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Dirk-Jan Faber (Victron Energy) posted Nov 29, '22

Node-RED: Fetch forecasted solar yield

As briefly shown during todays webinar, I've created a simple subflow that allows to easily fetch the predicted solar yield for a given location. I believe that this subflow can be very useful for determining how and when to charge your batteries.

Under the hood it uses the public API of https://forecast.solar/. That is free for 12 queries per ip per hour, but if you start using it, please consider creating a personal account.Configuration is straightforward:

All fields are quite self explanatory and the documentation of the modules also gives more information on each field. The api key is not required, but with it you will be able to query more often.

My subflow can be found on https://flows.nodered.org/flow/d83d3224f241ec4abf6f9f119bbee9cc. In order to get the flow into Node-RED, copy the flow from the linked page and import it into Node-RED (ctrl-i).

Of course there are alternatives out there:

- https://flows.nodered.org/flow/a1907502b7256a0e4bcf408fbee93411 uses solcast for doing the same thing. But that one requires a bit more manual adjusting and a (also free) registered API account.

forecast-solar.png (10.8 KiB)

forecast-solar-config.png (26.7 KiB)

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Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jun 22, 2023 at 07:40 AM

I have updated the flow with an example showing how to combine different solar arrays into a single graph.

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Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 09, 2023 at 03:11 PM

I've just updated the subflow to make it easy to generate a graph from it. All that is needed is to connect a line or bar chart to the second output of the subflow.

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nickdb ♦♦ Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 09, 2023 at 06:50 PM

I’ve been using the original subflow for a while now, just parsing it with a function to extract the forecast in kWh to make it easier to use for my purposes. It has been surprisingly accurate.

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wkirby ♦♦ Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 09, 2023 at 10:14 PM

Thanks for this. It seems to work pretty well.
I experiment with is some more. I wonder how well it will work long term for UK, it's hard to tell what the weather will be for the next 5 minutes let alone the next day!

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johnone commented · Jan 10, 2023 at 08:44 AM

Thanks for sharing this - obvs, forecasting is an important part of optimising self-consumption. First thing for me is exploring accuracy - yesterday's forecast accurately predicted the likely low yield today, given unrelenting rain (UK Midlands).

I have a 1770Wp free-standing, facing due south, array - will write a bit of python to collect the day's forecast at 1am (when E7 cheap rate starts) and, that evening, compare with the actual yield for the day. Will report back in a week.

Edit: sorry, should have said, going to use Solcast data as the forecasts are based on location, declination, azimuth/orientation AND weather (essential for the UK!). It seems the 'forecast solar' data is only based on location, declination, azimuth/orientation??

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nickdb ♦♦ johnone commented · Jan 10, 2023 at 09:51 AM

It takes weather into account, there is a separate estimate they provide called "clear sky" which gives the theoretical maximum.

From their site:

Restful API for Solar production forecast data and Weather forecast data

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Dirk-Jan Faber (Victron Energy) ♦♦ johnone commented · Jan 10, 2023 at 10:15 AM

According to https://forecast.solar/ : "The always weather based solar production forecast, historic averages and clear sky optimum provides watt powers and kilo watt energy data."

So that means that the weather is taken into account. Note that neither I or Victron have any affiliation with them. I just made the flow to use their api.

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johnone Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 10, 2023 at 10:22 AM

Thanks @Dirk-Jan Faber, it seems a little ambiguous to me. On the page you've linked to it says: "the current weather is thus not included in the calculation" (not is their bold). This is a critical issue for the UK. I think the bit @nickdb has very helpfully underlined refers to the option of subscribing to current weather data, in the 'Professional' package.

I'm not sure - is the 'forecast solar' data based on historic weather or current weather?

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nickdb ♦♦ johnone commented · Jan 10, 2023 at 10:29 AM

It definitely takes weather into account. I have been using this for some time on the public API and the forecasts have varied appropriately with weather, and not been too far off (considering we have lousy weather forecasts here).

I recently updated to the private plan for more calls mainly, where I noticed the clear sky option.

The pro plan gives detailed data about the actual weather but if you read their splurb below:

Solar production

(All plans)

The always weather based solar production forecast, historic averages and clear sky optimum provides watt powers and kilo watt energy data.

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johnone nickdb ♦♦ commented · Jan 10, 2023 at 10:53 AM

I emailed them for a definitive answer - it's not clear if the yield forecasts are based on historic weather data (which, here in the UK, is less useful given climate change) or if they are based on actual weather forecasts: i.e. 'awlays weather based' could mean based on historic data.

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johnone johnone commented · Jan 10, 2023 at 10:55 AM

Reply from forecast solar ...

Hi,

there are 2 parts used

- Historic data as the base for calculation

- Actual weather forecast for the location (with a possible offset because there are not so many stations around), here the cloud coverage factor and the temperature forecasts, the cloud coverage is the most important, the temperature is only relevant in hot summers when the modules can have > 70 °C which lowers the performance.

Regards

Knut

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nickdb ♦♦ johnone commented · Jan 10, 2023 at 11:01 AM

They seem to tap into wunderground. Fortunately we have no shortage of weather stations.

You'd think historic would be perfect for the UK - damp and dark every day ;)

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johnone nickdb ♦♦ commented · Jan 10, 2023 at 11:22 AM

Some days it's dark then damp. I too signed up to private for more calls - python requests and json proving a bit fiddly ...

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Dirk-Jan Faber (Victron Energy) ♦♦ johnone commented · Jan 10, 2023 at 11:24 AM

The professional account seems to add a weather forecast via the same API. See the example on http://doc.forecast.solar/doku.php?id=api:weather.

If the current weather predictions weren't taken into account, the 3 day estimate graph (from my home in NL) would not differ a lot on a daily basis:

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nickdb ♦♦ Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 10, 2023 at 11:35 AM

We are in a sunny spell, so it is correctly predicting some consistent yields.

The last few days it has been all over the show with cloud and showers.

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johnone commented · Jan 11, 2023 at 10:58 AM

Collecting hourly forecast.solar data in a db, but seeing big variation when doing an estimate/watthours/day/ call for my location and my little 45/0/1.77 array. In the last 3 hours:

{'2023-01-11': 3068, '2023-01-12': 829, '2023-01-13': 1472, '2023-01-14': 967}

{'2023-01-11': 2609, '2023-01-12': 824, '2023-01-13': 962, '2023-01-14': 1132}

{'2023-01-11': 1781, '2023-01-12': 946, '2023-01-13': 2230, '2023-01-14': 1051}

Yesterday the forecast for today went as high as 3303W - was wondering if I'd need sun cream, but no it's partly cloudy with rain on the way! Anyone else seeing this level of variation? Have I gotten something wrong??

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nickdb ♦♦ johnone commented · Jan 11, 2023 at 11:45 AM

Just make sure you have all the settings right, originally what threw me was the azimuth, North was -180 instead of 0 as I had expected. Being southern hemisphere our weather patterns are a tad different. I do see it sometimes being a little optimistic but it does seems to start correcting itself.

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johnone nickdb ♦♦ commented · Jan 11, 2023 at 03:38 PM

Thanks - it's all set-up ok. It's the wild variation from one hour to the next which is the problem. Today's 'Watt hours (energy)' forecast has gone from 3068W, down to 1781W, then back up to 3844W. On what is another rainy day with periods of heavy cloud, the current forecast is for 3546W from my little 1.77Wp array at 52 deg North! Cebo GX says total so far today is 700W at 15:30.

I'm trying to establish accuracy/reliability. I'm not that interested in 'it seems quite good' or 'it does seems to start correcting itself'. Instead, I'm looking at actual data. So far, it doesn't look good. I'll report back with a week's data - in the meantime, has anyone else seen wild variations? Has anyone else looked at accuracy/reliability?

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nickdb ♦♦ johnone commented · Jan 11, 2023 at 03:48 PM

You and I are probably on opposite extremes of the weather. I would imagine hourly forecasting in variable weather can be problematic. Would be interesting to compare to the other api.

Today I was forecast 35.1 kWh and I’ve used 32.5, granted some left unused but the temps were much hotter than forecast so I suspect they failed to calculate how much it would actually derate.

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wkirby ♦♦ johnone commented · Jan 11, 2023 at 08:58 PM

For me, Monday's and today's estimates were pretty close to what I actually got.
Tuesday's estimate was way off. My estimate was for 1.7kWh, but I actually got 600Wh. I wonder if the Forecastsolar people have any idea of just how dark and grim the British weather can actually get. A 1kW array is generating 1.7W at noon for example. Their idea of thick black cloud may less than Britain is actually capable of.
I'm recording each day's estimate and actual values to see if a reliable pattern can be formed.

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Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 11, 2023 at 03:55 PM

For the graph output there are some extra settings available:

Output in kWh - when checked output can be set to kWh instead of Wh
Show todays forecast - whether or not to include todays forecast
Days to forecast - the number of days to forecast (excluding today). Note that you can not get more days forecasted than your API key allows.
Widen graph - widen the graph to only show non-zero values

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nickdb ♦♦ Dirk-Jan Faber (Victron Energy) ♦♦ commented · Jan 12, 2023 at 01:09 PM

@Dirk-Jan Faber I think there is an error with the "estimate for the period" chart output, selecting the number of days to forecast doesn't seem to have an effect.

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Dirk-Jan Faber (Victron Energy) ♦♦ nickdb ♦♦ commented · Jan 16, 2023 at 03:11 PM

That should be fixed now.

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johnone commented · Jan 16, 2023 at 08:37 AM

Quick update on accuracy/reliability of forecast data. It's a good time to gauge accuracy as in the UK this morning there's a bog-standard Atlantic jet-stream produced weather system which has been accurately forecast - over the last 3-4 days - to arrive overnight. Currently, in the Midlands at 52 deg North it's overcast but clearing. Given yields over the last few weeks - with max sun altitude of 15deg, now 16 deg in mid-January - harvesting 1kW today looks very optimistic. Following data gathered by python script into a db:
Time of forecast / yield-for-the-day forecast for 1.77kWp free-standing, south facing array
Solcast
21:02 2448W
00:02 2783W
03:02 2794W
06:02 977W
forecast.solar
21:02 4196W
00:02 5112W
03:02 4129W
06:02 1501W

EDIT: actual yield for today: 960Wh (Pmax 1076W). Snowing at midday. Yes, Solcast was very accurate at 6:02 - of course, though, that's way too late for deciding whether or not to fully charge the battery in cheap rate period. 6:02 is more of an observation than a forecast!

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nickdb ♦♦ johnone commented · Jan 16, 2023 at 05:36 PM

I found it tends to overestimate, so I lowered the panel size slightly to where I was more confident in the forecast.

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johnone nickdb ♦♦ commented · Jan 16, 2023 at 06:50 PM

Yes, lots of examples where 6kWh is forecast for a day - figures only possible in our summer. The problem today, though, was both APIs not taking account of the weather forecast!

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johnone commented · Jan 17, 2023 at 10:27 AM

Another big clue to the inaccuracy - today is sunny (not cloudless, and started out misty). Max sun altitude for my location (52 deg N) today is 17 deg. Expected max yield for the 1.77kWp array is around 2.5kWh. Forecasts:
Solcast
21:02 6785Wh
00:02 7066Wh
03:02 7124Wh
06:02 6416Wh

forecast.solar

21:02 6398Wh
00:02 6417Wh
03:02 5905Wh
06:02 5645Wh

You'd have thought they would use the easily available 'clear-sky irradiance' data as a baseline, then factor in the weather. Here's a January daily 'average daily irradiance' graph for my location from PVGIS.

Clearly, 6-7kWh for my 1.77kWp array is simply not anywhere near possible in January - why don't the APIs use PVGIS data?

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krafty johnone commented · Mar 04, 2023 at 06:12 AM

I gave up fiddling about with 'forecast.solar' etc a while ago... I just tried it again (for today) and their forecast was out by 5 x.

Now I simply pull the (free) 3-hour cloud cover figures from openweathermap at 5am, average them, add ~20 and set that as my SOC target to hit before the cheap juice ends at 7am.

Last year, I had the various forecasts all feeding their (wildly!) varying figures into a db for a while for comparison. The method above emerged the clear winner. :D

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fish commented · Mar 04, 2023 at 04:50 PM

Dirk Thanks for the ideas, I've now got your bit and a Solcast one averaging and setting the charge levels each night for my kit dependant on the forecasts. It also emails the figures used each day... Cheers

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joolster fish commented · Mar 26, 2023 at 10:44 AM

Hi all,

What's your current best view regarding long term accuracy for solcast and forecast.solar?

From my calculations (and just for the last 3 days), it looks like both are +/- 30% (approx) on the real production values, and typically their forecast is optimistic.

Does this correlate with what you've been seeing?

If this is true then a sensible ESS off-peak charging scenario would be to take the solar forecast numbers, factor down to 75%, and subtract this from target SOC (obviously converting between kwh, battery capacity etc) and charge to this level off peak - this would minimise solar 'waste' (export to grid), maximise off-peak (lower cost charging) etc

What do you think?

Thx, Julian

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tomcat commented · Apr 07, 2023 at 01:37 PM

Hi, thanks for sharing.

Im relativly new to node red. Ist there a way to get the estimated production from doing the request to the end of the day?

If not I want to suggest it :)

Why im asking is, I try to consume as most as possible with different loads directly wihout touching the battery but still want to get the battery full at the end of the day.

So I need to know when to stop direct consumpten to get enough to get the battery full.

I know I could charge the battery first and start direct consumption after its full but my heat pump has high load from 4-6kw/h and Im running it whern sun deliveres enough to not touch the battery and in most cases the battery is not full when this period starts. I hope you understand what im trying to say achieve :D

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ronski tomcat commented · Apr 07, 2023 at 04:00 PM

I'm also very new to Node Red, and JSON, but I have done a bit of programming in the past so starting to pick it up.

What you're asking could be done, but it would be very inaccurate, mine is currently predicting 28.1kWh for today, it's 5pm and I've generated 41.23kWh so far - the weathers been great, pretty much as per the weather forecast, so not even a difficult day to predict.

I've logged a weeks worth of predicted values from the night before, and actual values generated, and they range from 20% to 50% more than estimated. I believe I have everything set correctly as well.

For me it's just an indication, a guide as such, but from Tuesday my game plan changes as I've moved to Octopus Flux.

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tomcat ronski commented · Apr 07, 2023 at 04:34 PM

Hm I was doing it manually for 2 weeks now. I charged to 60-70% turned the heat pump on and turened it off when remaing predicted solar power was enough to fill the battery up to 90-100%.

In 80% if my manuall tries I succeeded. The prediction of the whole day in the morning is not that accurate true but the next 2-4h is fine in most cases :D. For the cases where I get more than prediced I will install a 0-100% controlleable electrical heater into my buffer tank.

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