NREL PVWatts Calculator Accuracy

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AllPawDrive
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NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 8:38 am

Hello all,

Does anyone have experience using the NREL PVWatts Calculator https://pvwatts.nrel.gov/?

What’s your experience when comparing the projected PV power output with your actual array power output? Is it pretty accurate? Overly optimistic?

Do you have a PV power calculator you feel is more accurate?

Thanks,

Dave

raysun
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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 9:20 am

None of the calculators were particularly effective at predicting my solar harvest, but I'm in a unique locale and that's not accounted for in the models.

For me, NREL's calculator uses data from about 60 miles away and on the other side of 9700 cubic miles of mountain mass. Safe to say there may be some variation in results.

It also calculates much more AC power from PV harvest than our logs indicate. It not that the model is wrong, it's just that we don't use as much energy as the model assumes. One factor for us is being off-grid, we don't "sell back" to a power company.

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Re: NREL PVWatts Calculator Accuracy

Post by provo » Thu Aug 06, 2020 10:02 am

raysun wrote:
Thu Aug 06, 2020 9:20 am

One factor for us is being off-grid, we don't "sell back" to a power company.
That's my main problem with it -- the assumption you're pulling the maximum possible watts out of your system every minute of the day! I like it for comparing different array elevations and azimuths. Then it's off by a constant multiplier anyway.

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Re: NREL PVWatts Calculator Accuracy

Post by JRHill » Thu Aug 06, 2020 10:05 am

Seems there are plenty of cases of over or under sizing the string(s). Maybe start with Outback's tool and then plug the results into the NREL tool? Maybe start with Outback's tool and then plug the results into the NREL tool?

http://www.outbackpower.com/resources/t ... izing-tool

For example in my case being new to many of the system components, I had no way of factoring in battery options and FLA was the best choice in the day. I made a best guess at what battery would fill the bill after estimating the number of panels, and an appropriate inverter/charge controller. All worked great when everything was new. Four things have changed in the last 10 years:
- the panels have decreased in generation a bit;
- I've gone through two battery banks and the second is at end of life now;
- Battery options have changed, and;
- Generator options have changed.

Dang, why? And what are the priorities in the above? Well, that's a whole different story. But there are things that come back to bite 'ya despite best efforts and learning maintenance patterns. Suffice it to say I did not understand the proper charging AND DISCHARGING of a quality FLA battery. Next would be generator selection and as with battery habits and options, a lot has changed in the last ten years.

Bottom line is this forum is your edge. I doubt you are going to do anything that hasn't been dealt with before.

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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 10:42 am

raysun wrote:
Thu Aug 06, 2020 9:20 am
One factor for us is being off-grid, we don't "sell back" to a power company.
provo wrote:
Thu Aug 06, 2020 10:02 am
That's my main problem with it -- the assumption you're pulling the maximum possible watts out of your system every minute of the day!
I see. Thanks for that tip.

I hadn’t factored in full batteries with no where to store PV power in off grid situations when looking at the PVWatts calculator.

I’d imagine that as battery capacity relative to PV array size increases, the closer the PVWatts model would be accurate for an off grid application.

I’d originally planned to use only a propane on demand water heater, but reading some of the threads on this forum, doing some calcs, has convinced me to store some of the extra power in the summer by using an electric water heater seasonally. Should help bring my %usage closer to the grid tied model.

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Re: NREL PVWatts Calculator Accuracy

Post by JRHill » Thu Aug 06, 2020 10:53 am

AllPawDrive wrote:
Thu Aug 06, 2020 10:42 am
raysun wrote:
Thu Aug 06, 2020 9:20 am
One factor for us is being off-grid, we don't "sell back" to a power company.
provo wrote:
Thu Aug 06, 2020 10:02 am
That's my main problem with it -- the assumption you're pulling the maximum possible watts out of your system every minute of the day!
I see. Thanks for that tip.

I hadn’t factored in full batteries with no where to store PV power in off grid situations when looking at the PVWatts calculator.

I’d imagine that as battery capacity relative to PV array size increases, the closer the PVWatts model would be accurate for an off grid application.

I’d originally planned to use only a propane on demand water heater, but reading some of the threads on this forum, doing some calcs, has convinced me to store some of the extra power in the summer by using an electric water heater seasonally. Should help bring my %usage closer to the grid tied model.
That's a good observation. A little detail from the above is that I can hit the FLA batteries with a good charge rate. But my discharge rate is really light. But you are still designing. It may not be a consideration depending on the battery type.

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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 10:58 am

I have a 6kW array and a 14kW battery.

If my battery size was nearer my array size, the figures would be worse.

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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 11:00 am

JRHill wrote:
Thu Aug 06, 2020 10:53 am
That's a good observation. A little detail from the above is that I can hit the FLA batteries with a good charge rate. But my discharge rate is really light. But you are still designing. It may not be a consideration depending on the battery type.
I’ve pretty much settled on lithium iron phosphate batteries.

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Re: NREL PVWatts Calculator Accuracy

Post by provo » Thu Aug 06, 2020 11:16 am

AllPawDrive wrote:
Thu Aug 06, 2020 10:42 am

I hadn’t factored in full batteries with no where to store PV power in off grid situations when looking at the PVWatts calculator.

I’d imagine that as battery capacity relative to PV array size increases, the closer the PVWatts model would be accurate for an off grid application.

In theory, but I doubt many folks here have enough to store all the kWh's their panels could produce in a day (OTOH, some have too much battery!) That's what's great about being grid-tied -- they take anything you don't need! In an off-grid system, you want to get a good charge every day after pulling the batteries down a healthy amount (Li-ion doesn't care about that much, AFAIK), and you want to cover all your daily loads. Panels beyond that are wasted, and can actually cause damage to your system.

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Re: NREL PVWatts Calculator Accuracy

Post by JRHill » Thu Aug 06, 2020 11:23 am

raysun wrote:
Thu Aug 06, 2020 10:58 am
I have a 6kW array and a 14kW battery.

If my battery size was nearer my array size, the figures would be worse.
Missed my point, buddy. Of course you are correct. The recharge rate is very important. But discharge rate is also important. That's why I thought the resistance electric hot water heater would be a good idea as long as he had the ability to recover. Again, depending on the battery type.

AllPaw, you are going Lithium so this is a mute point.
Last edited by JRHill on Thu Aug 06, 2020 11:27 am, edited 1 time in total.

AllPawDrive
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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 11:24 am

provo wrote:
Thu Aug 06, 2020 11:16 am
In an off-grid system, you want to get a good charge every day after pulling the batteries down a healthy amount (Li-ion doesn't care about that much, AFAIK), and you want to cover all your daily loads. Panels beyond that are wasted, and can actually cause damage to your system.
Yes, given my array seasonal output variation, I’m coming around to the idea of somewhat over sizing my array for the summer and under sizing it for the winter. I’m planning on a quality diesel generator back up so I’ll just need to run it a bit more in the winter.

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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 11:26 am

AllPawDrive wrote:
Thu Aug 06, 2020 11:00 am
JRHill wrote:
Thu Aug 06, 2020 10:53 am
That's a good observation. A little detail from the above is that I can hit the FLA batteries with a good charge rate. But my discharge rate is really light. But you are still designing. It may not be a consideration depending on the battery type.
I’ve pretty much settled on lithium iron phosphate batteries.
"Hey dad, I passed my driver's test! Can I wreck the Ferrari instead of wrecking the Volvo?" 🤣🤣

You have confidence making the first battery an expensive LiFePo4.

On the plus side, Lithium is simpler to charge (sort of). On the minus side, Lithium is way more "twitchy" than Lead Acid.

If sticking with one of the major battery vendors, then there will at least be published charging parameters. What there won't be is a track record of how those parameters translate into cycle and service life, the batteries are all too new.

Having said that, many of us are switching from LA to Li. We're all hoping, in the long run, that it's a good choice.

Were I commissioning a brand new system as a brand new experience, I might consider hedging my bets at the outset, and buy a "throw away" lead acid battery to cut my teeth on.

That "if I only knew then what I know now" refrain will be less painful that way.

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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 11:44 am

Certainly some good advice, Raysun.

I’ll keep my eyes open for some “hoopty” batteries to start off with.

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Re: NREL PVWatts Calculator Accuracy

Post by provo » Thu Aug 06, 2020 11:47 am

AllPawDrive wrote:
Thu Aug 06, 2020 11:24 am

Yes, given my array seasonal output variation, I’m coming around to the idea of somewhat over sizing my array for the summer and under sizing it for the winter. I’m planning on a quality diesel generator back up so I’ll just need to run it a bit more in the winter.
Have you made a detailed assessment of your typical daily power consumption in each season? For me personally, that's the part I was most wrong about in my original system design.

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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 11:55 am

AllPawDrive wrote:
Thu Aug 06, 2020 11:44 am
Certainly some good advice, Raysun.

I’ll keep my eyes open for some “hoopty” batteries to start off with.
I tend to learn by denting things, it seems. With a battery the "Oh sh**" moment is better accommodated with less cash outlay.

Just my $0.02

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Re: NREL PVWatts Calculator Accuracy

Post by JRHill » Thu Aug 06, 2020 11:56 am

AllPawDrive wrote:
Thu Aug 06, 2020 11:44 am
Certainly some good advice, Raysun.

I’ll keep my eyes open for some “hoopty” batteries to start off with.
As has been stated in other threads the warranties for L/I are for much further in the future than the companies have existed. If publically offered you might want to buy some stock in them. New technology is really wonderful. When my OB equipment supports them I'll go to it. Thanks to all of you who are participating in the R&D.

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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 11:59 am

provo wrote:
Thu Aug 06, 2020 11:47 am

Have you made a detailed assessment of your typical daily power consumption in each season? For me personally, that's the part I was most wrong about in my original system design.
Yes, generated a summer, winter and critical load daily power consumption profile using the https://www.altestore.com/ load calculator.

I’d consider my current version a second draft. As I refine it and make it more detailed, I’ll be able to reduce my “fudge” factor.

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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 12:11 pm

JRHill wrote:
Thu Aug 06, 2020 11:56 am
AllPawDrive wrote:
Thu Aug 06, 2020 11:44 am
Certainly some good advice, Raysun.

I’ll keep my eyes open for some “hoopty” batteries to start off with.
As has been stated in other threads the warranties for L/I are for much further in the future than the companies have existed. If publically offered you might want to buy some stock in them. New technology is really wonderful. When my OB equipment supports them I'll go to it. Thanks to all of you who are participating in the R&D.
From my personal experience (your experience may vary)

I learned a long time ago in the tech sector: never buy the 1.0 release of expensive gadgets.

I learned more recently: never be the 1.0 release on expensive gadgets.

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Re: NREL PVWatts Calculator Accuracy

Post by AllPawDrive » Thu Aug 06, 2020 12:13 pm

raysun wrote:
Thu Aug 06, 2020 11:55 am

I tend to learn by denting things, it seems. With a battery the "Oh sh**" moment is better accommodated with less cash outlay.

Just my $0.02
I’m actually considering setting up a “lab” version of my off grid build in my current on grid home. To make those mistakes earlier rather than later.

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Re: NREL PVWatts Calculator Accuracy

Post by raysun » Thu Aug 06, 2020 12:18 pm

AllPawDrive wrote:
Thu Aug 06, 2020 12:13 pm
raysun wrote:
Thu Aug 06, 2020 11:55 am

I tend to learn by denting things, it seems. With a battery the "Oh sh**" moment is better accommodated with less cash outlay.

Just my $0.02
I’m actually considering setting up a “lab” version of my off grid build in my current on grid home. To make those mistakes earlier rather than later.
If you have the luxury of time and space for that, then it would be a great learning experience. If the system doesn't need to service anything but test loads,then the bench battery could be fairly small. So then, could the test array.

The devil is in the details on this stuff. Seeing the system perform, and scratching one's head over why its behaving as it is, is the principal activity of most all 'hands on' operators.

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Re: NREL PVWatts Calculator Accuracy

Post by pss » Thu Aug 06, 2020 4:30 pm

I love this discussion thread. In my own case, I have an Outback system with Radian, FM charge controllers, batteries, mate, etc. And my second system is a grid tied with a couple of Sunnyboy's on it. It has been my experience that watt for watt, the Sunnyboy system can dump watts into the grid much more easily than the Outback system can charge, store and then convert the DC to AC and up the voltage too. The Outback is working harder and by my own crude observations and calculations, this harder work is roughly 20% to do the job. So 1000 watts of PV from Sunnyboy to my greedtility is equal to about 800 watts from Outback being sent to the grid, even though 1000 watts was produced and lost in the resistance/heat equation. BUT, and this is a big BUT, Outback stores my power for when I want it! And there is a cost of doing business like that, one I readily accept. I value my PV myself, the greedtility sucks my other PV into their lair and tells me whet it is worth to them. Big difference.

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Re: NREL PVWatts Calculator Accuracy

Post by JRHill » Thu Aug 06, 2020 6:23 pm

'greedtility' I like that.

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