Newbie Basic Questions on Size and output calculations...

[kronic24601]

Hi Folks,

For starters, I'm just in the initial phase of thinking through solar. I typically do as much as I can DIY for most projects, and pay professionals for the parts that are really hard or dangerous. The questions I'm going to ask may not inspire confidence in my abilities, but I want to learn so any help is appreciated.

Desire: I'd like to put up solar to power 100% of my current electrical needs (roof mount). I've estimated that I use around 11,750 kWh annually. I live in the Los Angeles area (LADWP).

Setup I'm considering:
Inverters - Enphase C250 Microinverters
Panels - SolarWorld SW 345 XL Mono

Other factors
Annual Temp: (Fahrenheit)
* Record low - 20°
* Record high - 115°
* Average low - 40°
* Average high - 96°

Panel size = 200 Amp

Questions: I've been reading conflicting information on how one calculates system sizes and needs. I'm hoping someone can help me clarify what I'm missing.

(calculating average daily kWh) - 11750/365 = ~32kWh daily.
(factoring in for sun in my area) 32/5hrs of sun a day on average = 6.4

Part 1
Here is part ONE I'm confused about. How to factor in conversion from AC to DC.
I've heard some people say a flat 20%
While others are based on the inverter efficiency itself (in this case is 96.5% efficiency)

So, to generate 11,750 kWh of AC, do I need a (6.4/.8) 8kW system of a (6.4/.965) 6.6 kW system?

Part 2
When I started looking at the LADWP permit paper work, they use this formula to calculate the total AC output of the system.
#of Microinverters _______ x Inverter AC Output Current ______ x 240V = _______ W

This confuses me since there is no variance for the rating of the panel. You could have a 150watt panel or a 350watt panel and it would be the same calculation since it just factors in the Microinverter. Case in point, if I wanted 24 of the 345watt panels (and this 24 micro inverters), that is what I understand to be a traditional 8.28kW system (DC). However, since according to the M250 documentation, the nominal output current (the only one listed) is 1.0 A. So 24 x 1 x 240 = 5760 w or 5.76 kW. What am I missing?

Part 3
I was doing some drawings of my roof and sizing of panels to try and get a feel for how this might all come together. I started to try and calculate the feedback breaker size I'd need based on my 24 panel setup (which I'm bringing into question in Part 1 and 2 above). My current plan would be 12 panels on a branch (M250 microinverters appear to support up to 16).
What formula do I use to determine
Panel (Isc) = 9.75 A x 1.25 = 12.1875 A x 2 branches = 24.375 A
OR
# Inverters in a branch (12) x Inverter AC output (1) x 1.25 = 15 A x 2 branches = 30 A

Either way it would seem like I could squeeze in a 40 A feedback breaker on my 200 A panel without any issues (right?) however I'd like to know how to properly calculate this.

Part 4
What else am I missing, I haven't calculated the AWG size needed (which somewhat depends on my distance and temp factors). Ultimately I just want to hone in on what size I need and attempt to finalize a shopping list or any other "gotchas".

Thanks so much in advance for any help I receive.

[foo1bar]

"So 24 x 1 x 240 = 5760 w or 5.76 kW. What am I missing?

LADWP is looking at it from their perspective - what is the maximum that you can send back to the grid?
And the most you can send back to the grid is 5.76kW - you may have more DC wattage, but that just means you'll potentially be at 5.76kW for a longer time, but you won't be more than that.
BTW - 8.28kW of modules with 5.76kW of inverter capacity looks like there's too much of solar modules for the inverters - so I think you need to double check that the inverter and module will work together properly. And if they are rated that they'll work together, you need to decide if you're OK with the clipping - or I suppose even whether you'll have clipping. If they're not at a good angle you may never get clipping even with it being oversized at 145%

So, to generate 11,750 kWh of AC, do I need a (6.4/.8) 8kW system of a (6.4/.965) 6.6 kW system?

What does pvwatts say for your planned orientation/tilt?

Either way it would seem like I could squeeze in a 40 A feedback breaker on my 200 A panel without any issues (right?) however I'd like to know how to properly calculate this.

Is it an end-fed bus?
If so, then 40A (or 30A) going to a combiner/subpanel to join your two branches seems reasonable to me - sounds like you could use the 120% rule. I haven't done microinverters so didn't have to deal with combining AC.
As for size, the module's Isc is irrelevant - the module is only connected to the inverter. So Isc only important when looking at the wiring between the module and the inverter (which for microinverter is just the wires that come with the module and the wires that come with the microinverter. )

[kronic24601]

Thanks for the responses foobar! I have a few answers, and a few more questions.

1) I "think" I've confirmed that the module and inverter capacity is compatible with each other. At least in terms of measured the modules Voc/Vmpp/Isc, etc and comparing the specs on the Micro inverter. These 345 watt panels have a high DC voltage, so the Enphase C250 was the only one that met the criteria. However, I still feel like I'm missing something. I just assumed it wasn't true "clipping" and that I am reading something wrong. Otherwise, there are no micro-inverter setups in which you can use high wattage panels and get the full benefit.
2) PVWatts, which I may have filled out wrong, seems to imply I'll be fine. generating 11,728 kWh a year (pretty close to what I need). However I wasn't sure how much this is guess work or not. You just sort of say "14% system losses ... sure". Also, I didn't fully understand the defailt DC to AC Size Ratio of 1.1 that they gave me.
3) This should be an end-fed bus.

[J.P.M.]

Thanks for the responses foobar! I have a few answers, and a few more questions.

1) I "think" I've confirmed that the module and inverter capacity is compatible with each other. At least in terms of measured the modules Voc/Vmpp/Isc, etc and comparing the specs on the Micro inverter. These 345 watt panels have a high DC voltage, so the Enphase C250 was the only one that met the criteria. However, I still feel like I'm missing something. I just assumed it wasn't true "clipping" and that I am reading something wrong. Otherwise, there are no micro-inverter setups in which you can use high wattage panels and get the full benefit.
2) PVWatts, which I may have filled out wrong, seems to imply I'll be fine. generating 11,728 kWh a year (pretty close to what I need). However I wasn't sure how much this is guess work or not. You just sort of say "14% system losses ... sure". Also, I didn't fully understand the defailt DC to AC Size Ratio of 1.1 that they gave me.
3) This should be an end-fed bus.

On PVWatts: Read the help/info screens a couple of times, get the proposed orientation of the array close to right for tilt and azimuth and use a 10 % system loss parameter rather than the 14 % default. See the help/info screens and the loss calculator for more help on losses. On the DC to AC: read the help/info screens. It's all there, laid out in a straightforward fashion.

[foo1bar]

However, I still feel like I'm missing something. I just assumed it wasn't true "clipping" and that I am reading something wrong.

Clipping is when the module *could* produce more power - but the inverter isn't able to handle more power. So instead of a nice rounded hill when you look at power vs. time you see a ramp up in the morning a flat spot at mid-day when the inverter is maxed out (clipping) and a ramp down in the evening.
A little clipping is not a problem - you'd lose just a tiny bit of potential production right around noon - but it will probably be cost effective overall. (it'll probably be the cheaper way to get the kwh desired, rather than going with smaller modules and more of them and more inverter capacity)
And often if your orientation isn't "perfect" you won't even see clipping at slightly more module wattage than inverter capacity (ex. 290W module pointed west with a 280W inverter will probably never clip. Even 300W with 280W inverter might never clip)

Otherwise, there are no micro-inverter setups in which you can use high wattage panels and get the full benefit

I'd bet those 345W panels are 72-cell.
Generally 72-cell modules are targeted at commercial installs. And generally commercial installs do not use microinverters.
And because I suspect it is a 72-cell module, I would STRONGLY recommend you make sure that the enphase microinverter will work with that module.
Probably you are better off with 285W or 300W modules that are smaller but will give you the same or more energy for the same amount of area.

Also, I didn't fully understand the defailt DC to AC Size Ratio of 1.1 that they gave me.

In your case I believe it would be module size divided by inverter size - so 345W / 250W = 1.38
Read the help/info screens.

3) This should be an end-fed bus.

Are you planning on keeping your current main panel? If so, then is your main panel an end-feed setup?

[kronic24601]

So, the inverter: https://enphase.com/sites/default/fi..._EN_US_web.pdf states it's a 240 W (continuous power). But as you have stated 1/2 my panels will have a 15° tilt west facing (the other 1/2 will be SW facing). So I was assuming that the clipping would be minimal-to-none with my setup.

You are correct that it's a 72-cell panel. Oddly the price of a 72-cell panel is equal to or less than the 300 watt 60-cell equivalent. I of course thought, hey great. Less panels AND less inverters. Win-win. I have confirmed that the C250 is compatible with 72-cell panels, and that their max DC voltage input is compatible. However, as mentioned above... when I did the "math" that LADWP had me do it basically ignored the output wattage of the inverter and just asked about the output current Amps. So I felt like I must be missing something.

When you divided by the inverter size, would you use the "Peak Output Power" number (253 W) or the "Rated (continuous) output power" (240 W)?

Annnd as far as the panel goes (excuse my ignorance) 1) Yes I was hoping to keep it and 2) I'm pretty ignorant as to the different setup types. I know it's not a split setup panel. The 200 A breaker is on top. Then there are two columns of breakers under it. I've tried Googling around to read up on the different types of setups, but mostly searches for "end-feed setup" and the similar don't lead to clear answers. If you can point me in the right direction in how to tell what I have now that would be really helpful. To be clear, I would hire an electrician to do any final connections to the main panel (so don't be afraid that I'll do something stupid).

[foo1bar]

QUOTE=kronic24601;n334737]So, the inverter: https://enphase.com/sites/default/fi..._EN_US_web.pdf states it's a 240 W (continuous power). But as you have stated 1/2 my panels will have a 15° tilt west facing (the other 1/2 will be SW facing). So I was assuming that the clipping would be minimal-to-none with my setup.

You are correct that it's a 72-cell panel. Oddly the price of a 72-cell panel is equal to or less than the 300 watt 60-cell equivalent. I of course thought, hey great. Less panels AND less inverters. Win-win. [/quote]

Not really a win-win IMO.
a 72-cell 345W is equivalent to a 285W 60-cell.
And your inverter is limited to 240W or 253W - so why pay extra for a 345W module?
Plus you have to deal with 72-cell modules (60-cell are big enough - I would not do 72-cell myself)
Oh, and you'll need to check your racking requirements - I don't know how that changes for 72-cell vs. 60. Probably very similar, but I'd check.

I don't know what clipping would be in your setup - You can put in your information (orientation, 345W, etc) into pvwatts, get the hourly output from it, and see whether that 345W module is producing >240Wh in any given hour in that projection. And if so, how many kwh is being lost due to clipping. Since you have two different orientations, you'll have to do that twice - once for each orientation.

When you divided by the inverter size, would you use the "Peak Output Power" number (253 W) or the "Rated (continuous) output power" (240 W)?

Don't know.

200A breaker is on top. Then there are two columns of breakers under it. I've tried Googling around to read up on the different types of setups, but mostly searches for "end-feed setup" and the similar don't lead to clear answers. If you can point me in the right direction in how to tell what I have now that would be really helpful.

That is almost certainly an end-feed setup being fed from the top. (an electrician could remove the front and visually see the bus bar to confirm it. If there's a model number on a sticker on it you can look up that model number and find out that way)

Is there space at the bottom (farthest away from the 200A main) for a 30A or 40A breaker?
Is there an easy way to connect to the box? (ex. a knock-out that can connect a conduit to) I would guess you'll run conduit from it to a new box for combining your solar outputs. If the main breaker panel is inside, then maybe instead of conduit it'll be romex going out through the knockout and over to the combiner.

[kronic24601]

Thanks again. This is a lot of great info and I'll go back to the drawing board a bit on my concepts.

As for my panel, it's totally full so I'm going to have to make some room doubling up a few of the standard breakers to make room for a new double poll breaker. It's on an external wall, and I have access to it from behind (garage), so pretty straightforward there. I was planning on combining everything in the service disconnect box. Originally it would just be 2 strings combined. Not sure if there is a better way to do this with a microinverter setup (I know that String inverters have more required here).

[foo1bar]

Thanks again. This is a lot of great info and I'll go back to the drawing board a bit on my concepts.

As for my panel, it's totally full so I'm going to have to make some room doubling up a few of the standard breakers to make room for a new double poll breaker. It's on an external wall, and I have access to it from behind (garage), so pretty straightforward there. I was planning on combining everything in the service disconnect box. Originally it would just be 2 strings combined. Not sure if there is a better way to do this with a microinverter setup (I know that String inverters have more required here).

This microinverter setup would have *more* required.
I believe that with two microinverter strings you have to combine the strings. Which means a box with a breaker for each leg - then a breaker for the combination of the two strings.
A string inverter would have that combination of the strings in the inverter, and the output from the inverter would just go to a single breaker. (Possibly you'd need an AC disconnect between the inverter and the main panel- depends on the AHJ)

I was planning on combining everything in the service disconnect box.

Is there a fuse or breaker in the service disconnect box? I think maybe what you're calling the service disconnect is actually the main breaker panel, and your house panel is actually a sub-panel. (Hard to say without knowing more) It's likely you could put the solar's backfeed in either place - your electrician can probably figure out which is cheaper and/or easier to get AHJ to approve.