Array Oversizing

nwdiver · Feb 27, 2015

I'm helping a friend with what's probably going to be a 15.6kW array tied to a 11kW inverter. That's an oversize ratio of ~1.4. That seems to be the sweet spot with module prices <$1/w. I'm curious as to how common this practice has become... Array Oversizing. UL1741 inverters are tested to a ratio of 2.

FlasherZ · Feb 27, 2015

I think most of it is anticipating how long you'll be operating at peak.

One of my arrays is 30x300 (9 kW, pointed S @ 175 deg) fed into 8 kW inverter. The second array is 16x315 (5.04 kW, pointed SE @ 140 deg) fed into a 4 kW inverter. The third array is 8x315 (2.52 kW, pointed E @ 85 deg) fed into a 2 kW inverter. The fourth array is 7x250 (1.75 kW, pointed SW @ 215 deg) fed into a 2 kW inverter. I've tried to keep them at 1.25 or below.

There are times during the summer day when the first array will reach 8400 W generation for about an hour and you can hear the fans blazing on the SMA 8kW inverter. I can't imagine running a higher oversubscription for several hours without shortening the life of the inverter.

ChadS · Feb 27, 2015

My second system (installed in December) was 4.86kW into a 4kW inverter, so just over 1.2. I didn't pick the numbers (not enough knowledge), the solar company did.

nwdiver · Feb 27, 2015

FlasherZ said:
There are times during the summer day when the first array will reach 8400 W generation for about an hour and you can hear the fans blazing on the SMA 8kW inverter. I can't imagine running a higher oversubscription for several hours without shortening the life of the inverter.

The inverter doesn't dissipate the extra power... it increases the input voltage lowering the current, this simply lowers the power being pulled from the array. I believe the UL1741 limit of 2 is based on a fault condition. If there are no faults you should be able to tie an 8MW array to an 8kW inverter... as long as the input voltage is <600v. You would just be at 8kW from sunrise to sunset :wink:

RichardC · Feb 27, 2015

Unless you have diverse panel orientation, or a feed-in tariff limit or other regulatory constraints, I don't understand why you would go above 1.2 ratio. As reflected in the following (from the panels that first cleared themselves of snow), even a 1.2 ratio is leaving a significant number of electrons on the roof on sunny days.

nwdiver · Feb 28, 2015

RichardC said:
Unless you have diverse panel orientation, or a feed-in tariff limit or other regulatory constraints, I don't understand why you would go above 1.2 ratio. As reflected in the following (from the panels that first cleared themselves of snow), even a 1.2 ratio is leaving a significant number of electrons on the roof on sunny days.

There are multiple reasons to oversize your array... utility restrictions are probably the most common.

I was surprised how little is lost on an annual basis. For example... 14kW array tied to a 10kW inverter will produce ~23367kWh/yr. Upgrading to an 11kW array will yield ~250kWh more per year. It's ~$300 more for that extra kW.... sure it's only $300... but you're only making an extra ~$25/yr... that's a 12 year payback.

Why supersize?

FlasherZ · Feb 28, 2015

nwdiver said:
The inverter doesn't dissipate the extra power... it increases the input voltage lowering the current, this simply lowers the power being pulled from the array. I believe the UL1741 limit of 2 is based on a fault condition. If there are no faults you should be able to tie an 8MW array to an 8kW inverter... as long as the input voltage is <600v. You would just be at 8kW from sunrise to sunset :wink:

Sure - sorry, didn't mean to imply that it sank the power; instead, meant to imply that running at 100% load for many hours per day was likely to have an impact on the lifespan. The fans running are a side-effect of running the hardware at 100%, not to cool sinks. To Richard's point, flat-spotting the curve will lose some production, but I'm looking for more longevity from the equipment.

nwdiver · Feb 28, 2015

FlasherZ said:
Sure - sorry, didn't mean to imply that it sank the power; instead, meant to imply that running at 100% load for many hours per day was likely to have an impact on the lifespan. The fans running are a side-effect of running the hardware at 100%, not to cool sinks. To Richard's point, flat-spotting the curve will lose some production, but I'm looking for more longevity from the equipment.

It's really heat not power that degrades the electronics inside an inverter... I realize that more power = more heat but that pales in comparison to environmental factors. An inverter operating with an ambient temp of 20C & 70kWh/day will probably live longer than one operating with an ambient temp of 23C & 50kWh/day. Most inverters are 96 - 98% efficient. @ 10kW that's only 200 - 400w of heat. The actual 'guts' of my inverter are probably <5% of it's mass... discounting the transformer >50% of the weight is thermal mass. Dissipating heat appears to have been the top design priority.

It would be interesting to see a geographic representation of MTBF for inverters... I would expect there to be a strong correlation between temperature and failure.

neroden · Mar 7, 2015

nwdiver said:
There are multiple reasons to oversize your array... utility restrictions are probably the most common.

Well, I guess THAT makes sense. But barring "trouble from the utility company", it seems odd to me. My inclination would be to get all the power I can and sell the excess (if the utility wants it) or store it in batteries (if the utility doesn't). But I suppose you could put the battery storage on the DC side of the inverter. Actually you probably should, thinking about it....

nwdiver · Dec 30, 2020

If you're triggered by oversized arrays... look away.

It’s time to start wasting solar energy

'Last year, the 49 MW Tobacco Valley Solar in Connecticut began operation with a 1.84 inverter ratio (meaning it produces 84% more electricity than its connection to the grid can output during peak production).'

iPlug · Dec 31, 2020

Economics and grid needs are making this a better argument every year. Less peak electricity when it is not needed and more on the early morning and later afternoon shoulders when it is needed makes sense.

SageBrush · Dec 31, 2020

nwdiver said:
If you're triggered by oversized arrays... look away.

Funny headline

I've mostly come around to your view when the marginal inverter upsizing is expensive and the panels are cheap. Do you think that situation will last ?
Utility scale is a different animal because they often deal with a grid connection bottleneck. In that case the profit optimization is to keep the pipe full.

I would not be quick to say that what is good for utility scale PV is good for residential.

nwdiver · Dec 31, 2020

SageBrush said:
Do you think that situation will last ?

I expect it to only accelerate. I can see a time when it will cost you money to export for a few hours mid-day. The wholesale price of electricity already goes negative occasionally near noon in places with high levels of solar penetration. This will probably get worse. Oversizing arrays will probably get a lot more popular if clipping noon output could actually save money or at the very least that lost production is next to worthless anyway.

mspohr · Dec 31, 2020

My older arrays are undersized. Newer installations are oversized (~20%) since panels are so cheap now.

SageBrush · Dec 31, 2020

From our friends at Renvu

If I was putting up 6 or more kW, I think the largest of these inverters would be an easy choice.
Going by your earlier post that undersizing 1 kW loses 250 kWh a year, that is 3,750 kWh over a 15 year inverter lifetime. If the marginal inverter cost is $100 to collect that energy you end up paying 2.66 cents a kWh

iPlug · Dec 31, 2020

As an example, if it costs roughly the same to get 25% more solar panels installed as increasing inverter power capacity 25%, the former option is much better. More kWh net generated and more importantly, even more generated at times when it is more valuable to the grid. Properly priced TOU plans would encourage that at the residential level.

Understand this example is a huge exaggeration and inverter costs still net and incremental increases are substantially below that at this time.

mspohr · Dec 31, 2020

SageBrush said:
From our friends at Renvu

View attachment 622930

If I was putting up 6 or more kW, I think the largest inverter would be an easy choice.

That's a choice I made on one of my arrays. Since the extra cost for the larger SMA inverter was only $100, I went with it even though I didn't need the capacity at the time. Gives me the option to add another string in the future for just the cost of the panels.

SageBrush · Dec 31, 2020

By the way, PVWatts lets you set the DC:AC ratio in the advanced settings. Default is 1.2:1

nwdiver · Dec 31, 2020

SageBrush said:
From our friends at Renvu

View attachment 622930

If I was putting up 6 or more kW, I think the largest of these inverters would be an easy choice.
Going by your earlier post that undersizing 1 kW loses 250 kWh a year, that is 3,750 kWh over a 15 year inverter lifetime. If the marginal inverter cost is $100 to collect that energy you end up paying 2.66 cents a kWh

There's some 'step changes' you start running into. The first one is rather minor. Upgrading from ~3.8 to 6kW inverters is cheap. To get to 7.6kW requires a larger disconnect but that also doesn't cost too much. Over 7.6kW you need a 'line-side tap' which is why 7.6kW inverters are so common. The next cap is 48A or 11.4kW. More than that and you need a disconnect >60A which is uncommon for some reason.

The project we're working now is ~35kWdc and ~31kWac. We're using (4) 7.6kW inverters because as you point out the cost difference between a 6,7 and 7.6 is minor so might as well go with the 7.6. The interconnect we're using can support up to 160A so might as well use it. Check out the size of the AC disconnect you need to accommodate 128A on the AC side...

Screen Shot 2020-12-31 at 11.35.46 AM.png

So basically my design philosophy is sorta based around these step changes of 16, 32, 48, 80 and 160A.

SageBrush · Dec 31, 2020

nwdiver said:
Check out the size of the AC disconnect you need to accommodate 128A on the AC side...

LOL !
Looks to be about two Molly's

(apologies if I mangled her name.)

Array Oversizing

What's your solar DC/AC ratio?

<= 1

~1.2

~1.4

>= 1.6

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