Comparison of PV on AC or DC

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1

Hello everyone,

As part of renovating my house, I’m also planning to install a PV system and a battery, and I’m trying to familiarise myself with the subject. However, I’m currently stuck on a fundamental question.

What are the advantages and disadvantages of connecting the PV system on the AC or DC side?

Inverter on the AC side:

  • Usually allows a higher PV string voltage
  • Presumably lower losses when used directly, as conversion is only required once
  • Smaller cable cross-sections for the PV due to the higher voltage
  • Can still be used if the “battery system/DC side” fails
  • When storing energy and consuming it later: multiple conversion losses. PV DC → AC → DC → battery → DC → AC → consumer
  • An AC grid is required for a “cold start”

Inverter on the DC side:

  • Direct, straightforward conversion for storage in the battery
  • Victron devices and better integration
  • A “cold start” is also possible without an AC grid
  • Lower PV string voltage
  • Larger cable cross-sections due to the lower PV voltage

At the moment, for example, a Fronius Symo followed by an MP2 and a battery seems quite plausible to me. However, most people seem to connect the PV system on the DC side via SmartSolar MPPT. Why is that? What am I missing, or where is my mistake in reasoning?

Best regards, Sascha

2

That also depends on your household consumption… and on the size of the solar installation… and the number of MP IIs

in most cases, it makes sense to split the modules… e.g. 50% via MPPT and 50% via AC inverter…

3

If you have high consumption during the day, or are able to specifically control large loads during the day according to the available PV output, and only have a low base load in the evening/night, then AC PV is better.

If the large loads tend to run in the evening/night, when there is no sunlight anyway, and during the day the battery is only charged (and surplus power is not to be fed into the grid), then DC PV would be better.

But as Holger has already written, in most cases a combination makes sense.

4

Clearly, in the end, it all has to fit together. Why does it move in a different direction with higher consumption or more solar, and, above all, in which direction?
So, more solar → ? and higher household consumption → ?

I’m trying to understand the relationships better so that I can better assess how the systems fit together.

5

That is mathematically correct, but with the cable types on offer, it is vastly overstated. A 4mm² cable is more than sufficient. Even with a total length of 50m. That would be 0.225Ω. At a current of 12A, that would result in a power loss of 32W. At an MPP voltage of 120V, 1,440W are transmitted. So roughly 2% loss.

If shading is involved (which is almost always the case), several MPP trackers make sense. For example, three 450W modules in series connected to a SmartSolar 150/35, or two series strings in parallel connected to a 150/45. And, in addition, microinverters feeding into the MultiPlus via AC coupling. This gives you more power available in sunshine than the MultiPlus can deliver.

In the event of a grid failure, the MultiPlus has to ensure that the batteries are not overcharged. To do this, it slightly increases the frequency in that situation in order to throttle back the microinverters (including the Fronius) until they switch off. Since the MultiPlus can only reconnect to the grid, it has to synchronise the frequency to 50Hz. That is, when the grid returns. In the process, the AC inverters feed in at full power. The MultiPlus therefore has to leave enough room in the battery for the associated energy to fit in. So it must not charge the battery completely. The MPPs take care of that.

6

That is not the case when AC PV with a country code (as it actually should be) is connected to ACout.

7

I’d start by working on the roof layout and the string plan, including the cable routes.

That often points you in one direction or another.

8

Great, thanks for your suggestions and pointers.

I’ll get to work on the PV coverage and take it from there.

9

Take a look at the video by Helmut “SchattenPV” on this.

Basically, you’ve listed everything:

  • Direct consumption: AC-PV, e.g. surplus charging of an EV
  • Battery charging: DC-PV (MPPT/RS)
  • Shade: short strings
  • Low cost: long strings

Every string with its own MPPT requires a disconnect switch and surge protection. Parallel strings can be combined, e.g. 2s2p on one MPPT 150/45. But that’s only 4 panels as well.

Fronius is officially supported as AC-PV.
So, depending on the string voltages, the options are:

  • 150/250V Victron SmartMPPT (DC-PV)
  • 450V Victron RS (DC-PV)
  • 800V–1000V Fronius AC-PV
  • Individual panel/microinverter (AC-PV) as a DIY option for special cases

Rule of thumb:

  • Working away from home: 1/3 AC-PV, 2/3 DC-PV
  • Working from home with an EV: 2/3 AC-PV, 1/3 DC-PV
  • Exporting to the grid: lots of AC-PV, DC-PV for overnight demand
  • Black start: at least 60V or 2 panels in series for DC-PV, so that the string voltage is 5V above the battery voltage.