How many watts from PV on cloudy day

nschizzano

It's 7:00 PM here in Minnesota, April 29th. It's relatively well lit out side, but full thick clouds. Like pretty heavy winter clouds. How many watts should my 2, 12v 100w PV's produce? It cycles from 0w to 1w to 2w, does that seem correct?

BB.

In general... The formula I use for sunny weather is:

PV Watts Rating of panel (standard conditions) * 0.77 panel+controller derating = typical "best case" watts

On a average sunny day (maybe a few clouds in the sky, etc.)--It is not unusual to see a derating of 0.50 (50%) before I would be worrying about electrical contacts, panel integrity, etc. Our eyes are not good at estimating sunlight--And we can bearly see the difference of a factor of 2x difference (I.e., 100% full sun, and 50% half sun)... Just the way our eyes and the physics works for us humans.

You can use a current meter on a "shorted solar panel" (or get a small solar cell)--And the current will be proportional to the amount of sunlight (I.e., 100% of Isc--short circuit current--Is approximately full sun or 1,000 W/sq-meter). 

Or if you have (for example) two solar panels--Connect one panel directly to a 10 Amp current DMM (assuming your Isc is less than 10 amps)--And measure one panel, then the second panels (within minutes or seconds of measuring the first)--And both both readings should be pretty close to matching (if the panels are good, and you are seeing "reasonable" Isc readings).

Monitoring the Watts (or array current) works great for Grid Tied Solar Power systems--They are setup to supply 100% of the available solar energy (volts*amps=watts) of the solar array. For battery based systems, you are at the mercy of the battery bank state of charge, how much current the solar charge controller "thinks" the battery bank needs, your DC loads on the battery bus, etc.)... If the battery bank is below ~80% state of charge (for Flooded Cell Lead acid batteries), the battery bank will take a lot of current... For 90-95%+ state of charge, the battery bank will naturally limit its current draw.

For "dark cloudy weather", I have seen as low as 5% of array current/Watts rating (just as dark storm clouds are overhead) for my home GT System.

Just for the heck of it, I used PV Watts to calculate the available array Wattage for a typical April 29th day for Minneapolis Minnesota with the array tilted to 45 degrees and facing south... This is for a random day (seems to be sunny--Average day for 20+ years):

https://pvwatts.nrel.gov/pvwatts.php (PV Watts has a spread sheet output for average power per hour per day of year)

Month	Day	Hour of day	Forget Row	Watts from Array
4	29	0	0	0
4	29	1	0	0
4	29	2	0	0
4	29	3	0	0
4	29	4	0	0
4	29	5	16.43	1.643
4	29	6	123.708	12.3708
4	29	7	477.244	47.7244
4	29	8	877.571	87.7571
4	29	9	1185.87	118.587
4	29	10	1382.288	138.2288
4	29	11	1489.481	148.9481
4	29	12	1264.875	126.4875
4	29	13	928.797	92.8797
4	29	14	726.257	72.6257
4	29	15	750.801	75.0801
4	29	16	586.284	58.6284
4	29	17	208.328	20.8328
4	29	18	43.465	4.3465
4	29	19	0	0
4	29	20	0	0
4	29	21	0	0
4	29	22	0	0
4	29	23	0	0

So for a 200 Watt array, would expect around 20.8 Watts average for 5-6PM (not the "forget this row" is left over from spread sheet as NREL PVWatts only allows a minimum of 1 kWatt array--Those are for a 2,000 Watt array, the last column is 1/10 or 200 Watt array output estimate). This is for a GT Solar system--No battery/load "issues").

For 7pm (19:00 hours), very close to "zero harvest" for a south facing 45 Degree Tilt array....

-Bill

nschizzano

In general... The formula I use for sunny weather is:

PV Watts Rating of panel (standard conditions) * 0.77 panel+controller derating = typical "best case" watts

On a average sunny day (maybe a few clouds in the sky, etc.)--It is not unusual to see a derating of 0.50 (50%) before I would be worrying about electrical contacts, panel integrity, etc. Our eyes are not good at estimating sunlight--And we can bearly see the difference of a factor of 2x difference (I.e., 100% full sun, and 50% half sun)... Just the way our eyes and the physics works for us humans.

You can use a current meter on a "shorted solar panel" (or get a small solar cell)--And the current will be proportional to the amount of sunlight (I.e., 100% of Isc--short circuit current--Is approximately full sun or 1,000 W/sq-meter). 

Or if you have (for example) two solar panels--Connect one panel directly to a 10 Amp current DMM (assuming your Isc is less than 10 amps)--And measure one panel, then the second panels (within minutes or seconds of measuring the first)--And both both readings should be pretty close to matching (if the panels are good, and you are seeing "reasonable" Isc readings).

Monitoring the Watts (or array current) works great for Grid Tied Solar Power systems--They are setup to supply 100% of the available solar energy (volts*amps=watts) of the solar array. For battery based systems, you are at the mercy of the battery bank state of charge, how much current the solar charge controller "thinks" the battery bank needs, your DC loads on the battery bus, etc.)... If the battery bank is below ~80% state of charge (for Flooded Cell Lead acid batteries), the battery bank will take a lot of current... For 90-95%+ state of charge, the battery bank will naturally limit its current draw.

For "dark cloudy weather", I have seen as low as 5% of array current/Watts rating (just as dark storm clouds are overhead) for my home GT System.

Just for the heck of it, I used PV Watts to calculate the available array Wattage for a typical April 29th day for Minneapolis Minnesota with the array tilted to 45 degrees and facing south... This is for a random day (seems to be sunny--Average day for 20+ years):

https://pvwatts.nrel.gov/pvwatts.php (PV Watts has a spread sheet output for average power per hour per day of year)

Month	Day	Hour of day	Forget Row	Watts from Array
4	29	0	0	0
4	29	1	0	0
4	29	2	0	0
4	29	3	0	0
4	29	4	0	0
4	29	5	16.43	1.643
4	29	6	123.708	12.3708
4	29	7	477.244	47.7244
4	29	8	877.571	87.7571
4	29	9	1185.87	118.587
4	29	10	1382.288	138.2288
4	29	11	1489.481	148.9481
4	29	12	1264.875	126.4875
4	29	13	928.797	92.8797
4	29	14	726.257	72.6257
4	29	15	750.801	75.0801
4	29	16	586.284	58.6284
4	29	17	208.328	20.8328
4	29	18	43.465	4.3465
4	29	19	0	0
4	29	20	0	0
4	29	21	0	0
4	29	22	0	0
4	29	23	0	0

So for a 200 Watt array, would expect around 20.8 Watts average for 5-6PM (not the "forget this row" is left over from spread sheet as NREL PVWatts only allows a minimum of 1 kWatt array--Those are for a 2,000 Watt array, the last column is 1/10 or 200 Watt array output estimate). This is for a GT Solar system--No battery/load "issues").

For 7pm (19:00 hours), very close to "zero harvest" for a south facing 45 Degree Tilt array....

-Bill

Fantastic answer 

Great idea I will just pop off the back plastic on these 12v 100 watt renogy and test with a multimeter to see amps and volts and compare to mppt to check?

True. I assume my LFP bat is low but I'm still learning what is 10percent on it, it was at 12.70. Assuming my mppt should give full wattage/ampage.

The chart was really cool thanks Bill. Pretty accurate too. It's Ottertail Minnesota, so a few hours north of MNPLS I wonder if it changed the figures that much. It seemed about an hour behind, I started tracking at 8am but these little clouds are interfering with the experiment, additionally our panels are almost straight up and down, its just what I did for low winter maintenance.

BB.

The "best"--Get an AC+DC current clamp DMM... You can measure the current in each panel/string and compare. A couple of examples of clamp meters:

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4

And into more details... Depending on the specifications for your MPPT solar charge controller, typically, the "optimum" for a pair of "12 volt solar panels" is to put the two in series for Vmp-array~35 VDC. (assuming your Vpanel max is >60 VDC).

Details matter here (MPPT controller specs, panel specs, min/max temperatures, etc.).

A 90 degree vertical array in summer is not very efficient--Would be better at your latitude (about 46 degrees for best year round harvest--Tilting to vertical in winter with snow is very common--Watch that snow does not build up behind panels on roof, and that panels are high enough off ground to not build up a snow bank in front either).

If your two panels are in parallel, the MPPT controller is not going to help much (Vmp-array is pretty low).

I am not sure what you want to do... Simply short out the panel with a DMM set to 10 amp scale on the rear of the panels, or break a connection and connect a 10 amp meter in series, or what?

Imp (and Isc) are proportional to the amount of solar energy hitting the panels... But you also need a "load" that can sink that current at Vmp-array. That is what the MPPT controller does--It figures out the Vmp-array every few minutes.

At this point, you are looking for "close enough" numbers to see if the setup is working approximately correctly. With off grid system (I.e., battery systems), it gets confusing with battery state of charge, any DC loadings, what the MPPT charge controller thinks the battery bank "needs", etc..

If you are going to get into solar--The AC+DC current clamp DMM is really the way to go... Also very handy to check out the charging system on a car (just did that a couple days ago for my sister-in-law whose battery died in the parking lot).

The neat thing about Current Clamp Meters is they are very easy to use and very safe (no cutting/disconnecting wires, exposed to high voltage/current, etc.). Just clamp across the one wire at a time to measure the current through that wire.

-Bill

mahendra

The worst I have seen in my area is 20% from my panels on a really cloudy or rainy day.(I usually had about 7to 14days of this in an average year).
but 50% seems like the the years stick.
i just add panels to compensate .

faostephen

The first thing to confirm is if you are demamding enough wattage from the panels.

Your inverter will show you what you are requesting from the PV, not all that the PV gives. This happens mostly when your batteries are fully charged and you have low / np load. So, if you want the inverter to show all the PV gives, you have to load it enough to start using your batteries, then you can see what you are getting from the panels from the inverter readings.

If you are not willing to load your batteries, the only other way is to get a multimeter and take your voltage and current readings directly from the panels.

Solar panels are more effective when it is bright and cool than when it is all sunny and hot. We all tend to reduce load on 'not so sunny' days but you might be dazed how much PV power you are wasting when it is bright.

On such an evening as you described (hard to really put a number on that, just imaginations), I get only about 3% of the full rating of my panel array. Applying this to your rating, it means about 3W. Doesn't seem way off from your reality.

Bright evenings, however, are more tricky and less effective.