How much do your panels produce when it is overcast and raining?

[wattmatters]

What trips me out is how good the online chart is at predicting solar output for the given location and azimuth of the panels.

I mean it's literally dead to nuts correct.

It's optimistic for me as it has no capacity for fine tuning for various factors which impact production. It says we should have an annual generation of 16.148 MWh from our GT system.

I used Pylon Observer to get the projected long term performance but frankly the variance year to year in performance means they are no more than general indicators. Pylon Observer says an annual generation of 14.5 MWh.

This is our actual generation, can see substantial year to year variation, simply due to weather with both drought and flood years in there:



For short term forecasting (hours, days) of PV output I use Solcast, and have the API integrated into Home Assistant.

I use the forecast data to make calls on whether my off-grid PV may need supplemental battery charging assistance from the grid-tied PV, how much it needs and when to initiate it.



It's 7:45AM, dashboard shows battery at 68% SOC and needs 11.5 kWh to be fully charged (my battery cycles between 57% and 100%).

It also shows that my off-grid system today is forecast to have another 8.3 kWh of production remaining in the day, and since that's not enough on its own to complete a full charge, then the "Grid Charge Req'd" indicator is on and the automations handling supplemental charging are operating.

Can see it has (at that moment) assessed there is enough spare grid PV capacity to allow 20 A of supplemental charging. The EWMA (exponentially weighted moving average) Grid values tell it how much spare grid PV capacity there is (negative = exports) and so it adjusts the supplemental charge rate up and down to capture the excess while avoiding importing energy from the grid.

 

[Warpspeed]

No simple answer to all this at the planning stage.

If you have three crappy solar days in a row, more solar panels are very likely not going to help much.
If for example, you double up on panels, twice almost nothing is still almost nothing.

A bigger battery with four days of storage might (or might not) be a better solution.
But then you might get five crappy days in a row, and still run short.

All you can do is give it your best guess and see how it goes over at least one full year of operation.
Then try to figure out how to best and most economically upgrade the system.

If it all looks really terrible, do not overlook using a generator.
It may only need to run a very few times and for short periods through the very worst of mid winter, but if you can put up with a bit of inconvenience, it could save a lot of dollars on buying more very expensive batteries.

A couple of hours of charging now and again, might give you another 24 hours of extended battery capacity.

 

[wattmatters]

No simple answer to all this at the planning stage.

If you have three crappy solar days in a row, more solar panels are very likely not going to help much.

Our water heater is powered with a solar PV diverter. It uses the output from 1/3rd of our PV capacity, i.e. 3.6 kW. The water heater can store enough energy to get through consecutive poor days of production.

But how many days?

One might expect there may be a run of days with insufficient PV output for the water heater to recover enough energy (or worse store water at a temperature risking dangerous bacterial growth).

I have an automation to monitor that such that I can initiate a manual boost using grid power if necessary. So far I have not had to do that in the 18 months I've been running it.

We just don't often get a long run of consecutive days of poor PV output.

The most recent case we had was a run of poor output days in October 2022:
5.2 kWh
7.5 kWh
3.6 kWh
4.9 kWh
4.5 kWh
then
16.6 kWh

Considering the system needs to have a minimum export level before diverting, then about 2.5-3.0 kWh of production on those days is not diverted to the water heater. On average our water heater needs 5 kWh/day.

On that last day the system diverted 10.5 kWh and completed a full heat cycle, which is a bit over half its storage capacity. This means the tank was able to supply ample hot water for our needs despite the run of five consecutive poor output days.

But such instances are rare for us. On review of our daily data since October 2018 we've had just three instances of five consecutive poor days but never six consecutive days of poor output.

Someone much further from the equator though will more likely experience such events.

 

[wpns]

No simple answer to all this at the planning stage.

If you have three crappy solar days in a row, more solar panels are very likely not going to help much.
If for example, you double up on panels, twice almost nothing is still almost nothing.

A bigger battery with four days of storage might (or might not) be a better solution.
But then you might get five crappy days in a row, and still run short.

All you can do is give it your best guess and see how it goes over at least one full year of operation.
Then try to figure out how to best and most economically upgrade the system.

If it all looks really terrible, do not overlook using a generator.
It may only need to run a very few times and for short periods through the very worst of mid winter, but if you can put up with a bit of inconvenience, it could save a lot of dollars on buying more very expensive batteries.

A couple of hours of charging now and again, might give you another 24 hours of extended battery capacity.

This. There’s an economical solution that merges solar, battery, and generator to give you a working solution that will get you through most days without thinking about it but will allow some redundancy so the failure of one component won’t cause days of power loss.

I mean, I get that we are in this forum because we love to think about these things, but once I get my system running, I don’t want it to be my only thought between the time I wake up in the morning and the time I go to bed at night. I want to have done such a good job that I never _have_ to think about power or power bills or battery charge or insolation or anything else ever again. Yes, I will stare endlessly at graphs and invent SQL queries to determine how I’m doing, and my 42(!) daily status emails may grow by a few, but the power will always be on while I’m doing it.

Yeah, every once in a while I’ll have to hook up the generators if only to make sure they work, and there’ll be detailed instructions for operating the virtual transfer switch to re-enable the grid in case I get hit by a beer boat, but there’s more to (my) life than obsessing about power.

 

[Mattb4]

This. There’s an economical solution that merges solar, battery, and generator to give you a working solution that will get you through most days without thinking about it but will allow some redundancy so the failure of one component won’t cause days of power loss.

...

Yeah, but to come up with the solution where you don't have to think about it, you will have to have spent a great deal of thought in getting there. Which is pretty much one of the reasons for my Thread here. Perhaps at some point it will be all automatic, like self driving cars, the only thing to think about is style and color. No need to spend any thought on how all of it works.

 

[wpns]

Yeah, but to come up with the solution where you don't have to think about it, you will have to have spent a great deal of thought in getting there. Which is pretty much one of the reasons for my Thread here. Perhaps at some point it will be all automatic, like self driving cars, the only thing to think about is style and color. No need to spend any thought on how all of it works.

Sure, and that’s the fun part (for us). If you don’t have the skills to DIY you can always pay someone else, but that’ll be expensive and non-optimal (for you, it’ll be optimal for the turnkey installer ).

 

[Warpspeed]

Yeah, but to come up with the solution where you don't have to think about it, you will have to have spent a great deal of thought in getting there. Which is pretty much one of the reasons for my Thread here. Perhaps at some point it will be all automatic, like self driving cars, the only thing to think about is style and color. No need to spend any thought on how all of it works.

Spend not only a great deal of thought, but usually money as well.

All our circumstances are different, sometimes very different.
There is a definite limit on useful advice people can give, that can actually apply.
Its really all a giant learning curve, and it can all become quite an interesting and self satisfying adventure.

 

[Mattb4]

It is the first cromulent rainy day since I put my new Bifacial panel array into service. At 11am I would have to say there is no improvement over standard type panels when it comes to production. They are barely pulling in 26w out of 800w rating. A dismal 3%.

 

[JRH]

It is the first cromulent rainy day since I put my new Bifacial panel array into service. At 11am I would have to say there is no improvement over standard type panels when it comes to production. They are barely pulling in 26w out of 800w rating. A dismal 3%.

I think it may almost impossible to guess what a given cloud my produce …ya just have to take what ya get…
A few days back , it was heavy low clouds all day .. nasty ,rainy and cold… I was getting about 5 % all day..
The next day had basically identical conditions and was up to 10- 20 % all day .. even 2 hours longer than when the sun ususlly is behind the trees…

Beats me……J.

 

[seneysolar]

Fluke Solar Irradiance Meter

FREE shipping. The Fluke FLK-IRR1-SOL Solar Irradiance Kit comes with a meter, temperature probe and soft carrying case.

www.fluke.com

Something like this would be nice for comparison sake

 

[Brucey]

It is the first cromulent rainy day since I put my new Bifacial panel array into service. At 11am I would have to say there is no improvement over standard type panels when it comes to production. They are barely pulling in 26w out of 800w rating. A dismal 3%.

I have a surplus of available SCC amperage, going to try out a few of those 100W bifacials.

 

[wheelman55]

If you like to tinker, here’s a useful tool that will help site your array to maximize solar radiance at your site for any day of the year:

Solar Pathfinder - Solar Pathfinder

It’s good for use in areas with hills, tall buildings and trees. You can move the tool up a foot or more to see how much more production you can get. Same with moving sideways. It’s excellent to show you production differences for each month of the year at your site.

Using an irradiance meter along with the Solar Pathfinder and PV Watts can show you optimal array placement and tilt angle for maximum annual production.

A fun fact. The irradiance meter will show you that maximum solar production on overcast days happens when the modules are horizontal.

We use a single axis tracker for our array. When it’s overcast the modules are pointed face straight up (flat/horizontal) no matter what time of day.

Enjoy the journey.

 

[wattmatters]

This month's daily production and forecast by Solcast:

View attachment 200968

A full month example showing difference between good and not so good days: