No solar during extended outage?

Yesterday my new solar/battery/generator system was finished being installed and they explained how it works. I have a 23kw array, 48 Enphase IQ8A microinverters, one SPAN panel, 2 Powerwalls, and a Kohler 20kw generator.

The installer said that as it is now, my home will run on solar until the battery dies. Then when the battery does die it totally shuts down the solar and switches over to generator. He said that the generator cannot charge the battery though. So at this point because the battery is dead the solar cannot kick back on even if the power outage is a 3 week outage or 3 month outage, and I am stuck on generator indefinitely because the generator will not charge the battery. Does that sound right / make sense?

The way that I want it to operate during an outage is to have the battery run until there is 5% battery left and then intelligently/automatically kick on the generator, with the generator charging the battery back to 100% before shutting off again, and me never being in situation where I have a $100k solar install in that is totally unusable when I need it most in the middle of an extended power outage. I just can't fathom that we could spend that much money on solar and then have it be totally unusable when we need it most.

What he explained as a work-around is that the only way (as my system is built today) to keep from losing the operation of my solar in an extended outage is to manually walk downstairs and flip the switch to OFF on the Powerwall when it reaches 5%, then next day if/when the sun comes out, turn it back on. That seems silly to me since the whole system is incredibly connected, intelligent, and automated. Is he giving me bad information or do I just need to add some kind of additional equipment to get it to work how I want it to?

Supposedly, the powerwall goes into standby at 10% soc, and looks for solar. You could use the loss of power to trigger a standard generator transfer / start switch.

Span panel may have a gen start feature.

Seems to me that the installer forgot to say that the batteries will recharge when the sun is out. According to your post, the installer says if you let the battery go down to 0% it'll stay off and solar can't bring it back to life. What are you supposed to do then if nothing can bring it back? Is it a throw-away powerwall? If the powerwall disconnects due to extremely low voltage (as determined by the battery BMS), there is always a method to wake up the BMS so that solar can resume charging. But the inverter should shut off way before that and switch over to your generator so you won't have to worry about the BMS at all.

If your equipment doesn't do this now, there should be a setting for it. Look for a setting that stops the battery discharge below a certain voltage or SoC. Every inverter has this, although some of the cheaper ones are a fixed voltage and not programmable.

Your solar + batteries is your main backup. The generator is your secondary backup. In dire situations, you probably don't want your secondary backup to power your main backup. You'd want the secondary backup to power whatever essential devices that you need in the house. That should be your priority and not to charge up your main backup. Besides, you'll lose at least 10-20% efficiency going from generator to batteries. In dire situations, you definitely don't want that.

AaronSSH said:

The way that I want it to operate during an outage is to have the battery run until there is 5% battery left and then intelligently/automatically kick on the generator, with the generator charging the battery back to 100% before shutting off again

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Was this requirement stated in your brief to the installer before they quoted for a system?

What was the operational promise they made to you about how the system would operate?

Surely if you are spending $100k you are going through such details before anything is installed?

Powerwalls should be able to keep some of the PV operational and charge during a grid outage. That's a pretty normal set up and if it doesn't then that can be rectified. There is a limit to how much solar PV a Powerwall can manage in an off-grid state (usually no more than ~ 7 kW per Powerwall), so there may need to be some system redesign. A Powerwall keeps a small reserve capacity which enables it to wake up and test for solar PV availability for a "black start".

But Powerwalls are not capable of being charged by a generator. If you wanted a battery which could be charged by a generator (so that the battery could continue to power the home) then you bought the wrong battery system.

Your installer does not understand the system they have installed, it should do everything that you are hoping it would do.

JWLV said:

What are you supposed to do then if nothing can bring it back? Is it a throw-away powerwall? If the powerwall disconnects due to extremely low voltage (as determined by the battery BMS), there is always a method to wake up the BMS so that solar can resume charging. But the inverter should shut off way before that and switch over to your generator so you won't have to worry about the BMS at all.

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Keep in mind that in an AC coupled system, you need enough battery power to 1) bring the whole AC side back up (with loads shed if needed) 2) wait for the reconnect timer to time out for the inverters and 3) start to charge again. It's an issue with AC coupled systems; if the BMS cuts it off at night it is difficult to restart it even when the sun is out again. Powerwalls should keep a reserve that lets you do this (see above) so as long as that's working . . . .

wattmatters said:

Powerwalls are not capable of being charged by a generator

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Can you elaborate why/how? My Powerwall DOES charge from the grid if there is no solar (in my setup it does at least, some power companies / states don't allow it). So if it can charge from the grid it can charge from a generator, power is power. If wired in the right way with the right transfer switches, the Powerwall wouldn't be able to tell whether its power coming from the grid or coming from a generator and would start charging again.

Quattrohead said:

Your installer does not understand the system they have installed, it should do everything that you are hoping it would do.

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That is really what I am hoping. It was designed by an engineer back at their HQ and he is just the installer.

billvon said:

Keep in mind that in an AC coupled system, you need enough battery power to 1) bring the whole AC side back up (with loads shed if needed) 2) wait for the reconnect timer to time out for the inverters and 3) start to charge again. It's an issue with AC coupled systems; if the BMS cuts it off at night it is difficult to restart it even when the sun is out again. Powerwalls should keep a reserve that lets you do this (see above) so as long as that's working . . . .

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That is exactly how I understand it based on what the installer was saying. Here I thought I was being "smart" by getting the micro-inverters for a variety of reasons, but if the downside is that the solar system can't power itself back up because its AC coupled, well that is a big downside that I was totally not aware of.

JWLV said:

Seems to me that the installer forgot to say that the batteries will recharge when the sun is out.

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I understand that is how DC to Powerwall systems work, but my understanding is that since I have micro-inverters (AC to the Powerall) my situation is or may be different. That part still is not entirely clear to me and what I am trying to better understand.

AaronSSH said:

So if it can charge from the grid it can charge from a generator, power is power.

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No, it's not. The Powerwall operates in different modes when grid is and is not available and the AC output is not the same in each mode.

In off-grid mode the PW will raise the frequency of the AC output, especially if the battery is at a high state of charge. I can't imagine what would happen if a 60Hz generator was connected while the Powerwall is operating at 65Hz.

In normal grid mode the Powerwall operates in parallel with the grid it is connected to. It will attempt to balance loads and supply and can back feed the grid even if momentarily (imagine a large load is switched off). If you try making your generator act as "the grid" then you'll let out the magic smoke when the PW tries to backfeed the generator.

For a grid-tied battery to be charged from a generator it requires a dedicated AC input not linked with the grid input. The Powerwall has no such input.

When there is a grid outage then the Powerwall (Gateway) disconnects the home from the grid and the Powerwall then forms its own microgrid. It uses AC frequency shifting to control the amount of power generated by the AC coupled PV inverters. It's not designed to work with generators in this mode.

wattmatters said:

Was this requirement stated in your brief to the installer before they quoted for a system?

What was the operational promise they made to you about how the system would operate?

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Explaining what I wanted was done verbally to the salesperson almost one year ago (long lead time for installs here) so I don't recall details with 100% certainty. I don't know if I just assumed that is how it would work (likely) or if I actually requested that is how it should work. I do have a very thorough written signed 34 page proposal that lists all equipment, specs, expectations, warranties, etc, but nothing in that document addresses this specific detail of operation.

wattmatters said:

No, it's not. The Powerwall operates in different modes when grid is and is not available and the AC output is not the same in each mode.

In off-grid mode the PW will raise the frequency of the AC output, especially if the battery is at a high state of charge. I can't imagine what would happen if a 60Hz generator was connected while the Powerwall is operating at 65Hz.

In normal grid mode the Powerwall operates in parallel with the grid it is connected to. It will attempt to balance loads and supply and can back feed the grid even if momentarily (imagine a large load is switched off). If you try making your generator act as "the grid" then you'll let out the magic smoke when the PW tries to backfeed the generator.

For a grid-tied battery to be charged from a generator it requires a dedicated AC input not linked with the grid input. The Powerwall has no such input.

When there is a grid outage then the Powerwall (Gateway) disconnects the home from the grid and the Powerwall then forms its own microgrid. It uses AC frequency shifting to control the amount of power generated by the AC coupled PV inverters. It's not designed to work with generators in this mode.

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Thank you for that detailed explanation. There is so much more to it than I realized!!

AaronSSH said:

Thank you for that detailed explanation. There is so much more to it than I realized!!

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The Powerwall is a highly functional battery system and can do many things when set up correctly. They are mostly designed to do the thinking for you.

As to operating the Enphase PV (at least some of it) during a grid outage, that is definitely is possible and the installer (or Tesla) should be able to sort that out. Having the Powerwall form a microgrid and controlling operation of the PV inverter(s) during a grid outage is a pretty normal set up. There are some cases where that isn't feasible (e.g. with a multiphase string PV inverter or with inverter capacity well beyond the PW control spec).

But with the generator, then no it cannot operate in parallel with the Powerwall. That will be a case of one or the other. Hopefully you won't ever need it.

In grid outages it is also normal for only selected circuits to be powered, general power and lighting typically. You avoid use of high power draw appliances due to inverter power output limits of the PW (as well as to conserve energy). So large ducted aircon systems, resistive element heaters, electric water heaters, EV charging etc, generally you don't want to be using the battery for those during outages.

AaronSSH said:

but nothing in that document addresses this specific detail of operation.

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That's unfortunate.

wattmatters said:

No, it's not. The Powerwall operates in different modes when grid is and is not available and the AC output is not the same in each mode.

In off-grid mode the PW will raise the frequency of the AC output, especially if the battery is at a high state of charge. I can't imagine what would happen if a 60Hz generator was connected while the Powerwall is operating at 65Hz.

In normal grid mode the Powerwall operates in parallel with the grid it is connected to. It will attempt to balance loads and supply and can back feed the grid even if momentarily (imagine a large load is switched off). If you try making your generator act as "the grid" then you'll let out the magic smoke when the PW tries to backfeed the generator.

For a grid-tied battery to be charged from a generator it requires a dedicated AC input not linked with the grid input. The Powerwall has no such input.

When there is a grid outage then the Powerwall (Gateway) disconnects the home from the grid and the Powerwall then forms its own microgrid. It uses AC frequency shifting to control the amount of power generated by the AC coupled PV inverters. It's not designed to work with generators in this mode.

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Maybe this is a stupid / naive / ridiculous idea, but could some portion of the generator power go into an inverter to produce DC, and that DC could be fed back into the Powerwall as if it was coming from a DC solar array? I guess the question is: can a Powerwall can handle both micro-inverter AC solar input and DC solar input, or does one have to force it into one mode or the other?

AaronSSH said:

Maybe this is a stupid / naive / ridiculous idea, but could some portion of the generator power go into an inverter to produce DC, and that DC could be fed back into the Powerwall as if it was coming from a DC solar array? I guess the question is: can a Powerwall can handle both micro-inverter AC solar input and DC solar input, or does one have to force it into one mode or the other?

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That might work.

However often MPPT DC inputs are proscribed by manufacturer from being connected to anything other than solar panels. You also need a DC power supply (probably power factor corrected to help out the generator) that outputs above the MPPT minimum.

Another thing folks here on this forum do sometimes is connect batteries to a microinverter to boost grid tie exports in a … naughty way let’s say. In this case the reason to do it is to have the microinverter (AC coupling capable of course) take the damage from any potential failure modes from having an un sanctioned input source. Also microinverters have 16-60V ish input and it is much easier to find equipment in that power range.

Whether this is code compliant in your part of the world is an exercise for the reader

You might consider for instance an HMS 2000 or HM1500 for this science fair experiment. They can handle a lot of power in one microinverter. And there is a whole thread here on that one.

There is also a thread on Enphase 215/250 generation but I don’t think that is as good bc those I believe are pre UL1741SA so do not throttle/do high frequency ride through

For sure the HM and HMS, being newer and having to comply with modern grid requirements, are fine on those respects

I just came across this official Tesla PDF which explains that a Powerwall can be charged from a generator.

If the Powerwall system reaches a low state of energy, the generator will automatically turn on - this threshold will be set at the time of commissioning. Once the generator turns on, it will be able to supply power to home loads and charge the Powerwall. The generator will not turn on if solar panels are producing power. The default thresholds are 20% and 40% of the Powerwall's state of energy, how ever these are variable and will result in longer runtime or more frequent generator starts depending on the site.

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So I guess the question is, can I do it this way and use the grid (even if it is a manual grid/off-grid switch)? Or is a Powerwall strictly "programmed" as for grid use or off-grid only?

You’ll probably not find many powerwall specific config experts on this forum, and that’s what you need here — someone that knows how this logic is implemented. Try like r/teslasolar?

It feels mutually exclusive to me. With the inverters more commonly used on this forum there are two separate inputs for grid AC and generator AC. Or, the devices with a single input have readily accessible offline config tools for the user.

All the talk of configuring it at commissioning time implies it is hard to switch between on grid and off grid modes.