Powerwall2 and Enphase Microinverters

[NOLA_Mike]

Enphase recently published this on their blog:

Tesla’s Solar Roof: Worth the wait?

Based on that, I don't think Enphase is going to be hung ho about supporting Tesla's Powerwall. It looks like they are going to push their own battery powered "solution" instead (which looks suitable to power a tiny house with no heat or air conditioner).

Any experts have an opinion on some of their published concerns? I'm most alarmed about the increased fire potential with solar roof if that turns out to be true.

Mike

 

[SageBrush]

A. I assume that the PowerWall 2.0 has an internal inverter and has a HVDC input for one or more solar strings. I also assume that they will support some kind of DC optimizers, so there would be no advantage to using Enphase inverters at all. In fact, it would likely be a cost penalty and add another layer of integration complication. The generator input is the biggest open question.
B. If your usage figures are correct, a single PowerWall and the solar you have specified should keep you down to a handful of days that you would need to fill in with the "generator". You would definitely need some kind of alert or notification system when the PowerWall reaches some low setpoint where you would want to go connect and turn on the generator. In this case, you would have to turn on the car, turn on the 12V inverter and run the 120V power cord to the generator input.
C. If you want to charge the Prius Prime from this system, you probably want to get an EVSE that can vary the pilot signal or just use one that is always low power, like 240V 12A. EVSEupgrade or OpenEVSE is a good fit for this. You will have to pay attention to weather forecast and the SOC of the PowerWall to decide if you're going to charge the car. Maybe there could be some way automate this so you just plug in the car and the system decides when and how much to charge the car. There will be a lot more days that you won't have surplus energy to charge the car than the days where the house alone falls short and you need to add energy from the generator.
D. When you get the Model 3 you will have to seriously think about how many miles per day you want to recharge from this system and where you will charge it when you are short on energy at home. Of course, you can add more solar and another PowerWall, but that still won't be able to recharge a Model 3 from flat empty when you return from a trip. If you had 2 PowerWalls and 10kW of solar, you would probably never have to connect the generator, but there would be many winter days that you would have to charge the Model 3 elsewhere.

What are my choices in dealing with the situation where the PV is generating energy that is greater than the house/car load and the battery is full ?

 

[miimura]

What are my choices in dealing with the situation where the PV is generating energy that is greater than the house/car load and the battery is full ?

The system should automatically move the solar off the optimum operating point so that it does not over-generate. The only thing you can do is get creative with "dump loads" that do something useful with the surplus energy. Some have suggested things like Bitcoin mining because they had the hardware sitting around and the only cost is electricity.

 

[Ampster]

What are my choices in dealing with the situation where the PV is generating energy that is greater than the house/car load and the battery is full ?

Heat water.

 

[SageBrush]

Heat water.

The current system is through an NG boiler also used for space heating. I'm stuck with it. I have been thinking about putting in a heat pump for space heating/cooling but that will not be this year. There is only so much I can ask of my landlord at one time ;-)

And while an off-grid system may waste electricity this year, an M3 will provide more than enough use/storage to take up any extra generation from 2018 so I'm not concerned with the overall system design so long as I don't damage the system this year.

 

[tga]

What are my choices in dealing with the situation where the PV is generating energy that is greater than the house/car load and the battery is full ?

What's the issue?

 

[SageBrush]

What's the issue?

My question was: IS there an issue ?

Perhaps a better question would be, where is the energy dissipated ?

 

[Saghost]

What are my choices in dealing with the situation where the PV is generating energy that is greater than the house/car load and the battery is full ?

I don't know how the Powerwall is programmed, but the most common solution I've seen for this is to start reducing the solar power. One way they do that is by forcing the reference AC out of range - too high or low a frequency causes the micro inverters to shut down.

 

[brucet999]

There are Powerwall 2 specs out there which list both AC inverter and DC inverter round-trip efficiencies (with DC inverter efficiencies being a few points higher) which indicate that the Powerwall 2 will function as a standalone unit capable of charging from the grid.

This also means that the Powerwall 2 will integrate with any solar PV system since the AC Powerwall 2 does not even need to know that the system exists.

Tesla Powerwall 2 is a game changer in home energy storage: 14 kWh w/ inverter for $5,500

Good catch finding the AC input spec.
The AC input capability suggests that a homeowner without solar roof panels might benefit by being able to disconnect from Grid power during peak rate hours, running household loads off the Power Wall, then re-charge the Power Wall at night from the Grid at lowest rates - typically 1/3 of peak rates.

Of course, the amortization of $6500 installed cost over the estimated 10 year service life of Power Wall means whatever value one derives from power Wall must exceed $650 per year.

 

[brucet999]

I don't see how the AC from the enphase micros can charge the powerwall battery unless another invertor is used.

Technically, that would be a rectifier, a device that converts AC to DC. Presumably, the Power Wall has both an inverter in the output side and a rectifier for AC current to the input side. The difference in charging efficiency between DC input and AC input would come from losses in the rectifier.

 

[Ampster]

I'm don't see how the AC from the enphase micros can charge the powerwall battery unless another invertor is used.

I do it with a simple DC power supply (which techncally has a rectifier) that runs off AC and provides DC to charge my batteries. It is on a timer to only charge during super off peak rates. I think Enphase is working on a system to "AC couple" their inverters to high voltage batteries.

 

[simplesolid]

I ordered the PW2, hopefully it will work with my system. I have 44 panels with enphase micro inverters that slightly over 10kw.
Currently selling to power company however I sell to them cheap just so they can sell it back to me at a higher rate.
Hoping that the PW2 will let me save more money by storing my own solar power and just selling any surplus is any.

Sounds like a great product. Considering ordering an additional one.

 

[Ampster]

I ordered the PW2, hopefully it will work with my system

Keep us informed on how it is integrated with your Enphase inverters. Do you already have a critical loads sub panel or are you planning on running your whole house on the Powerwall?

 

[roblab]

My question is whether 12v DC current from the accessory battery of the Prius can be used to fill up the Tesla battery in cases of multiple days of no sun and therefore no PV.

Sunlight panels (aka "solar" panels or "PV") make power when there is light. We are having a cloudy day here, "no sun", but my panels are putting out almost one kW, a little less than ten percent. But that's still power. As to "filling up the Tesla battery", that's a job. Most homes use a fraction of what the battery holds, and solar is sized for home use.

I wouldn't want to try to "fill my battery" with the PV alone as it would take days. And if I had a partially full battery when the power went off, I'd probably not worry about filling it unless an emergency dictated I needed to drive more than a hundred miles. The Prius battery is only about a tenth of the Tesla battery, and very likely the Prius' gas tank would have enough to get anywhere anyway. (Although we sold the Prius, only have the one Tesla.)

 

[SageBrush]

I wouldn't want to try to "fill my battery" with the PV alone as it would take days.

Sorry for the lack of clarity. I meant the PW 2.0 when I wrote Tesla battery.

Your comments about partial sun days are some of the reason why I think one PW 2. 0 will cover my home for 95+% of the year.

 

[miimura]

I ordered the PW2, hopefully it will work with my system. I have 44 panels with enphase micro inverters that slightly over 10kw.
Currently selling to power company however I sell to them cheap just so they can sell it back to me at a higher rate.
Hoping that the PW2 will let me save more money by storing my own solar power and just selling any surplus is any.

Sounds like a great product. Considering ordering an additional one.

As long as the grid is up, you will have no problems and things should work as expected. However, I don't think a single PowerWall, even the 2.0 version will be able to cope with 10kW of independent inverters like the Enphase micros when there is no grid to sink that power into. The general rule of thumb that I've seen is that the battery inverter has to be larger than then connected AC coupled solar inverters. Certainly you have several circuits of connected micro-inverters, so they could be rearranged so that some panels could be islanded with the PowerWall if you cared about solar generation to keep you going longer during a power outage.

 

[SageBrush]

As long as the grid is up, you will have no problems and things should work as expected. However, I don't think a single PowerWall, even the 2.0 version will be able to cope with 10kW of independent inverters like the Enphase micros when there is no grid to sink that power into. The general rule of thumb that I've seen is that the battery inverter has to be larger than then connected AC coupled solar inverters. Certainly you have several circuits of connected micro-inverters, so they could be rearranged so that some panels could be islanded with the PowerWall if you cared about solar generation to keep you going longer during a power outage.

Would the house load reduce the power the PW inverter sees ?

 

[brucet999]

Enphase recently published this on their blog:

Tesla’s Solar Roof: Worth the wait?

Any experts have an opinion on some of their published concerns? I'm most alarmed about the increased fire potential with solar roof if that turns out to be true.

Mike

One doesn't have to be an expert to read the linked article about solar tiles catching fire. The problem seems to have occurred only with one product that was recalled in 2014. Referencing that is clearly a FUD tactic similar to articles sometime back referencing Tesla fires, that ultimately stemmed from external events.

 

[miimura]

Would the house load reduce the power the PW inverter sees ?

Sure, but remember we're talking about off-grid and you need to be sure that the system is stable regardless of variable loads. For example, when you AC couple solar to a Outback Radian, it is connected to a separate input on the Radian and it has an internal relay that can cut off that branch circuit completely when the batteries are full. On the same system, even at the same time, solar connected through solar charge controllers would be programmed to taper off the solar generation so that the batteries would not be over-charged. That is much more elegant than just cutting off the whole circuit of micro-inverters. There are other techniques for managing solar generation too. For example, a SMA Sunny Island (battery inverter) will vary the frequency on its AC mini-grid when the batteries are full and that signals SMA Sunny Boy inverters to reduce their output. If these stimulus-response systems are documented, then Tesla could take advantage of them. However, we would have to see white papers or other documentation to see how Tesla Energy has engineered their products to inter-operate with others.

 

[Nevetsyad]

Nevetsyad: thank you for the explanation. So, if I understand correctly, in case of a power outage (and usual caveats about the sun shinning), the powerwall2 will be able to supply power to, let's say, critical loads (appropriately wired, of course) AND recharge itself as long as the panels are producing power. Am I right?

Correct, if the inverter can handle it. Charging a Tesla at 40 amps and running your AC off of a single PowerWall 2 will not work. If you have 10kW of solar and a clear sky, plus a full PowerWall 2, you can do that for a while most likely.