16.32KW System Requires 2 Inverters?

[VslavZman]

Hi all -

First time stumbling upon this forum. I'm in the middle of the process, having just received permits and now awaiting installation timing. In the final design recommendation, I was surprised to see that the team recommended 2 SolarEdge7600 inverters for this install, for a total AC size of 15.2KW with estimated DC production at 16.12KWh. The 15.2KW output exceeds Massachusetts' net metering cap of 10KW, so I'll be taking a bit hit in the net metering benefit... which may be offset by the extra production, but more on that in a second.

Frist Question: When I asked why I can't use one SolarEdge10K, I was told that 1 inverter can't handle that size system? Is that true? I was told I'd have to drop down to the 12.2KW system size if I wanted to stay under the 10KW cap with one inverter, but I'd obviously lose a lot of production. Is there a happy medium to get me the maximum DC production without exceeding the 10KW AC net metering cap?

Second question: Is there a simple calculator out there that anyone can share, where I could more easily decide on the benefit of over-producing (and taking the "penalty" on the wholesale rate vs. retail rate for net metering in the month I over-produce) vs. staying under the cap and getting the maximum rate on the net metering? This is probably a Mass. specific question that also factors in the SMART program, but I'm hoping someone else was already in the same position I am.

Love the forum, thanks in advance for any responses!

Vlad

 

[darhall993]

SolarEdge does makes an 11.4 Inverter, I am using one today. There are limits to the size of array you can attach though and many considerations, you might want to read up here: https://www.solaredge.com/sites/default/files/inverter_dc_oversizing_guide.pdf

 

[MorrisonHiker]

From a reliability standpoint, I think you would want multiple inverters so that your system would still produce partially even if an inverter goes out.

When we had our 16.5 kW system installed, they put in 3 Delta Solivia inverters. When we expanded our system to 20.5 kW, they added a SolarEdge inverter. We don't have to worry about overproducing and don't have to deal with clipping.

 

[wwhitney]

Frist Question: When I asked why I can't use one SolarEdge10K, I was told that 1 inverter can't handle that size system? Is that true?

Apparently true for SolarEdge. Per the document in the second post, and per the datasheet, the SE10000H-US inverter has a maximum DC power connected of 15,500 watts.

I was told I'd have to drop down to the 12.2KW system size if I wanted to stay under the 10KW cap with one inverter, but I'd obviously lose a lot of production.

That just sounds like a cookiecutter Tesla response. There's no technical reason you couldn't put 15 kW of panels on a 10 kW inverter that is rated for it, like the SolarEdge. I would hope that if you find the right person, they can arrange for a number of panels that isn't a mulitple of 4 kW.

You can use PVWatts (PVWatts Calculator) to model the behavior or your system with a 10 kW inverter and different DC powers from 10 kW to 15 kW. On the "System Info" page, go to Advanced Parameters, and set the DC/AC size ratio to anywhere from 1.0 to 1.5. Set the DC System Size to 10 times that. Then you can compare the results from runs with different size ratios.

Cheers, Wayne

 

[BrettS]

Frist Question: When I asked why I can't use one SolarEdge10K, I was told that 1 inverter can't handle that size system? Is that true? I was told I'd have to drop down to the 12.2KW system size if I wanted to stay under the 10KW cap with one inverter, but I'd obviously lose a lot of production. Is there a happy medium to get me the maximum DC production without exceeding the 10KW AC net metering cap?

Often the DC part of the system is oversized as compared to the AC capacity of the inverter, however, a 16.12kW DC system would be way oversized for a 10kW inverter. I think it would probably be outside the specs of the inverter and even if it wasn’t you would lose a significant amount of your production. You would effectively wind up paying for panels that you can’t use.

If you don’t want to exceed a 10kW cap then you probably need to look at getting fewer panels and aim for maybe a 12kW DC system with a 10kW inverter. Solaredge makes a 9.6kW inverter that would work for this.

But as far as the number of inverters go, it’s very common to use two (or more) smaller inverters on large systems.

The other thing to look into is exactly how the net metering works in MA. This can vary widely from state to state, and even from one utility to another in the same state.

Usually with net metering it will look at your production over an entire year. So if you produce excess power in the summer when it’s sunny out and use power from the grid in the winter when the days are shorter then the amount of power you use in the winter will be offset by your overproduction in the summer. Then once a year they will look at all of the power that you sent back to the grid over that year and all of the power that you drew from the grid that year. It’s what happens at the end of the year that can vary a lot from one utility to another. In all cases, if you used more power that you sent back then you will get a bill. However, if you sent back more power than you used then what happens can vary. With some utilities you can get money back for overproduction (although it’s often at very low wholesale rates) and with other utilities any excess production is lost.

So, if you are saying that if you completely lose net metering when you go over a certain size and you can’t even use your excess summer production to offset winter use, then that seems like a very bad deal and you probably want to look at other options. However, if you are saying that you at the end of the year you won’t get credit for any overproduction then that’s not really such a terrible thing. You would want to try to size your system such that it’s annual production will be very close to your annual demand and then you would have no or very little excess production that gets lost.

 

[jboy210]

Hi all -

First time stumbling upon this forum. I'm in the middle of the process, having just received permits and now awaiting installation timing. In the final design recommendation, I was surprised to see that the team recommended 2 SolarEdge7600 inverters for this install, for a total AC size of 15.2KW with estimated DC production at 16.12KWh. The 15.2KW output exceeds Massachusetts' net metering cap of 10KW, so I'll be taking a bit hit in the net metering benefit... which may be offset by the extra production, but more on that in a second.

Frist Question: When I asked why I can't use one SolarEdge10K, I was told that 1 inverter can't handle that size system? Is that true? I was told I'd have to drop down to the 12.2KW system size if I wanted to stay under the 10KW cap with one inverter, but I'd obviously lose a lot of production. Is there a happy medium to get me the maximum DC production without exceeding the 10KW AC net metering cap?

Second question: Is there a simple calculator out there that anyone can share, where I could more easily decide on the benefit of over-producing (and taking the "penalty" on the wholesale rate vs. retail rate for net metering in the month I over-produce) vs. staying under the cap and getting the maximum rate on the net metering? This is probably a Mass. specific question that also factors in the SMART program, but I'm hoping someone else was already in the same position I am.

Love the forum, thanks in advance for any responses!

Vlad

With 2 inverters you have a backup, and can shut one down to under the 10kW cap.

But does Mass have a program for more than 10kW solar production?

 

[BrettS]

can shut one down to under the 10kW cap.

I’m not sure this is an effective solution. The limit seems to be the system size. Just turning off an inverter wouldn’t reduce the size of the system... just it’s output while the inverter is off.

 

[SoCal Dave]

10kW isn't a large system. Quickly looking at the Mass Net Metering site, it looks like it is 2MW. Is there someone else who is limiting it to 10kW?

Net metering eligibility

 

[VslavZman]

SolarEdge does makes an 11.4 Inverter, I am using one today. There are limits to the size of array you can attach though and many considerations, you might want to read up here: https://www.solaredge.com/sites/default/files/inverter_dc_oversizing_guide.pdf

Thanks for the reply. The 11.4 still has me running into the 10KW net metering cap in Massachusetts, so it doesn't really solve the problem. The 10K unit would probably be ideal, but I'd have to back off to 15.5KWDC (45 panels), otherwise I'd be maxing it out.

 

[VslavZman]

Apparently true for SolarEdge. Per the document in the second post, and per the datasheet, the SE10000H-US inverter has a maximum DC power connected of 15,500 watts.


That just sounds like a cookiecutter Tesla response. There's no technical reason you couldn't put 15 kW of panels on a 10 kW inverter that is rated for it, like the SolarEdge. I would hope that if you find the right person, they can arrange for a number of panels that isn't a mulitple of 4 kW.

You can use PVWatts (PVWatts Calculator) to model the behavior or your system with a 10 kW inverter and different DC powers from 10 kW to 15 kW. On the "System Info" page, go to Advanced Parameters, and set the DC/AC size ratio to anywhere from 1.0 to 1.5. Set the DC System Size to 10 times that. Then you can compare the results from runs with different size ratios.

Cheers, Wayne

Thanks for the reply. You're absolutely right... The representative I was speaking to didn't seem to know anything about flexibility in the system specified. You would think they would recommend 44 or 45 panels to keep me under the 15.5K input and allow me to use the SolarEdge10000 to stay under the net metering cap. On the surface, it seems like the most cost effective approach for me, as I'd be giving up about 1000 watts of annual production, but getting full retail on net metering. How do get to someone more competent on the design team directly so I can have these types of discussions?

 

[VslavZman]

Often the DC part of the system is oversized as compared to the AC capacity of the inverter, however, a 16.12kW DC system would be way oversized for a 10kW inverter. I think it would probably be outside the specs of the inverter and even if it wasn’t you would lose a significant amount of your production. You would effectively wind up paying for panels that you can’t use.

If you don’t want to exceed a 10kW cap then you probably need to look at getting fewer panels and aim for maybe a 12kW DC system with a 10kW inverter. Solaredge makes a 9.6kW inverter that would work for this.

But as far as the number of inverters go, it’s very common to use two (or more) smaller inverters on large systems.

The other thing to look into is exactly how the net metering works in MA. This can vary widely from state to state, and even from one utility to another in the same state.

Usually with net metering it will look at your production over an entire year. So if you produce excess power in the summer when it’s sunny out and use power from the grid in the winter when the days are shorter then the amount of power you use in the winter will be offset by your overproduction in the summer. Then once a year they will look at all of the power that you sent back to the grid over that year and all of the power that you drew from the grid that year. It’s what happens at the end of the year that can vary a lot from one utility to another. In all cases, if you used more power that you sent back then you will get a bill. However, if you sent back more power than you used then what happens can vary. With some utilities you can get money back for overproduction (although it’s often at very low wholesale rates) and with other utilities any excess production is lost.

So, if you are saying that if you completely lose net metering when you go over a certain size and you can’t even use your excess summer production to offset winter use, then that seems like a very bad deal and you probably want to look at other options. However, if you are saying that you at the end of the year you won’t get credit for any overproduction then that’s not really such a terrible thing. You would want to try to size your system such that it’s annual production will be very close to your annual demand and then you would have no or very little excess production that gets lost.

Thanks for the reply. I was told the net metering cap is based strictly on the system you install, not on actual production. Exceed 10KWAC with your system and you only get the wholesale rate for your net metering. That's not to say you get nothing back in credits... in MA. the wholesale rate is $0.04 vs. $.23 for retail. Would love to hear if anyone in MA using National Grid has had more experience near the cap.

 

[VslavZman]

10kW isn't a large system. Quickly looking at the Mass Net Metering site, it looks like it is 2MW. Is there someone else who is limiting it to 10kW?

Net metering eligibility
View attachment 580171

Thanks. You are right that I am still eligible for net metering, but in MA, if you go over 10KWAC with your system, the rate at which they credit any over-production drops from the retail rate ($.19-$.25) to just $.04 per KWH. It's a big drop, and only worth it I'm consuming most of what I would over-produce. Running the numbers, it's pretty close to a wash, but I'd love to see if someone in MA. has created an effective calculator to find the break-even point for exceeding the cap.

 

[wwhitney]

if you go over 10KWAC with your system, the rate at which they credit any over-production drops from the retail rate ($.19-$.25) to just $.04 per KWH.

Is that on a billing interval basis (15 minutes, an hour, whatever) or on annual basis?

Cheers, Wayne

 

[VslavZman]

Is that on a billing interval basis (15 minutes, an hour, whatever) or on annual basis?

Cheers, Wayne

It is based on net monthly consumption: (Total electricity consumed in a month - total electricity generated in a month)

 

[DrSmile]

I'd just reiterate that it's not a great idea to overpanel too much unless you have odd shading or orientation issues. If your panels are producing 15 KW and you only have a 10KW inverter you'll experience clipping, meaning that the excess energy is lost and you're only producing 10KW. You can't use it in the house or feed it back to the grid. You're "maxing out" when you hit the A/C rating, not the DC. Just because it's rated at 15.5 KW DC doesn't somehow mean you'll make more A/C power than the rating. My 11.4 KW Solaredge inverter delivers below 11 KW in the Summer when the line voltage drops. I'm overpaneled at 13.2 KW DC and wish I had two 6KW inverters instead.

 

[MorrisonHiker]

I'd just reiterate that it's not a great idea to overpanel too much unless you have odd shading or orientation issues. If your panels are producing 15 KW and you only have a 10KW inverter you'll experience clipping, meaning that the excess energy is lost and you're only producing 10KW. You can't use it in the house or feed it back to the grid. You're "maxing out" when you hit the A/C rating, not the DC. Just because it's rated at 15.5 KW DC doesn't somehow mean you'll make more A/C power than the rating. My 11.4 KW Solaredge inverter delivers below 11 KW in the Summer when the line voltage drops. I'm overpaneled at 13.2 KW DC and wish I had two 6KW inverters instead.

Our panels are only rated at 20.5 kW but thanks to our 4 inverters, we've seen production highs of 22 kW. No clipping here.

 

[BrettS]

I'd just reiterate that it's not a great idea to overpanel too much unless you have odd shading or orientation issues. If your panels are producing 15 KW and you only have a 10KW inverter you'll experience clipping, meaning that the excess energy is lost and you're only producing 10KW. You can't use it in the house or feed it back to the grid. You're "maxing out" when you hit the A/C rating, not the DC. Just because it's rated at 15.5 KW DC doesn't somehow mean you'll make more A/C power than the rating. My 11.4 KW Solaredge inverter delivers below 11 KW in the Summer when the line voltage drops. I'm overpaneled at 13.2 KW DC and wish I had two 6KW inverters instead.

This is true to an extent. But keep in mind that when clipping happens it only happens during peak production times, so at the worst you might clip for an hour or two a day, and maybe not even that much depending on the weather. But during all of the rest of the production time you will be able to take advantage of the higher output of your larger system. So your 13.2kW system with an 11.4kW inverter will produce 16% more power than an 11.4kW system during most of the day.

You might get a small amount of clipping, but if you had a bigger inverter you might get an extra 1kWh or so on bright sunny days, but the question is... is it worth paying more for a larger inverter to get an extra 1kWh of power?

This only works for a system that’s 10 or 20% oversized. Once you get much more than that then it probably makes sense to upgrade the inverter.

 

[wwhitney]

I'd just reiterate that it's not a great idea to overpanel too much unless you have odd shading or orientation issues.

Well,it sounds like the OP's situation falls under your "unless" clause. If they have regulatory reasons to cap the inverter size at 10 kW, it's a simple question of "what is the payback period on DC power above 10kW?" Obviously the payback for each additional kW is increasing. But it still may be worth 13, 14 or even 15 kW of DC. The PVWatts program I mentioned can easily model the results, so the OP can decide the proper tradeoff.

Cheers, Wayne

 

[Southpasfan]

This may have been answered elsewhere, but in a new all-Tesla install it would seem to me that any "clipping" would be cause to have them out and switch out an inverter, yes?

I understand its not "that much" of a loss, but isn't one of the implied parts of the contract that you get all the solar the panels can produce?

 

[BrettS]

This may have been answered elsewhere, but in a new all-Tesla install it would seem to me that any "clipping" would be cause to have them out and switch out an inverter, yes?

I understand its not "that much" of a loss, but isn't one of the implied parts of the contract that you get all the solar the panels can produce?

I don’t believe that’s the case. They sell you a system with specific inverters and panels and estimate the annual production and you can decide if it’s a system that you want or not. They don’t have any guarantee that there will be no clipping. I believe that it’s reasonably common practice in the industry to design systems such that the DC capacity is greater than the AC capacity, which would allow for some clipping to happen.