selecting correct inverter size for Sunpower panels

[incubus0h]

Hi all,

Thanks for all the replies I got about my other topic on this forum, mounting the inverter in garage. I didn't expect such a good technical explanation on the topic and suggestions to avoid the problems.

Well, I do have a related question - I had received a quote from another Sunpower dealer who has the Aurora Power-One inverter in its proposal (in reality, it will be the Sunpower-branded version). As I have realized the advantage of using the Power-One inverter over the SMA inverters, I am considering their proposal once again. They seem to be more conservative in their shading analysis, compared to the other dealer. So, they proposed PV solar system of size 4905 watts - 15 Sunpower 327W panels (SPR-327NE-WHT-D) with a Power-One PVI-4.2-OUTD-US-240/Sunpower-branded SPR-4200p-TL-1 inverter. All the 15 panels are on the roof facing in the south-west direction. They are going to put 8 panels on 1st MPPT and 7 panels on the 2nd MPPT.

The online Power-One configuration tool Power-One configuration tool shows the "Inverter utilization ratio" of 102.2% for this setup. The dealer says this is okay. Their reasoning is that (1) the losses in the system due to line losses and losses due to extreme roof temperatures will cause the system to perform under this threshold even during the first year when the panels are producing at their optimum, and (2) the panels tend to degrade year after year. Hence, maximizing the capacity of the inverter will ensure that the panels produce enough voltage to run the inverter for as long as the inverter lasts.

If I change the inverter size to PVI-5000-OUTD-US-240, the configuration tool shows the "Inverter utilization ratio" of 95.2%. The dealer says that that might be little too high for the 15 panels, and suggests to stick with the PVI-4.2-OUTD-US-240 model.

I am not fully convinced of their argument. What do you guys think - does the lower size of the inverter really make such difference ?

Thanks in advance.

[Naptown]

In reality no. You are well within the design limits of either inverter. Remember that peak power will only be hit for a few hours a day and when it is cool out.
Several things happen.
When the sun is strongest during the summer months and the panels are more likely to be perpendicular to the sun the temperature is also high which reduces the output of your panels. So they are actually producing less than nameplate rating.
Inverters are most efficient when running at close to peak output. By downsizing the inverter a bit it will increase the annual production of the array. Remember that peak production only happens for a few hours a day then array output drops off or ramps up. What you will lose on a few days a year will be made up more during the off peak hours the rest of the year.

[KRenn]

In reality no. You are well within the design limits of either inverter. Remember that peak power will only be hit for a few hours a day and when it is cool out.
Several things happen.
When the sun is strongest during the summer months and the panels are more likely to be perpendicular to the sun the temperature is also high which reduces the output of your panels. So they are actually producing less than nameplate rating.
Inverters are most efficient when running at close to peak output. By downsizing the inverter a bit it will increase the annual production of the array. Remember that peak production only happens for a few hours a day then array output drops off or ramps up. What you will lose on a few days a year will be made up more during the off peak hours the rest of the year.

I agree with Naptown.


I think you can go eitherway, both inverters will give you sufficient output, the 4.2 is close enough that you shouldn't suffer any clipping effect.

[incubus0h]

I agree with Naptown.


I think you can go eitherway, both inverters will give you sufficient output, the 4.2 is close enough that you shouldn't suffer any clipping effect.

KRenn, what do you mean by the clipping effect ? Can you explain it a little bit more ?

[Naptown]

When inverters are fed too much power from the panels they will not use all of the power available. this is known as clipping.
This happens mostly when the array STC rating is more than 20% larger than the inverter capacity.
In most instances the occurrence is rare as conditions need to be just right. (Cool weather, very bright, low humidity, smog free day, Panels exactly perpendicular to the sun etc.).

[billvon]

I am not fully convinced of their argument. What do you guys think - does the lower size of the inverter really make such difference ?

Thanks in advance.

No. 102% is very reasonable.

1) You won't ever actually get 100% of your panel output due to dirt, haze in the air, higher panel temperatures etc.
2) Even if you do get occasional periods of 100% output (due to cloud lensing or snow or something), you are only losing 2% of your output for perhaps an hour or two a day. It won't be noticeable in the long run.

[incubus0h]

No. 102% is very reasonable.

1) You won't ever actually get 100% of your panel output due to dirt, haze in the air, higher panel temperatures etc.
2) Even if you do get occasional periods of 100% output (due to cloud lensing or snow or something), you are only losing 2% of your output for perhaps an hour or two a day. It won't be noticeable in the long run.

I have decided to go with the proposal using the 16 Sunpower panels (system size: 16 X 327W per panel = 5232 W). The installer wants to use the PVI-6000-OUTD-US-240 inverter in the system. I put these values in the String Tool from the PowerOne. It gives me the inverter utilization ratio of 84.7% when the panels are mounted using the two MPPTs (8 on one MPPT and the other 8 on the second MPPT). Is that okay ? The installer says that the 6000W inverter costs them the same as the 5000W inverter.

I just want to make sure that 5-10 years down the line, I don't have to replace the inverter because the panels are not producing enough power.

[KRenn]

I have decided to go with the proposal using the 16 Sunpower panels (system size: 16 X 327W per panel = 5232 W). The installer wants to use the PVI-6000-OUTD-US-240 inverter in the system. I put these values in the String Tool from the PowerOne. It gives me the inverter utilization ratio of 84.7% when the panels are mounted using the two MPPTs (8 on one MPPT and the other 8 on the second MPPT). Is that okay ? The installer says that the 6000W inverter costs them the same as the 5000W inverter.

I just want to make sure that 5-10 years down the line, I don't have to replace the inverter because the panels are not producing enough power.

16x327 means you should use the 5000 watt inverter, there's absolutely no reason for them to be using the 6000 unless you're planning on adding more panels down the road.

[Elkhornsun]

Another option is to go with factory panels that include their own integrated micro-inverters and produce AC output, like the SunPower SPR-225E ACPV 240 W panel. These AC PV'sy are 30" wide so in terms of watts per area they are comparable to the 327 Watt 41" wide panels. AUO AC Unison PM250MA0 250-watt panel has an integrated micro-inverter and more manufacturers will be adding these to their product lines for residential use.

The micro-inverters have the same 25-year warranty as the panels, quite unlike a central inverter. Each inverter is dealing with a fraction of the power that the central inverter has to handle and a fraction of the heat and it is heat that kills electronic devices. With AC outputs the cable diameters can be a lot smaller and the voltage is a lot less than with a DC system.

The drawback to the micro-inverter is that it can handle a maximum of 250 Watts so it needs to be mated to lower output panels. SunPower provides 225 W and 240 W panels with the integrated micro-inverters in light of this limitation. I would expect that as average panel output increases there will be new micro-inverters developed to handle them. In the meantime it is whether to buy more expensive PV's with integrated inverters or to buy standard panels and install a single large central inverter. Over a period of 25 years the micro-inverters are covered by the manufacturer's warranty whereas with a central inverter I would expect after 25 years to be on my third inverter for the house. The smaller the gird the more the economics favor micro-inverters over a central inverter.

[KRenn]

Another option is to go with factory panels that include their own integrated micro-inverters and produce AC output, like the SunPower SPR-225E ACPV 240 W panel. These AC PV'sy are 30" wide so in terms of watts per area they are comparable to the 327 Watt 41" wide panels. AUO AC Unison PM250MA0 250-watt panel has an integrated micro-inverter and more manufacturers will be adding these to their product lines for residential use.

The micro-inverters have the same 25-year warranty as the panels, quite unlike a central inverter. Each inverter is dealing with a fraction of the power that the central inverter has to handle and a fraction of the heat and it is heat that kills electronic devices. With AC outputs the cable diameters can be a lot smaller and the voltage is a lot less than with a DC system.

The drawback to the micro-inverter is that it can handle a maximum of 250 Watts so it needs to be mated to lower output panels. SunPower provides 225 W and 240 W panels with the integrated micro-inverters in light of this limitation. I would expect that as average panel output increases there will be new micro-inverters developed to handle them. In the meantime it is whether to buy more expensive PV's with integrated inverters or to buy standard panels and install a single large central inverter. Over a period of 25 years the micro-inverters are covered by the manufacturer's warranty whereas with a central inverter I would expect after 25 years to be on my third inverter for the house. The smaller the gird the more the economics favor micro-inverters over a central inverter.

Completely irrelevant as that option is not available with the Sunpower lease. It also substantially more expensive, $1.00 a watt more typically, meaning on 5 kilowatt system, you're paying $5000 more, or essentially the cost of about 3 PVI central inverters.

[inetdog]

With AC outputs the cable diameters can be a lot smaller and the voltage is a lot less than with a DC system.

It seems to me that with a DC series string, so that the voltages are higher, you will have lower current on the DC side with corresponding lower cable diameters, while the advantage of lower output current from each of the micro inverters will be lost once you parallel their outputs into the AC harness. If the total amount of power remains the same, lower voltage means higher current and vice-versa.
With micros on each panel, you have to bring them together in a common grid connection point eventually.

(AC fuses and breakers can definitely be cheaper though.)