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How Much Surplus Solar Power Do You Actually Have - And How to Find Out!


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With the average and reasonably well planned solar setup, there should be an excess of solar power on a reasonable to well lit day.

This actual "excess" power is hard to determine and goes to waste, once batteries are full, the fridge is cold, etc etc etc, the system then starts drawing less from the panels via one means or another, waiting for load conditions to change... How do you know when, and how much, can be loaded onto your inverter at this point? Remember, this "point" that I refer to, is a dynamic point - This is what makes it tricky - Nasty things like clouds, mist, dust, sun angle, to name but a few, move our available solar target around quite a bit.... Now add to this, the deep freezer cutting in at seemingly random periods, the fridge playing games with the inverter whenever someone visits for a "cold one", and the problem actually starts to get quite "tricky"....

But, it is actually quite simple to see what you have available at any given time - You have to throw a bit of hardware at it, unfortunately, but we are not talking about a big pile of lightning soluble silicon - It is, at best, 3 small components (or sets thereof) that you need in addition to each bank of oriented panels - ie, N panels all at same inclination, 1 set components, E panels at a particular inclination, another 1 set of components, and so on ....

Disclaimer: Note the Forum Title - Home Automation - This post assumes you have some form of home automation in place and working, and you are monitoring and controlling your household in some way already....

The components needed per Bank of Panels:

1xSonoff Basic Flashed with Tasmota, 1xADS1115 Ad Converter hooked to it, 2x Resistors to act as a voltage divider, 1x Tiny little solar cell, match your Resistors to divide the voltage to slightly less than the maximum acceptable input on the ADS1115.

Hook the above together, (There are plenty of internet tutorials to do it.) and get the numbers feeding back from the Sonoff/ADS onto your wifi network and into your logging system...

Next, put the little "metering panel" in the least advantaged spot on your array of choice(Last lit, first shadowed), parallel with, and at the same inclination of the panels themselves - this is important!

Now start watching the numbers, recording them if you can... There is just going to be too much data to do it properly on the fly, but it isnt that complex...

As the sun strikes the metering panel, you will get digital values back from the metering sonoff corresponding to the solar intensity available - At any given time, these numbers indicate the actual solar available - Lets say the metering numbers max out to 28532 on a crystal clear day with no clouds(This is your max nominal value).  Later in the day, you check it out and the metering number is now 12555... That means that at that point in time, under those specific atmospheric conditions, you are getting 12555/28532, ie around 50% of the maximum available power for that particular meter, and as such, for that particular bank of panels that it is associated with...

Does it work? Definitely yes...

Is it accurate? Reasonably so, I have been running my system with one of these on the main bank for around a year and it has allowed me to "forecast" what I can load onto the inverter very nicely.

Calibration: This can be tricky and requires patience - Well worth doing to get the best out of your system that you can.

To do the calibration, you need to bear in mind that it is best done when the batteries are fairly low in SOC and not being charged at maximum allowable current as set by your inverter parameters - This means that all available power from the panels is being used at that particular moment in time.

Look at the power being delivered by your panels - Lets say its 3450W - Look at the numbers from the solar meter - Lets say it is, for example, 18505 -

Now, open your calculator and do some maths - 18505/28532 = 0.65 ie 65% of total max solar availability - We also know now that 65% of total availability is 3450W... therefore the max we would nominally get from the bank is around 100/65 *3450 ie 5307W .... Now look at the spec of your mppt/inverter .... Although the panels can supply the power, will the inverter or charger limit what is available - If so, use the lowest limit under these circumstances...

Real World Application:

The sun's shining, the batteries are full, the beer is cold.... The panels are putting out 800W,.... It's midday... There are no clouds... The metering system says 26222...

This means that at that particular moment in time, the solar power availability is: 26222/28532 ie 0.92 or 92%... The power that is available from the panel bank is .92 x 5307, ie 4877W .... But hey, our inverter is only capable of 4500W, so in total, we have 4500 less the 800 we are already using.... So we have an additional 3.7Kw we can draw for something useful.... Time to automate your geyser, aircon, fish pond, pool pump, irrigation system, grey water recycling.... And let it use the power that would otherwise just be ignored.....

This is NOT a theoretical scenario, I am running my system in this manner and I have found that my solar harvest has dramatically improved by doing so... To do the switching manually, however, is not practical, it does need a home auto system to get the best out of it.... Especially with those pesky clouds......

Cheerz

Ed

Here is a section of graph as pulled 5 mins ago...

Legend:

Orange: Unused Solar/Blue: Used Solar/Red: Inverter Output/Purple: Inverter ACVA In/Light Blue line-Metering Estimate

now.jpg

Edited by EdDee
Legend and graph
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  • 3 weeks later...
2 hours ago, Sc00bs said:

Thanks for the great explanation of your solution @EdDee your project answers one of my many solar questions and is something I am going to have to build 🙂 

Pleasure.... Just went from Pb to Li Batteries this weekend.... This solar "forecasting" method is still doing its thing, very well in fact, not switching things in as much with the Li as with the Pb... But still useful nevertheless!!

The graphs I was using on the Pb now look a little "disconcerting" with the Li... The Li can soak up the "excess" power for charging way better... I was very gentle on the Pb, limiting charge and DOD on them to keep them healthy as possible for as long as possible... They are topping 2 to 3years now, still about an average of 80% health when checking them with a battery analyzer (28x638SMF in 7 strings of 4 giving about 490Ah total).... They have already found a good home and have gone a fair way into recovering the ZAR for the crossgrade... (Up yours, Eskom and ZAR!!) ....

Cheers

E

 

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