I’ve executed the Forecast.solar API to calculate the solar forecast of today and it returns: 6.6kWh.
The Victron VRM platform is estimating the following forecasting:
It is estimate somewhere between 20 to 24 kWh. This is about 3 time higher as the Forecast.solar does. Why is ther such a big difference? What am I doing wrong?
6.6kw left?
That would take you to 20kw (adding to your already produced 14kw) in the vrm forcast range.
No, I don’t believe that that is the case. I’ve updated the time to 0:00 so that it would calculate for the entire day. The number doesn’t change. The API I’m using is: “Get estimated watt hours sum per day” so it is for the entire day in my view.
And one source is from Victron VRM and the other source is from Forecast.solar !
Do you have more than one mppt in the system?
Maybe it is total for one device?
Well, currently I’ve one MPPT in the system. Will be expanded with two extra MPPTs in about a weeks time.
The Victron VRM system is based on the current/history information and the Forecast.solar API is executed based on the same amount of panels, same location.
Today the Victron VRM forecast is 10 to 12 kWh while the Forecast.solar API gives me 5.6 kWh. So way off compared to the VRM forecast. And the actual until now (14:38) is already 8 kWh.
I’m doing something wrong in the API call, not sure what.
I don’t know that you are. i set up mine and it is throwing out some figures i don’t agree with.
Sorry, I don’t understand your response. Can you elaborate a bit more on this? Can you clarify?
Is your azimuth, angle and everything else correct?
South is 0° so north is 180° (on the api)
They want you to set that to the ° when the sun is vertical on your panels.
Yes, to my understanding it is. The 0 in the URL is for the south facing panels. The azimuth I’ve not specified in the URL as it doesn’t matter that much. I’ve played around with it but the number doesn’t change that much. So the effect is minimum on the outcome.
It matters. It changes it alot.
It definitely predicts significantly smaller yields than VRM.
Maybe it needs time to settle. I will run for a while and see.
Sorry, the Azimuth is included, is it the 15 in the URL.
FOr 5 I get: 2025-05-28 6694
For 15 I get: 2025-05-28 6682
For 20 I get 2025-05-28 6636
For 50 I get 2025-05-28 5843
Conclusion, there is difference of course, but it is not major.
Victron VRM platform is predicting 11 to 14kWh for today. So double the amount compared to Forecast.Solar.
Just thought I would chime in here, i am also seeing a massive difference between the victron solar forecast and the Forecast solar forecast, so what I have done is ad an adjustment factor into the node red code, so what happens is that the system will learn the under value (or over value) of the forecast solar forecast, and then calculate an adjustment factor based on the actual generation for the day, it obviously learns this over several days, and adjusts the forecast based on the actual hourly generation, that should alter what is reported from forecast solar.
I hope that makes sense.
So for today we generated 55kWh, and the forecast was 50 - 61kWh
My forecast solar display (fetched from Forecast.Solar) reads 22.50 kWh (this is for both my arrays, the victron system reads it as one total, so the same in effect)
And you can see the forecast confidence in the top left corner, its 50% at the moment, which means no adustment, thats because I have been playing around with the code today and reset the adjustment, but that confidence will adjust based on the actual generation, and that will then adjust the forecast.
That, over several days, should give a far better and more accurate forecast figure closer to the VRM number.
