 97.5%. That is a DC coupled system. This makes sense because whatever circuitry you can put into a microinverter you can scale up and put into a string inverter ad would have similar performance. The awful 87-92% number is just wrong for PV to AC for a string inverter.

As for AC couple, that implies microinverters on the panels. Microinverters do achieve about 97% efficient (Enphase IQ7 number for reference) which is similar to a string inverter. If, as is usual, the inverter power is less than the panel power, clipping, you do lose that energy as well.

Net result is that PV to grid, microinverters do not do better than a string inverter, at best, they are about the same, and with clipping, they can be worse.

Now let's analyze going through battery storage.

For the string inverter, the PV input comes in as DC and charges the DC battery. There is a voltage conversion, but it is DC to DC directly and can be quite efficient, 18KPV is 95% doing that. Then to brig the power out of the battery, the string inverter goes DC to AC just once. The 18KPV is 94% efficient doing that. So the round trip efficiency PV to battery to grid is 89%.

For the microinverter system, the PV panels make AC first, then the AC gets converted back to DC to charge the batteries and then makes AC again to power the output. Enphase IQ Battery 10 lists a round trip efficiency of 89% (only at half load, so maybe cheating a bit?). Total including the microinverter loss (but discounting any clipping) is 86%.

Net result is that the string inverter wins on delivering energy through a battery since it has only one DC to AC conversion.

Overall, string inverters are as good on PV to grid, and they win on PV to battery to grid versus microinverters.

Mike C.

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No, the AIO inverters with HV DC bus have more conversions than you think.

For PV directly to AC: PV goes DC to DC from PV to HV DC bus, then DC to AC. Still comparable to a "string inverter" type grid tie or micros.

For PV to batteries: you have DC to DC PV to HV DC bus, then DC to DC HV bus to battery.... which is lower efficiency than a straight up charge controller.

For battery to AC: you hav DC to DC from battery to HV DC bus, then DC to AC from HV DC bus to AC.

As long as probably 50% or more goes directly from PV to AC, you are further ahead that way. Once you are under 50% direct to AC, you are way more efficient using charge controller and "low frequency" grid sell capable inverter such as Schneider XW or even Midnite Rosie (I know, that one is technically "high frequency.")

By the way on an AC coupled system, you have an inverter like the Schneider XW, or something similar, with either a "string inverter" type grid tie inverter, or micros, on the critical loads side "AC coupling" to the output of the battery inverter, or grid when the battery inverter is connected to grid.

An 18kPv is not just a string inverter! It is a hybrid string inverter with battery option. Normally what is referred to as a "string inverter" is simply an grid tie inverter that gets a "string" of solar panels connected to it, whereas a micro inverter only gets connected to anywhere from 1 to 4 solar panels. With a string inverter you have 1 (or sometimes a few, on bigger systems) inverter that handles all the solar panels. A micro inverter will have 1 (or sometime more, up to 4) solar panels plugged directly into it.... benefits are less issues with partial shading. Drawbacks far outweigh benefits.... but that's a whole other can of worms. More components to go bad, more electrical connections to go bad, more heat for mocros because of being mounted on or under the panels, etc....

Anyway... at the end of the day, when using batteries (actually cycling them, that is.....) AC coupled or HV AIO inverters are usually both not as good an option as good old charge controller and inverter....

Cmiller said:

Still comparable to a "string inverter" type grid tie or micros.

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My 3-phase Fronius string inverter has an efficiency rating of 98%.

Cmiller said:

For PV directly to AC: PV goes DC to DC from PV to HV DC bus

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MPPT efficiencies are very high because the voltage difference is low if your strings are properly configured. This is why it is best to build strings with Vmp of about 350 volts, the PV to DC HV bus conversion is then very low loss. EG4 18KPV lists MPPT efficiency as 99.9%, effectively no power loss.

As to microinverters, they also have to make a HV DC bus, but they start with a single low panel voltage. Much harder to take 35 volts and make it 350 volts than to start with 350 volts and make it 350 volts.

Cmiller said:

then DC to AC.

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Same for string or microinverters. You need a HV bus for either.

Cmiller said:

For PV to batteries: you have DC to DC PV to HV DC bus, then DC to DC HV bus to battery....

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That's a design choice, you can go PV to battery charging directly if you design it that way, basically MPPT + charger in one.

But in the end, it is simpler to go PV to DC HV since the string is already HV and the MPPT is really no power loss, then optimize the battery charger from the HV bus to battery voltage.

The microinverter has to go from DC LV to DC HV, then DC HV to AC, then AC to DC battery voltage.

Cmiller said:

For battery to AC: you hav DC to DC from battery to HV DC bus, then DC to AC from HV DC bus to AC.

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This is the same for all LV battery systems using HF inverters (that is, not a big honking transformer at the end). All HF inverters need ~350 volts to make 240 VAC.

To review:

PV to AC is easier with a string due to higher PV voltage to start with. Microinverters have to boost low panel voltage to DC HV first, the make AC.

PV to battery in a string inverter goes through one HV to LV conversion, the battery charger. PV to battery in a microinverter setup goes through PV LV to HV, HV to AC, AC to LV battery.

Battery to AC in any HF inverter setup is the same for both systems, LV battery to HV bus, to AC output.

Clipping seems to be much more prevalent in microinverter setups than string setups. Clipping is a fancy word for "lost power".

Cmiller said:

Anyway... at the end of the day, when using batteries (actually cycling them, that is.....) AC coupled or HV AIO inverters are usually both not as good an option as good old charge controller and inverter....

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I don't see why that would be if your strings are not setup to be way off the 350 volt operating point.

Mike C.

wattmatters said:

My 3-phase Fronius string inverter has an efficiency rating of 98%.

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Yes, I think most straight grid tie inverters will tend to be a little bit more efficient than a Hybrid AIO one. The reason I said it is still comparable is because the general idea is pretty close to the same. The grid tie units are basically PV to HV DC bus (MPPT), then HV DC bus to AC. The hybrids are DC to DC (MPPT included in this) PV to HV DC bus, then HV DC bus to AC.

mciholas said:

That's a design choice, you can go PV to battery charging directly if you design it that way, basically MPPT + charger in one.

But in the end, it is simpler to go PV to DC HV since the string is already HV and the MPPT is really no power loss,

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The rated efficiency will normally be the peak efficiency. I seriously doubt 99.9% is a valid number for a normal install!

mciholas said:

then optimize the battery charger from the HV bus to battery voltage.

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From my personal experience, I don't think this end of it tends to be very efficient on most of the AIO inverters!

mciholas said:

To review:

PV to AC is easier with a string due to higher PV voltage to start with. Microinverters have to boost low panel voltage to DC HV first, the make AC.

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I agree, this makes total sense.

mciholas said:

Cmiller said:

Anyway... at the end of the day, when using batteries (actually cycling them, that is.....) AC coupled or HV AIO inverters are usually both not as good an option as good old charge controller and inverter....

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I don't see why that would be if your strings are not setup to be way off the 350 volt operating point.

Mike C.

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It doesn't have to do with how you string the panels. It has everything to do with the whole internal process in the inverter between PV and batteries, and batteries and AC. I can assure you that in off-grid installs where a large portion of the PV power goes to the batteries, then comes back out, the efficiency is pretty bad on the Sol-Ark inverters specifically, and I have no reason to think other brands such as Eg4 are any different!

Here is an example of an on grid system that is set in a limited to home mode with the batteries discharging to avoid buying power from the power company.

This screenshot is for the month of March of this year.

So we have a total load of 1012.6kWh that was supplied by a combination of 999.9kWh PV and 139.2kWh bought from power company. The import went straight to AC loads because of how the system is configured (no battery charging from grid), so that gives us 1012.6- 139.2= 873.4kWh supplied from PV alone.

873.4 load/ 999.9 PV = a net efficiency of 87.3%

I have seen even worse than this! The more that goes straight to AC loads, the better the efficiency, but the efficiency just plain is not great when cycling the batteries on these AIO systems!

A Tesla Powerwall 2's RTE is 89%. That's about as much as you can expect from an AC coupled battery.

Cmiller said:

873.4 load/ 999.9 PV = a net efficiency of 87.3%

I have seen even worse than this! The more that goes straight to AC loads, the better the efficiency, but the efficiency just plain is not great when cycling the batteries on these AIO systems!

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AC coupled systems don't fix this, they make it worse since they have to make AC first to charge the battery.

The best system would be a string inverter with a high voltage battery. 360 volts strings, 360 volt batteries, 360 volt AC inverter. The power conversions would be highly efficient due to voltage parity among the sources.

Mike C.

mciholas said:

AC coupled systems don't fix this, they make it worse since they have to make AC first to charge the battery.

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Yes, that is correct. In my case only about 14 kWhs a day of my 51 kWhs produced by AC coupling is battery charging and the rest serves the AC loads including EV charging, with the balance being exported. My DC coupled solar only provides another 6 or 7 kWhs a day.
In a perfect world I would have installed more DC coupled solar at a higher voltage than the two small strings I added later. Then I would have incrementally added AC coupling in smaller increments without losing efficiency.

mciholas said:

AC coupled systems don't fix this, they make it worse since they have to make AC first to charge the battery.

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I agree. I was simply stating that when a person uses less than 50% or so of their PV directly as it's produced, and is charging/discharging batteries with the rest of the PV, then it is more efficient to stick with a charge controller and inverter charger, rather than either AC coupled OR hybrid AIO with HV DC bus.

I know for a fact that hybrid AIO units with HV DC bus are not much more efficient than AC coupling, when you are cycling batteries.

That was all that I was saying.

Cmiller said:

I know for a fact that hybrid AIO units with HV DC bus are not much more efficient than AC coupling, when you are cycling batteries.

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I wonder how that can be since the full power path is low volts panel DC, boost to high volts DC, then make AC, all in the microinverter. Then, we take the AC, make high volts DC from it, buck it down to battery low volts.

That is a lot of conversions to charge the battery. The string inverter does basically only one, high volts panel string to low volts battery charging.

I suspect you think AC couple isn't much worse due to not being aware of the panel input power being lost in the microinverter first. That is, you are looking at the "input" being the AC from the microinverter, not the panel power itself.

Mike C.

mciholas said:

I wonder how that can be since the full power path is low volts panel DC, boost to high volts DC, then make AC, all in the microinverter. Then, we take the AC, make high volts DC from it, buck it down to battery low volts.

That is a lot of conversions to charge the battery. The string inverter does basically only one, high volts panel string to low volts battery charging.

I suspect you think AC couple isn't much worse due to not being aware of the panel input power being lost in the microinverter first. That is, you are looking at the "input" being the AC from the microinverter, not the panel power itself.

Mike C.

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When I said AC coupling, I was actually referring to a string inverter such as a Fronius. For instance, at home I am AC coupled using a Fronius 7.6. I have 2 strings of (6) 450w panels. This brings a voltage of ~264VDC to the Fronius, where it is then converted to AC and backfed (AC coupled) to my Victron battery based inverters.

I honestly don't have much experience with micro inverters, but I am sure that micros are less efficient than a string inverter (whether it is an AIO or just a grid-tie).

Personally, I'm not a fan of micros. I would use optimizers over micros, although I feel those end up wasting a bit of power as well. Best thing to do is to find an install location with little to no shading and south facing, to avoid partial shading. Then run a higher voltage string for PV DC.