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Max power limited by utility

VictorV

New Member
Joined
Dec 21, 2020
Messages
8
Hi,
I'm located in Idaho and our provider is Idaho Power (IP). They limit the system output to 25kw based on
the "label" on the inverter. I'm considering using micro-inverters so I believe IP would use the max output
of each inverter. Looking at the Enphase IQ 7 it's max output is 240 VA or 290 VA depending on model.

Questions:
1) Is the 240/290 the same as "watts"?
2) Do I want to select panels that are as close to the max inverter input value so that the max output is generated
and I don't "waste" available watts?
3) How do I determine what size panel would result in the max inverter output?

Here's a link to the Enphase datasheet. https://enphase.com/sites/default/files/downloads/support/IQ7-IQ7plus-DS-EN-US.pdf

Victor
 
The spec sheet says panels up to 350w and 440w respectively. Bigger is better because only about 2 hours per day can a panel do rated output without a sun tracker. I have 340w panels on my 300w micros....different brand.

You can translate VA to watts in this case. Technically it is a different unit but for this use it is close enough.
 
Since my system will be fixed at 20 deg tilt, the max panel output will be limited. If I use a 440w panel
and the inverter spec says the max output is 290 watts that means that 150 watts are
wasted for each panel. Is that a correct assumption? I'm also trying to find out if you need
a 440 watt panel to get the max 290 watt output or can you get the max output from a smaller panel.

The 25kw limit results in some interesting tradeoffs. For 290 watt max inverter output I can have 86
inverters and 38kw of panels (assuming 440 watt panels). Unfortunately I only have room for around
73 panels (depending on size).
 
Since my system will be fixed at 20 deg tilt, the max panel output will be limited. If I use a 440w panel
and the inverter spec says the max output is 290 watts that means that 150 watts are
wasted for each panel. Is that a correct assumption? I'm also trying to find out if you need
a 440 watt panel to get the max 290 watt output or can you get the max output from a smaller panel.

The 25kw limit results in some interesting tradeoffs. For 290 watt max inverter output I can have 86
inverters and 38kw of panels (assuming 440 watt panels). Unfortunately I only have room for around
73 panels (depending on size).
I suggest you spend a few minutes with: https://pvwatts.nrel.gov/
You can input your exact location, orientation and tilt. Then do a couple different scenarios, changing the panel size and number ( DC system size in PVWatts terminology). Since you are assuming fixed ac size ( 290 watts/ panel) then you need to enter the correct dc to ac ratio in the advanced parameters. (440 / 290 = 1.52). It will then tell you the kWh per year for that configuration. The whole process won’t take 10 minutes. Once you know the kwh for each choice then it is up to you to decide whether the extra production justifies the cost.
 
You can get 285w panels for $80 each and run them on the 240va inverters. 73 panels would put you at a 17520w array. Not your 25, but in the ball park. What are you going to do with that much power? :p
 
You can get 285w panels for $80 each and run them on the 240va inverters. 73 panels would put you at a 17520w array. Not your 25, but in the ball park. What are you going to do with that much power? :p
I have a fairly decent sized house and a small winery that has temp-controlled barrel room. Last year our usage was 44,000 kwh and our average bill was $400/month. We have a big power usage spike in Dec/Jan due to our HVAC but I'm not sure it's working correctly. Our lowest use
months are May and Oct at around 2800 kwh. Max was Dec at 6564 which seems like something is wrong. I'm in the process of measuring
what's being used with each of our circuits. Our HVAC consists of 2 heat pumps with electric heat backup. The electric backup is what's
using all the power in the winter. Before we buy a solar system we have to get our usage straightened out first.
 
Ouch. That's a crap load of power. My highest month ever was 1500kw and that was 100+ temps 111 days straight so the AC basically just stayed on. Do you have land you can ground mount to?
 
Ouch. That's a crap load of power. My highest month ever was 1500kw and that was 100+ temps 111 days straight so the AC basically just stayed on. Do you have land you can ground mount to?
We have 5 acres but not much available at this point. We looked into a 2-axis tracker but they are really ugly. We just finished building
a barn and we oriented it with the intention on putting solar on the roof. There's 1950 sq ft of roof that's south-facing so there should
be plenty of space. The primary issue at this point is that Idaho Power limits the system to 25kw.
 
I have a fairly decent sized house and a small winery that has temp-controlled barrel room. Last year our usage was 44,000 kwh and our average bill was $400/month. We have a big power usage spike in Dec/Jan due to our HVAC but I'm not sure it's working correctly. Our lowest use
months are May and Oct at around 2800 kwh. Max was Dec at 6564 which seems like something is wrong. I'm in the process of measuring
what's being used with each of our circuits. Our HVAC consists of 2 heat pumps with electric heat backup. The electric backup is what's
using all the power in the winter. Before we buy a solar system we have to get our usage straightened out first.
44,000 KWH per year is an average 121KWhper day.
25KW inverters can produce 150KWh per day... or more...
 
We have 5 acres but not much available at this point. We looked into a 2-axis tracker but they are really ugly. We just finished building
a barn and we oriented it with the intention on putting solar on the roof. There's 1950 sq ft of roof that's south-facing so there should
be plenty of space. The primary issue at this point is that Idaho Power limits the system to 25kw.
So if you only have room for about 73 panels and you use Enphase 290 VA converters, then that is 21.17 kw no mater which panels you pick. Idaho power is not the primary issue. your roof area constrains you. You can’t exceed Idaho’s limit.
 
Since my system will be fixed at 20 deg tilt, the max panel output will be limited. If I use a 440w panel
and the inverter spec says the max output is 290 watts that means that 150 watts are
wasted for each panel. Is that a correct assumption? I'm also trying to find out if you need
a 440 watt panel to get the max 290 watt output or can you get the max output from a smaller panel.

The 25kw limit results in some interesting tradeoffs. For 290 watt max inverter output I can have 86
inverters and 38kw of panels (assuming 440 watt panels). Unfortunately I only have room for around
73 panels (depending on size).
As the sun comes up, then overhead, then goes down, there is a bell curve power output that happens for any fixed panel. The panels only peak for an hour or two per day. Peak doesn't mean "make rated power", it means the peak of that curve. If the panels are hot from the sun, they make less than their rating even when the sun is at a perfect angle. Only when it is breezy and not real hot or when it is cold, can the panels make their rating and then it is only when the sun is at a good angle. So to think using a 440 watt panel on a 290 watt inverter "wastes" 150 watts is not so. What is so, is that you are more likely to make 290 watts for a lot longer (more minutes) in the day with a 440 watt panel than with a 300 watt panel. Again, the 300 watt panel will only make 300 watts rarely and only for a short period of time.
 
PV panels are pretty cheap, Microinverters cost more than string inverters (but not so much more if string requires RSD box per panel.)
So over-paneling can be useful

If the area calculation is correct that you can only fit enough panels for 21 kW, consider more smaller panels of highest efficiency to reach 25 kW of inverter. Rules on what area of roof has to be kept clear for firemen has changed over the years. Make sure your planned installation is compliant.

I take it this is the only location, only face of roof, only one orientation that can be used for PV?

If you might want battery backup, plan for it now by selecting equipment compatible with a battery inverter and affordable batteries.
e.g. Enphase offers a battery solution. SolarEdge inverter could instead have StorEdge installed up front (batteries can be added later.)
Several other brands to choose from. Best choice is based on what kind of loads you need to power, then cost, reliability, safety. Possibly peak-load shaving, if it is useful to game the time of use rates.
 
I don’t think fireman access rulings apply to a standalone garage or barn.
People do not live in it, firemen will not try to rescue anything in a fire.
I would love clarification if any is known.
 
If they have to enter it to put out a fire, they might want to vent the roof. So I would expect it to apply but don't know.
At least for commercial buildings, a "shed" roof with no ridge the fire marshal might approve a different venting method.
So I've pondered a re-roof, which would also give me twice the usable area. High windows that could be busted. If it is accepted in residential.

Maybe sprinklers in the barn would be a good idea.

California,


Looks like barn doesn't need it, and for house exception can be made if access not required.

"
Exceptions:

  1. Detached, nonhabitable structures, including but not limited to detached garages, parking shade structures, carports, solar trelisses, and similar structures, shall not be required to provide roof access.
  2. Roof access, pathways, and setbacks need not be provided where the fire code official has determined that rooftop operations will not be employed.
  3. These requirements shall not apply to roofs with slopes of 2 units vertical in 12 units horizontal (2: 12) or less.
"
 
If I use a 440w panel
and the inverter spec says the max output is 290 watts that means that 150 watts are
wasted for each panel. Is that a correct assumption?

No.

A fixed orientation panel will only produce at its rated output for a brief period during the middle of the day and only during times of year when the sun is at a suitable angle above the horizon and the sky is clear and sunny and the temperature is not hot. Mornings, afternoons and during the broad winter months the panel's output won't be anywhere near its rated output. Even with an ideal sun angle in summer, output will often be lower than peak rating due to heat derating.

The peak output rating of a panel is under a standard set of ideal environmental and insolation conditions. For the vast bulk of a panel's life it will output only a fraction of its peak power output.

Accordingly it is normal and good practice to "overclock" the panel's peak rated output relative to the inverter's rated output. Each inverter will have a specification on the peak DC input it can handle, so don't exceed that. In some places there may also be regulatory limits on the panel DC to inverter AC ratio. Enphase IQ7+, with a peak output of 295W, I think can accept panels rated up to 440W. That's a 1.5:1 DC:AC ratio and is pretty typical DC:AC limit for many solar PV inverters.

Of course there will be some nice clear sunny days in the summer months when the panel's output will be "clipped" by the inverter. Don't think of this as being "wasted", rather by having the panel capacity oversized means greater capacity utilisation of the inverter. This way the inverter is being used far more efficiently for longer periods of the day and year.
 
We have 5 acres but not much available at this point. We looked into a 2-axis tracker but they are really ugly. We just finished building
a barn and we oriented it with the intention on putting solar on the roof. There's 1950 sq ft of roof that's south-facing so there should
be plenty of space. The primary issue at this point is that Idaho Power limits the system to 25kw.
Just Don't grid tie the extension of the system. Get a few of the new Hybrid Inverters - and use the grid as backup. You can have infinite amounts of off-grid power. Depending on your local government you don't even need permits (check on that)

Florida limits me 10KW :p which is a joke

So I'm just building a grid supported -Off-Grid system for the whole farm. Run on solar most of the time, and use the grid when I got 3-4 days of little sunshine.
 
Just because your Big Power Co limits how much Grid Tie feed you can do does not limit what you can do for yourself. Quite a lot of Power Co's have older & degraded equipment which limits capacity both for intake & output and therefore may have no option but to limits input transmissions. This is not new BTW.

That being said, there is nothing stopping you from being able to export up to 25kW and then pushing the rest into a Battery Bank to use in your home... IF properly planned in advance with the right equipment, the design could be such that you could incrementally add battery packs and become more self-sufficient over time and peeling away from giving out power to Big Power Corporate. 25kW is a lot of juice that can charge a heap load of battery and provide a lot of stored energy.
 
44,000 kWh/year / 365 days/year = 120 kWh/day

20 kW of PV at optimum orientation could be a little bit short depending on orientation and location.
His orientation may be less than optimum. 25 kW would come closer.

If time of use rates peak late in the day when PV production is low, extra production in the morning could cover that.
 
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