70amp solar on a 200 amp main

[jaggedben]

I'm just not seeing how this could be considered a continuous (3 hour or more) ac current flow in a grid tie system without batteries. If each day we are only getting 3 solar hours in the course of say 8 to 10 hours of daylight that means the inverter is only going to operating at max current on the AC side for a brief period and the rest of the day at considerably less output.

I don't know why you keep insisting that 3 hours is relevant. 1) That's super unrealistically low for most solar systems in the summer. I question the figure. 2) Average sunlight is irrelevant. From a design a safety standpoint, you essentially need to size for the output that the system is going to have on the sunniest day in its life.

 

[Joethemechanic]

I don't know why you keep insisting that 3 hours is relevant. 1) That's super unrealistically low for most solar systems in the summer. I question the figure. 2) Average sunlight is irrelevant. From a design a safety standpoint, you essentially need to size for the output that the system is going to have on the sunniest day in its life.

Ok, I just checked, Average summer solar hours is 4.44, so that means a one kilowatt solar panel will produce on average 4.44 kilowatt hours of energy in a day. Am I correct?

Maybe a mean average is the wrong way of looking at it for electrical calculations. So for how many hours will a solar panel in my area produce it's max rated output on the best day of the year?

 

[electrofelon]

Ok, I just checked, Average summer solar hours is 4.44, so that means a one kilowatt solar panel will produce on average 4.44 kilowatt hours of energy in a day. Am I correct?

Maybe a mean average is the wrong way of looking at it for electrical calculations. So for how many hours will a solar panel in my area produce it's max rated output on the best day of the year?

Did you see post #36? Whether the panels are putting out their full output is generally irrelevant to the inverter output.

 

[Joethemechanic]

Did you see post #36? Whether the panels are putting out their full output is generally irrelevant to the inverter output.

But how can an inverter produce more than what is coming in? They aren't magic. One kilowatt in means somewhat less than one kilowatt out due to entropy

 

[jaggedben]

Ok, I just checked, Average summer solar hours is 4.44, so that means a one kilowatt solar panel will produce on average 4.44 kilowatt hours of energy in a day. Am I correct?

Source? That still seems low to me for summer total sun hours. That might be a 'peak hours' number, meaning a 1kW solar panel will produce about 1kW for 4.4 hours, and produce more total during the same.

From experience, I know that 7kWh per kW per day is common for resi solar systems in good conditions.

Maybe a mean average is the wrong way of looking at it for electrical calculations. So for how many hours will a solar panel in my area produce it's max rated output on the best day of the year?

Yes, mean average is definitely the wrong way of looking at it. I just looked at my own solar system for last month as an example: highest day was 27% higher than mean average. The longer a time period you look at for the mean average, the farther it's going to be from the max. We're interested in the maximum, not thr average.

The other thing (which has already been mentioned) is that most solar arrays are oversized to their inverters, sometimes by a large factor (1.5 to 2 even). So the number of hours that the inverter runs at max output is greater than the number of hours that the panel would output at/near peak if inverter nameplate were higher.

 

[jaggedben]

But how can an inverter produce more than what is coming in? They aren't magic. One kilowatt in means somewhat less than one kilowatt out due to entropy

What's coming in can be a lot more than the inverter's rating. The code goes by inverter rating because that's the max that is possible regardless of what's connected to it.

 

[Dennis Alwon]

I didn't read it all but if possible you can change the main breaker as we did. Took out the 200 amp breaker and installed a 175 amp breaker.

That will allow a 60 amp breaker or if the buss is rated 225 then you have 90 amps available

I assume there is no "next size up rule"

 

[jaggedben]

...

I assume there is no "next size up rule"

No, the next size up rule essentially still applies. For the 120% rule, 705 uses '125% of power source output circuit current', not overcurrent device rating. As long as the breaker protects the wire per 240, the breaker can be bigger than the allowed backfeed. This also means for example that if the solar is fed through a subpanel then the subpanel feeder can be larger than the solar output.

 

[Joethemechanic]

What's coming in can be a lot more than the inverter's rating. The code goes by inverter rating because that's the max that is possible regardless of what's connected to it.

I don't think you understood me.

One kilowatt in means somewhat less than one kilowatt out

The output of the inverter will always be less than the input from the panels. It's not a magic box that breaks the laws of thermodynamics

 

[ggunn]

Ok, I just checked, Average summer solar hours is 4.44, so that means a one kilowatt solar panel will produce on average 4.44 kilowatt hours of energy in a day. Am I correct?

Maybe a mean average is the wrong way of looking at it for electrical calculations. So for how many hours will a solar panel in my area produce it's max rated output on the best day of the year?

None of that makes any difference; the NEC requires the AC side of a PV system to be designed to the maximum nameplate output current of the inverter(s) and it be considered continuous. There are no exceptions for low DC:AC ratio and/or hours of sunlight.

 

[ggunn]

None of that makes any difference; the NEC requires the AC side of a PV system to be designed to the maximum nameplate output current of the inverter(s) and it be considered continuous. There are no exceptions for low DC:AC ratio and/or hours of sunlight.

Case in point: I had a small commercial PV system to design with SolarEdge inversion which would have fit into an SE40KUS inverter. We could not get an SE40KUS, all we could get was an SE80KUS. I asked the AHJ if we could wire it as a 40kW inverter if we only connected DC to one of the two 40kW blocks of the SE80KUS.

No.

 

[jaggedben]

I don't think you understood me.



The output of the inverter will always be less than the input from the panels. It's not a magic box that breaks the laws of thermodynamics

Okay, 'potential' input from panels.
Let me know if my post #45 didn't address your questions.

 

[Joethemechanic]

None of that makes any difference; the NEC requires the AC side of a PV system to be designed to the maximum nameplate output current of the inverter(s) and it be considered continuous. There are no exceptions for low DC:AC ratio and/or hours of sunlight.

I understand you have to follow the code. I'm trying to understand the reasoning behind it. Maybe all the guys who wrote the code for solar live in Arizona and think it's the same way everywhere

 

[Joethemechanic]

most solar arrays are oversized to their inverters,

Why? are the inverters that expensive?

 

[jaggedben]

Why? are the inverters that expensive?

In a nutshell, yes. Adding more panels is cheaper these days.

Why design a system with any clipping of PV production?

It is pretty common to oversize the PV array in relation to the inverter capacity. This often results in some amount of "clipping" at the highest output days. I don't understand why you'd want to clip at all when the inverter size can be chosen to handle the highest output of the PV array...

forums.mikeholt.com

 

[ggunn]

I understand you have to follow the code. I'm trying to understand the reasoning behind it. Maybe all the guys who wrote the code for solar live in Arizona and think it's the same way everywhere

I think it's more that systems must be designed to be able to handle statistical outliers with a uniform set of rules. If everyone tried to scale their AC sizing based on what they expected the output current to be, coupled with the driving force to save money, there would be a lot of problems out there.

 

[jaggedben]

I understand you have to follow the code. I'm trying to understand the reasoning behind it. Maybe all the guys who wrote the code for solar live in Arizona and think it's the same way everywhere

The essential thing here is, inverter output is a fixed variable that you can write a short and clear code rule around. Solar insolation is not.

 

[ggunn]

Why? are the inverters that expensive?

Yes, DC capacity is significantly cheaper than AC capacity

 

[winnie]

The output of the inverter will always be less than the input from the panels. It's not a magic box that breaks the laws of thermodynamics

If I understand the issue you are discussing, you are saying that 1 kW of solar panel will only produce 1kW of electricity for a very short portion of the day. Further you state that for every 1kW of DC entering the inverter, less than 1kW of AC will exit the inverter. You are then using this to claim that the inverter output will be at its peak for only a very short portion of the day.

The first point is correct. Unless your panels are on a tracker, they will only produce peak rated power for a small portion of the day.

The second point is also correct. Inverter efficiency is always less than unity.

But the claim about inverter output relative to its peak is not correct because it misses a key factor: inverter DC to AC ratio.

It is very common for the DC rating of the panels to significantly exceed the AC rating of the inverter. Inverters have separate ratings for maximum DC panel capacity and maximum AC output, with the DC rating greatly exceeding the AC rating. When the AC limit is met, the inverter adjusts the DC voltage so that the panels are not operating at their maximum power point. The reason for doing this is to maximize the energy production vs the total cost of the system; inverters are not free so it makes sense to loose some of the potential production of the panels in order to better use the investment in the inverter.

But what this means is that the inverter will be running at maximum AC capacity for a greater portion of the day than the panels are running.

To take an extreme case, imagine a 10kW PV array connected to a 5kW inverter. The inverter will be at maximum output once the panels are at 50% of peak insolation.

More realistically the DC:AC ratio will be in the range of 1.2-1.5.

-Jon

 

[Joethemechanic]

I had no idea the inverters were the limiting factor. I never looked much at solar installations. Probably because all of the solar installers I've met around here are not doing your industry's reputation any favors. Little to no knowledge of electrical theory, and giving customers the impression that one kilowatt of panel is going to be producing one kilowatt from dawn until dusk.