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SageBrush said:
johnlocke said:
Micro inverters convert to AC at the panel so DC losses are minimal. SMA string inverters are cheaper by a few hundred dollars then the equivalent Micros but not having to work with HVDC and no single point of failure balance that out for DIY'ers.

The wire resistance losses are proportional to the square of the current. 10 awg copper resistance is about 1.25  ohms per 1000 ft. Your string has about 2x the current as a a DC solution, and then 4x more current after two strings are combined. A rooftop is almost certainly close enough to the inverter to not bother considering but a ground mount may be a different story.

E.,g. if two 20 Amp strings are combined near the panels and then 40 Amps run 250 feet to the inverter, the loss will be
40^2*2*1.25*250/1000 = 1000 watts. 100 watts every 25 feet of home run.

My SMA string inverter cost ~ $1,200 for 8 kW so about 15¢ a watt. AND I get an (admittedly modest, but still very useful) grid-down solution for free.

Each solution has trade-offs. Anybody who says one or the other is always the right answer is missing information or is pushing an agenda.

---
addendum: 10 awg looks too small for 40 Amps. What is awg of your wires after the strings are combined ?
Your math is off. Each micro inverter is two inverters in one box, 600w max, 500w typically. 2.1A @ 240 VAC per box, 5 boxes per string. 10-12A per string (12 AWG). Loss on the branch circuit is a max of 13w. loss on the line from the breaker panel to the house (8 AWG) is 400w at max power and less than 300w at typical power midday. That's figuring a 250 ft. run.

The inverter string is 12 AWG both on the trunk and from the junction box to the breaker panel. The max distance from an inverter to the panels about 35 ft for the furthest set,20 ft. for the middle set and 5 ft. for the nearest set.. That's a max of 12A on each string at a max distance of 35 FT. From the panel to the house I ran 8 AWG. Most of the time current is about 30A or less. I'm just fine with a slight loss under max power. I had 8 AWG left over from another project so I used that. 6 AWG would have been better but wasn't worth the extra cost. Also I would have had to dig up existing conduit or bury new conduit to accommodate 6 AWG. It wasn't worth the effort. The other set of panels is on a Sunnyboy 8K inverter and is run on 6 AWG over a bit longer distance (50 feet further). The micro inverters and panels actually do better because they are better situated and are set to a higher angle of inclination. If I could do It all over again I'd set all the panels to 45 degrees for better power levels in the winter and more consistent power generation all year long instead of summer peaks and winter troughs.

Keep in mind that I have 18KW of panels. It would be nearly impossible to put that much on a south facing roof of a typical house. Putting them out in a field let me set them for the best operation. The2-3% loss due to distance is made up by the improved siting.
 
(my bolding)

johnlocke said:
The inverter string is 12 AWG both on the trunk and from the junction box to the breaker panel.

Aren't you combining strings for the run from box to panel ? I'm surprised the 12 awg can handle the current.

We do seem to agree though that the lion's share of the losses for a microinverter array at a distance from the panel will be the run from combiner box to panel, and that run will be at 240v and somewhere around 20 - 40 Amps, depending on configuration. The current here calculates out to P(ac) / 240

As for your calc of 400 watt losses, doesn't the 250 feet run have 500 feet of wire for the circuit ?
 
SageBrush said:
(my bolding)

johnlocke said:
The inverter string is 12 AWG both on the trunk and from the junction box to the breaker panel.

Aren't you combining strings for the run from box to panel ? I'm surprised the 12 awg can handle the current.

We do seem to agree though that the lion's share of the losses for a microinverter array at a distance from the panel will be the run from combiner box to panel, and that run will be at 240v and somewhere around 20 - 40 Amps, depending on configuration. The current here calculates out to P(ac) / 240

As for your calc of 400 watt losses, doesn't the 250 feet run have 500 feet of wire for the circuit ?
The trunk lines do converge at a breaker box on the PV mount. Each trunk line goes into a separate junction box and is connected from there to 12g wire which goes into the breaker box mounted on the PV structure. There is a 40A master breaker and 3 20A breakers, one for each string. From there the power is on 8g wires to a 40A breaker in the house panel. And yes I did calculate the 500 ft. of wire. The trunk lines only carry 12A max each. 12a@240VAC=2880w= the output from 10 panels. Typical output from these panels at noon is more like 240-250 watts. About 10A per string, 3 strings. 500ft. of 8g has a resistance of 0.3141 ohms. At 30a the loss is 282.69w. At 36A, the loss is 407w.

I may have confused you with my other descriptions, hopefully this clears everything up.
 
johnlocke said:
The trunk lines do converge at a breaker box on the PV mount. Each trunk line goes into a separate junction box and is connected from there to 12g wire which goes into the breaker box mounted on the PV structure. There is a 40A master breaker and 3 20A breakers, one for each string. From there the power is on 8g wires to a 40A breaker in the house panel. And yes I did calculate the 500 ft. of wire. The trunk lines only carry 12A max each. 12a@240VAC=2880w= the output from 10 panels. Typical output from these panels at noon is more like 240-250 watts. About 10A per string, 3 strings. 500ft. of 8g has a resistance of 0.3141 ohms. At 30a the loss is 282.69w. At 36A, the loss is 407w.

I may have confused you with my other descriptions, hopefully this clears everything up.

^^ This looks right -- about 2 - 3% losses from the higher current on a long run home. That is ballpark the shading advantage that micros have over serial DC, but it is quite a bit more expensive, locked into a vendor, and has worse reliability. The main upside is panel level monitoring.and simpler troubleshooting. I should add that troubleshooting a serial array is pretty simple. It would take me 3 - 4 DMM checks to find the bad panel.

Regarding cost ...
If the system is enphase, then I think these costs are ballpark:
$125 per microinverter
$3/foot for the run from the combiner box to the panel, based on $1/foot per wire * 3 wires
$20 per trunk cable
Envoy proprietary monitoring hardware and software. -- ?? $300 - $500

A SMA SunnyBoy serial w/ central inverter setup:
15 - 20¢/DC watt, depending on Array size
$1.5/foot home run, based on two strings, 33¢/foot, and 5 wires that are 12 awg

My arithmetic says for a 10 kW array that is 250' from the panel:
SMA: $1,875
Enphase: ~ $4,000
 
SageBrush said:
This looks right -- about 2 - 3% losses from the higher current on a long run home.
The AC vs DC choice on a long run from a remote array has various tradeoffs. Microinverters lock you into AC, but sometimes with a string inverter you'd choose to put it at the array anyway. The 2-3% losses you mention could obviously be mitigated with larger (more expensive) conductors, so that's just another tradeoff.

I.e. there's no right answer, just a bunch of tradeoffs. Depending on your goals, either microinverters or string inverters may end up a better choice.

Cheers, Wayne
 
SageBrush said:
johnlocke said:
The trunk lines do converge at a breaker box on the PV mount. Each trunk line goes into a separate junction box and is connected from there to 12g wire which goes into the breaker box mounted on the PV structure. There is a 40A master breaker and 3 20A breakers, one for each string. From there the power is on 8g wires to a 40A breaker in the house panel. And yes I did calculate the 500 ft. of wire. The trunk lines only carry 12A max each. 12a@240VAC=2880w= the output from 10 panels. Typical output from these panels at noon is more like 240-250 watts. About 10A per string, 3 strings. 500ft. of 8g has a resistance of 0.3141 ohms. At 30a the loss is 282.69w. At 36A, the loss is 407w.

I may have confused you with my other descriptions, hopefully this clears everything up.

^^ This looks right -- about 2 - 3% losses from the higher current on a long run home. That is ballpark the shading advantage that micros have over serial DC, but it is quite a bit more expensive, locked into a vendor, and has worse reliability. The main upside is panel level monitoring.and simpler troubleshooting. I should add that troubleshooting a serial array is pretty simple. It would take me 3 - 4 DMM checks to find the bad panel.

Regarding cost ...
If the system is enphase, then I think these costs are ballpark:
$125 per microinverter
$3/foot for the run from the combiner box to the panel, based on $1/foot per wire * 3 wires
$20 per trunk cable
Envoy proprietary monitoring hardware and software. -- ?? $300 - $500

A SMA SunnyBoy serial w/ central inverter setup:
15 - 20¢/DC watt, depending on Array size
$1.5/foot home run, based on two strings, 33¢/foot, and 5 wires that are 12 awg

My arithmetic says for a 10 kW array that is 250' from the panel:
SMA: $1,875
Enphase: ~ $4,000
SMA no longer makes a 10KW inverter. You'd need two 5KW inverters at $1200 each plus the monitor box @ $900. It's a little cheaper but if you're mounting on a roof you also have to add the rapid shutdown modules for another $1000. If you are ground mounting, you really don't want to run multiple 600VDC lines back to an inverter at the house so you still have the AC transmission losses. You lose MPP tracking per panel and panel level monitoring. I used APS dual micro inverters (15 @ $185 ea) plus $400 for the wireless monitor. Add in the wiring, breaker box, conduit, etc. and you come out at about $4K. For a ground mount the SMA is a little cheaper but on a roof it would cost more and you lose features.

If I were doing an array with high wattage panels (over 350W), I'd use a central inverter just because microinverters won't handle the power. For a small array where a single inverter would work, I'd consider it. For anything else, I'd use microinverters.
 
johnlocke said:
If you are ground mounting, you really don't want to run multiple 600VDC lines back to an inverter at the house

Not my setup, but I am curious why you say this.
 
SageBrush said:
johnlocke said:
If you are ground mounting, you really don't want to run multiple 600VDC lines back to an inverter at the house

Not my setup, but I am curious why you say this.
Extreme DC in an underground conduit. I worry enough about someone or something cutting or chewing into a 240VAC line. 600V would fry anything on the spot. Also It's a lot harder to find a rated DC breaker especially if you need something over 500V. Can't pick it up from Home Depot. I'd want a breaker or disconnect box at the panels and a breaker somewhere to limit current. 240AC could hurt you, 500-600VDC will kill you.
 
johnlocke said:
SMA no longer makes a 10KW inverter. You'd need two 5KW inverters at $1200 each plus the monitor box @ $900.
That was news to me about SMA. Related to supply issues, or a sign of the time ?

In any case, there are other manufacturers that sell 10 kW for about $1,500:
https://www.renvu.com/Products/Solar-Inverters/Residential-String-Inverters/custitem56/9773.91to16113.09

My SMA comes with its own built in monitoring and comms. I'm not sure about these others, and I'm reasonably sure that they do not have an island AC supply when the grid is down.
 
Enphase IQ7+ micro-inverters claim to support up to 440W panels with a max output power or 290VA. I don't know exactly how that matches with high output panels though.

The Enphase monitoring hardware and software is not required to use the system AFAIK, but if you pay for the micro-inverters it is nice to see the individual outputs for monitoring and showing off on your phone, etc. :mrgreen:

In my case, it easily shows which panels are producing what and when. I do have partial shading so it was a no-brainer in my case. I'll have to think harder about the cost trade-offs when I do a ground mount system but so far I have been impressed with their design and performance.
 
goldbrick said:
Enphase IQ7+ micro-inverters claim to support up to 440W panels with a max output power or 290VA. I don't know exactly how that matches with high output panels though.

The Enphase monitoring hardware and software is not required to use the system AFAIK, but if you pay for the micro-inverters it is nice to see the individual outputs for monitoring and showing off on your phone, etc. :mrgreen:

In my case, it easily shows which panels are producing what and when. I do have partial shading so it was a no-brainer in my case. I'll have to think harder about the cost trade-offs when I do a ground mount system but so far I have been impressed with their design and performance.
290w inverters on 440w panels will cause a lot of clipping. I wouldn't use anything higher than about 330-350w panels in that case. The extra money spent on the high output panels is wasted. I have 320w panels on 300w nominal inverters but in warm weather they clip at 286w due to the heat.

NEP has high wattage micro inverters suitable for up to 440W panels. both single and dual panel models. They come with built in AC trunk cabling. PLC comm links between modules and the gateway. That can be a problem in noisy environments or where GFCI outlets are involved. While not necessary, monitoring capability is handy for troubleshooting.

Even for ground mount systems, shading is still an issue particularly if you have a small suburban installation. Lots of trees that can throw surprisingly long shadows in the mornings and evenings.
 
johnlocke said:
290w inverters on 440w panels will cause a lot of clipping. I wouldn't use anything higher than about 330-350w panels in that case.

Best not to generalize. There is some clipping, but you get more power in the morning and afternoon. The choice depends on the marginal cost compared to marginal generation in general, and more generation at what be more valuable times. YMMV
 
SageBrush said:
johnlocke said:
290w inverters on 440w panels will cause a lot of clipping. I wouldn't use anything higher than about 330-350w panels in that case.

Best not to generalize. There is some clipping, but you get more power in the morning and afternoon. The choice depends on the marginal cost compared to marginal generation in general, and more generation at what be more valuable times. YMMV
Assuming that 440w panels don't cost any more per watt than 330w panels, you'd still be better off with 12 330w panels than 9 440w panels assuming that you use 300w micros. You'd limit the output to 300w per panel for 3-4 hours each day. On the 330w panels that's not much of a limitation but the 440w panels would lose nearly 900w/hr. for that time period. You might expect to make up some that with better production in the morning and evening but the 330w panels will still match the 440w panels during that time. Extra production from the 330w panels will easily offset the extra cost of 3 more micros.

Good rule of thumb is the inverter should be able to handle 85-90% of the rated capacity. Less than that and you get clipping. more than that and you are just wasting money. Matching the panel wattage to the inverter just makes sense and saves money.
 
johnlocke said:
Good rule of thumb is the inverter should be able to handle 85-90% of the rated capacity.

Check out this analysis
https://www.youtube.com/watch?v=nvN6s1EpdS8

There is a presumption here that the extra DC wattage is 2.5 - 3 $/watt. I was thinking more along the lines of $0.5/watt
 
SageBrush said:
johnlocke said:
Good rule of thumb is the inverter should be able to handle 85-90% of the rated capacity.

Check out this analysis
https://www.youtube.com/watch?v=nvN6s1EpdS8

There is a presumption here that the extra DC wattage is 2.5 - 3 $/watt. I was thinking more along the lines of $0.5/watt
I figure about $1/w for a DIY ground mount before Incentives given the current market. That analysis is for commercial arrays and includes site costs and manpower cost. It may make sense to use higher ac/dc ratios when you include those costs to give the best ROI. Not really applicable for a DIY sweat equity project. I'm looking for the most power for the least money. I will pay extra for the convenience of low voltage installation and enhanced monitoring. I do realize that a central inverter is cheaper than micros.
 
johnlocke said:
SageBrush said:
There is a presumption here that the extra DC wattage is 2.5 - 3 $/watt. I was thinking more along the lines of $0.5/watt
I figure about $1/w for a DIY ground mount before Incentives given the current market. That analysis is for commercial arrays and includes site costs and manpower cost. It may make sense to use higher ac/dc ratios when you include those costs to give the best ROI. Not really applicable for a DIY sweat equity project. I'm looking for the most power for the least money. I will pay extra for the convenience of low voltage installation and enhanced monitoring. I do realize that a central inverter is cheaper than micros.
The low marginal costs of adding a few more PV panels to a DIY residential job occur because the fixed costs are paid for. The costs you mention for commercial either do not apply to our scenario, or are fixed so the conclusions are even stronger for us.

This is all a YMMV. Back when I bought my SMA central inverter the difference between the 7 kW and 10 kW versions was about $150, or about 5¢ a watt. Panels cost me 45¢ a watt. Add racking and that was the marginal cost for my project before tax credit. Even if the extra panels only produce 1/3 of ideal production (a very low estimate), they can generate electricity for as low as 3¢ a kWh. That is attractive, but at least for me the more important benefit is the extra generation on cloudy days and in the morning and late afternoon.

In your case the inverter is paid for so the marginal cost can come down to extra watts in a higher efficiency panel, or extra panel watts in a bigger panel and additional racking.
 
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