Micro-inverters a better choice with solar-coupled Powerwalls?

[nwdiver]

I guess that's assuming your utility is your battery

Even if you're off-grid it's still generally going to be cheaper to add generation in the timeframe it's required than add storage to load-shift. That's the cost disparity between solar and storage. I've often found it ironic that it's cheaper to generate electricity than store it.

The discussion earlier on vertically mounted PV to capture more evening generation could be thought of as a type of 'clipping'. You're sacrificing overall kWh for kW generated at a specific time of day when it's needed most. That's what array over-sizing does.

 

[PBBear]

A little closer to the original topic, it seems to me that the issue of clipping would be a selling point in favor of string inverters with optimizers. Correct me if I'm wrong, but with a right-sized string inverter, it seems that clipping won't occur unless all or nearly all of the panels are in full sun. With micro-inverters, there is clipping on individual panels even if half the array is in shade and the other half of the panels are in full sun.

My system has a central string inverter (6kW), fed by 7.4 kWp panels fitted with optimisers (largely because the 22 panels are on 3 different orientations, with half of them subject to some morning shade). I deliberately increased the panels from the original design knowing it would cause clipping in summer - the extra kWh generated from the 'wings' before/after midday easily exceeds the amount clipped during peak, especially since clipping only occurs during summer months. The extra power in the early morning and later evening extends the time window when you are generating more than you are using and increases self-consumption earlier and later. Also, you get full value of the extra panels on cloudy days.

A "right-sized string inverter" should still experience clipping during summer midday fullsun days, or the inverter is overspecced. Maybe deliberately so, if there are plans to add more panels in future.

In terms of the original topic header - "Micro-inverters a better choice with solar-coupled Powerwalls?", the answer I think is no - string inverters are also very good with powerwalls, batteries is not a micro-inverter vs string-inverter issue.
Behaviour under partial shade is a micro- vs string- consideration - As you said above, With some panels under shade, micros will still clip each panel experiencing full sun, while a string-inverter will capture the full panel energy so long as the overall collection of panels isn't clipping the inverter. But this has nothing to do with suitability for batteries or not.

 

[nwdiver]

My system has a central string inverter (6kW), fed by 7.4 kWp panels fitted with optimisers (largely because the 22 panels are on 3 different orientations, with half of them subject to some morning shade). I deliberately increased the panels from the original design knowing it would cause clipping in summer - the extra kWh generated from the 'wings' before/after midday easily exceeds the amount clipped during peak, especially since clipping only occurs during summer months.

Can't wait until this system is finally online so I can see what the production curve looks like. 22.77kW on (2) 7.7kW inverters. 2 Strings facing West and 1 facing East on each inverter.

 

[abasile]

In terms of the original topic header - "Micro-inverters a better choice with solar-coupled Powerwalls?", the answer I think is no - string inverters are also very good with powerwalls, batteries is not a micro-inverter vs string-inverter issue

While I mostly agree, I still think that a case can be made for micro-inverters when the panels may be subject to heavy snow that melts off the panels in a onesy-twosy fashion, our exact situation. And only in the US where Powerwalls must only charge from solar because of the rules for the 30% tax credit. The key here is that heavy snow blocks more light than shade or clouds and thus prevents string inverters from starting up.

Were I to redo our system from scratch, though, I might use string inverters with optimizers for the rooftop. The money saved would then help pay for a couple of vertical panels on our south wall with micro-inverters, enabling the Powerwalls to charge even when the rooftop is buried by feet of snow.

 

[Musterion]

At another family home, however, we have basic, commodity panels with a SolarEdge inverter and power optimizers. The roof on that house is much more accessible and has a gentler pitch, so I was able to go on the roof and clear six out of 14 panels. However, even after a couple of days of sun, we saw no solar production, and the inverter remained "asleep". Today, I went back up and cleared two more panels, or 8/14. Finally, the inverter started up. My solar installer confirmed that if the majority of the panels are not getting significant light, the inverter will not start even if a minority of the panels are in full sun. That would obviously not be ideal if we were trying to keep Powerwalls from going down to 0%.

@abasile
Have you tried lowering the DC start voltage threshold on the SolarEdge to give you some more flexibility in this situation?

 

[abasile]

@abasile
Have you tried lowering the DC start voltage threshold on the SolarEdge to give you some more flexibility in this situation?

Thanks for the suggestion. Do you know if there's a way for the homeowner to accomplish this? I just perused the SolarEdge monitoring API, and I didn't come across any references to the DC start voltage: https://www.solaredge.com/sites/default/files/se_monitoring_api.pdf

By the way, the SolarEdge panel-level and inverter monitoring that's available to the homeowner is in my opinion far better than the monitoring that SunPower offers with their micro-inverter systems. That was a selling point in favor of the SolarEdge inverter with optimizers when we spec'd out the system on the other house. Anyone who cares about monitoring and chooses Tesla as their installer should demand that Tesla agree, in writing, to allow them access to the full SolarEdge monitoring.

 

[NuShrike]

Interestingly, SunPower corporate just called us out of the blue to set up an appointment to upgrade our micro-inverters (to their "3.1" version) and upgrade our monitoring system. Since they're going to swap out the micro-inverters anyway (wow!), I asked if it might be feasible for us to upgrade the rated AC wattage on the inverters and pay the difference. They might not want to do this because it'd change the AC rating on the overall system, but I figured I might as well ask.

Hey, I'm really interested in this. Can you ask are they doing this in general, you're getting special attention, or this is for installations circa WXYZ?

If they're swapping out SolarBridge inverters for Enphase to get everybody in the line to reduce backwards-compatibility support costs, or to support the newer Rule21 changes that can't be supported by firmware updates, I'm all for it.

 

[abasile]

Hey, I'm really interested in this. Can you ask are they doing this in general, you're getting special attention, or this is for installations circa WXYZ?

If they're swapping out SolarBridge inverters for Enphase to get everybody in the line to reduce backwards-compatibility support costs, or to support the newer Rule21 changes that can't be supported by firmware updates, I'm all for it.

Sure. Thus far, I've only spoken with a scheduling person, who promised to have someone more knowledgeable call me within the next several days. I don't see any reason for SunPower to give me special attention. We'll see.

 

[abasile]

In our discussion here, we've been assuming that purchasing a solar system with a string inverter and power optimizers is significantly cheaper than buying micro-inverters. However, that might not necessarily be the case. In evaluating ENPH stock (Enphase) as an investment, I decided to price out solar system kits both ways.

$8981 for a 6.7 kW system kit with micro-inverters:
6.7 kW Grid‑Tied Solar System with Enphase IQ7+ Microinverters and 20x JinkoSolar 335w Panels

$8459 for an equivalent 6.7 kW system kit with optimizers:
[URL='https://www.wholesalesolar.com/1890911/wholesale-solar/complete-systems/6.7-kw-grid-tied-solar-system-with-solaredge-and-20-jinkosolar-335-watt-panels']6.7 kW Grid‑Tied Solar System with SolarEdge and 20 JinkoSolar 335 watt Panels[/URL]

While the kit with optimizers is $500 cheaper, it only comes with a 12 year warranty on the inverter. The micro-inverters, on the other hand, have a 25 year warranty. Considering how close the long term costs are, assuming those kit prices are representative, it seems to me that deciding whether to go with optimizers or micros is probably more just a matter of preference. Micros have the edge in overall system robustness and producing when much of the system is buried by snow, but they have the disadvantage of clipping at the module level.

 

[nwdiver]

In our discussion here, we've been assuming that purchasing a solar system with a string inverter and power optimizers is significantly cheaper than buying micro-inverters. However, that might not necessarily be the case. In evaluating ENPH stock (Enphase) as an investment, I decided to price out solar system kits both ways.

$8981 for a 6.7 kW system kit with micro-inverters:
6.7 kW Grid‑Tied Solar System with Enphase IQ7+ Microinverters and 20x JinkoSolar 335w Panels

$8459 for an equivalent 6.7 kW system kit with optimizers:
6.7 kW Grid‑Tied Solar System with SolarEdge and 20 JinkoSolar 335 watt Panels

While the kit with optimizers is $500 cheaper, it only comes with a 12 year warranty on the inverter. The micro-inverters, on the other hand, have a 25 year warranty. Considering how close the long term costs are, assuming those kit prices are representative, it seems to me that deciding whether to go with optimizers or micros is probably more just a matter of preference. Micros have the edge in overall system robustness and producing when much of the system is buried by snow, but they have the disadvantage of clipping at the module level.

Warranties aren't always representative of expected life... and you can extend the warranty of the 6kW SE HD Wave inverter to 25 years for $184. Now compare that to a string inverter w/o optimizers and the cost drops another $600... ~$1300 if you don't need rapid shutdown.

 

[abasile]

Warranties aren't always representative of expected life... and you can extend the warranty of the 6kW SE HD Wave inverter to 25 years for $184.

Good to know! With that warranty extension, the system with optimizers is still $338 cheaper than the system with micros. On the other hand, with a string inverter (optimizers or not), you have a larger component (the inverter) to hang on the wall of the house somewhere. So, again, the overall cost seems close enough that it's probably just a matter of preference for the homeowner.

Now compare that to a string inverter w/o optimizers and the cost drops another $600... ~$1300 if you don't need rapid shutdown.

If partial shade is of minimal concern and the panels are all oriented identically, then a cheaper string inverter would clearly be the way to go.

 

[nwdiver]

Good to know! With that warranty extension, the system with optimizers is still $338 cheaper than the system with micros. On the other hand, with a string inverter (optimizers or not), you have a larger component (the inverter) to hang on the wall of the house somewhere. So, again, the overall cost seems close enough that it's probably just a matter of preference for the homeowner.

Also each string of Enphase inverters is limited to 16A or 3.8kW. If you want more you need another breaker which means a sub panel, which means more $$$.

 

[abasile]

Also each string of Enphase inverters is limited to 16A or 3.8kW. If you want more you need another breaker which means a sub panel, which means more $$$.

Good point. Our SunPower AC panels are on two circuits. However, if the homeowner is also having Powerwalls installed, then there will typically be a subpanel for "critical loads" and solar anyway.