Load Balancing for Rooftop Solar

[Skotty]

The deeper I get into investigating rooftop solar, the more confusing some of it seems to get. For this thread, I want to focus on how to ensure I can integrate a load balancing battery system like the PowerWall into a rooftop solar installation.

The solar installer I am currently talking to can install Sonnen battery systems and claims they can retrofit battery systems at a later date (desirable to me to spread out the cost a little over time), but claims and realities do not always coincide.

What do I need to do to ensure the system I have installed can be used in combination with a load balancing battery system? Will that differ depending on whose battery I use (PowerWall vs Sonnen)? And who is the right person to even ask? If I want a PowerWall, do I need to ask Tesla about this?

Thanks for any thoughts.

 

[nwdiver]

I would ask explicitly what they mean by 'retrofit'... any battery system can be AC coupled into an existing PV system but that's probably not your interpretation of retrofit...

In terms of Sonnen vs Tesla... Sonnen appears to use Lithium Iron instead of the Lithium Cobalt used by Tesla. IIRC LiFe is a more robust chemistry at the cost of lower energy density...

 

[Skotty]

I would ask explicitly what they mean by 'retrofit'... any battery system can be AC coupled into an existing PV system but that's probably not your interpretation of retrofit...

In terms of Sonnen vs Tesla... Sonnen appears to use Lithium Iron instead of the Lithium Cobalt used by Tesla. IIRC LiFe is a more robust chemistry at the cost of lower energy density...

By retrofit, I just mean install basic rooftop solar without battery, then at a later date, add a load balancing battery to the existing system. What would be another way to interpret it?

 

[nwdiver]

By retrofit, I just mean install basic rooftop solar without battery, then at a later date, add a load balancing battery to the existing system. What would be another way to interpret it?

Basically is the inverter designed to work directly with a battery...

I can 'retrofit' any PV system with a battery... simply by adding another inverter and making it 'AC coupled'. OR by adding a charge controller.

As opposed to the new Fronius and Solaredge systems that you simply plug a battery into...

 

[Skotty]

Basically is the inverter designed to work directly with a battery...

I can 'retrofit' any PV system with a battery... simply by adding another inverter and making it 'AC coupled'. OR by adding a charge controller.

As opposed to the new Fronius and Solaredge systems that you simply plug a battery into...

Here's what one of their techs said in a follow up email:

We are a Sonnenbatterie exclusive partner and have installed several of their units. We are currently installing them as critical loads battery backups because of the fact that as you mentioned load balancing is not an economic necessity with the existence of net metering. With that being said, the second part of your question was can they be installed at a later date and the answer to that is yes. The norm in battery installations with solar is a technology called AC coupling. This allows you to install batteries to nearly any AC output from any generation means. In theory one could AC couple a battery backup without solar and just use the grid provided power. The AC coupling allows you to add batteries to any renewable energy technology at any time.​

 

[nwdiver]

Here's what one of their techs said in a follow up email:

We are a Sonnenbatterie exclusive partner and have installed several of their units. We are currently installing them as critical loads battery backups because of the fact that as you mentioned load balancing is not an economic necessity with the existence of net metering. With that being said, the second part of your question was can they be installed at a later date and the answer to that is yes. The norm in battery installations with solar is a technology called AC coupling. This allows you to install batteries to nearly any AC output from any generation means. In theory one could AC couple a battery backup without solar and just use the grid provided power. The AC coupling allows you to add batteries to any renewable energy technology at any time.​

LOL... yeah... it's a stretch to call that retrofitting...

If storage is important to you I would wait a few months until the new Fronius inverters come out or go with Solaredge... that would save you the cost of a second inverter...

 

[Skotty]

LOL... yeah... it's a stretch to call that retrofitting...

If storage is important to you I would wait a few months until the new Fronius inverters come out or go with Solaredge... that would save you the cost of a second inverter...

So with the hybrid, would that be DC coupling? And does that mean the AC coupled approach has to go through a DC-->AC-->DC conversion to get into the battery, but the DC coupled can go straight from the panels to the battery? Or maybe with the hybrid there a DC-->DC conversion. What would the efficiency difference be?

 

[nwdiver]

So with the hybrid, would that be DC coupling? And does that mean the AC coupled approach has to go through a DC-->AC-->DC conversion to get into the battery, but the DC coupled can go straight from the panels to the battery? Or maybe with the hybrid there a DC-->DC conversion. What would the efficiency difference be?

Yes... with AC coupled to charge your battery it's DC => AC => DC... each step is ~97% efficient for ~95% efficiency.

With the hybrid system from Fronius and Solaredge the battery is charged DC-DC with a ~98% efficiency since the step change is very small... ~350v => ~350v the only real variation would be to control the SOC of the battery. But the real savings would be the initial capital... one inverter is generally going to be cheaper than two.

 

[strider]

You can always add a separate micro grid inverter/charge controller later (the AC coupling nwdiver described above). IMO the slight reduction in efficiency will be offset by being able to buy MUCH cheaper individual units (buy a regular grid-tie inverter now, charge controller with battery system later). Here's what I'm planning to do...

Sunny Boy grid tie inverter into a 400A panel (I have 200A service - I'm upsizing the panel to get the beefier bus bar to handle the solar input - have not done the math on this but making an assumption at this point) with my critical needs (basically all my 120V circuits) into another panel (hopefully it will actually be my existing service panel but it may need to go up to 400A too - depends on load calcs and how much solar I end up installing) that will hold my 240V circuits (car outlets, HVAC, stove, and ovens). That panel will be connected to the meter.

Then when battery prices drop (I'm thinking it's going to be 5 years or so to get the Gigafactory cranked up and prices to really drop) I'll simply break the connection between the two panels and install a micro grid inverter/charge controller. When the grid goes down the micro grid inverter will supply 60Hz to the critical loads panel from the batteries. This will cause the solar inverter to think the grid is still there and it will happily generate power to run the house and charge the batteries. If the batteries are fully charged the micro grid inverter will alter the frequency slightly to cause the solar inverter to shut off. Once the batteries drain down it'll adjust the frequency back and the solar inverter will come back on and charge the batteries.

So unless you need the backup now because your grid is unreliable (due to storms or whatever) I would wait on batteries but go ahead and get your panels and circuits laid out now during your PV install so you can easily add batteries later.

 

[nwdiver]

IMO the slight reduction in efficiency will be offset by being able to buy MUCH cheaper individual units (buy a regular grid-tie inverter now, charge controller with battery system later).

It was certainly true previously that two separate units were ofter cheaper... and actually more efficient as well but the new generation of inverters combine all functions for about the same cost. The price of the new Fronius Primo hybrids hasn't been released yet but they're expected to be about the same as the current grid-tie line...