Ratted wattage of solar panels

[Rodneyheaney]

Very Simple question.. About wattage rating of solar panels,,,,,

I’ve bought numerous solar panels on eBay over the years...
None ever produce anything like the rated wattage....
I’m from Australia I bought a more expensive one recently from an Australian provider.
The provider claims his 300w pane will only produce max 16.6 amps into my 12v battery system...
and I should be happy with my tested 15amps (200w) results I get.
i connected a dc watt meter and the best I can get prior to (and after) going through an mppt solar controller or charger is 200w (maybe about 13v * 15 amps )....
They claim it’s about the Pmax which is 19v and how everyone rates there panels...
This means that only can produce max 16.6 amps and i only get that at my batteries voltage which means that in perfect conditions I’ll only get maybe 200 - 215 watts max from my panel....
. (and if a 19v battery existed and I had a charger for it it would get the full 300w) he also states charging a 24 v battery (as my mppt can charge a 24v battery) it would not be any better performance as It wouldn’t get the 300w as it has to step up the voltage...

Am I stupid is it really correct a 300w panel with a pmax of 19 will only put up to around 200w (13v 15 amps). Into my 12v battery??

Also is it correct when Will is referring to 100w panels connecting to 12v systems only put in say 70-80w at say 12-13v. ?????

 

[Bud Martin]

The MPPT SCC (Solar Charge Controller) is basically a efficient Buck Converter that Converter high PV panel Voltage Low current into lower Voltage and higher current to charge the battery, if the battery is getting full or fully charge then the battery will pull less current from the Panel/SCC, you will also have conversion and system loss. The panel rating is done in a lab with controlled environment and calibrated equipment.
What does the SCC show (Power, current, etc) if you add the load to your system?

 

[mikefitz]

connected a dc watt meter and the best I can get prior to (and after) going through an mppt solar controller or charger is 200w (maybe about 13v * 15 amps )....

I suspect your controller is not MPPT if the input and output current readings are the same, a PWM controller would show these results.

Your panel is rated 300 watts, max current 16.6 amps ( this value may be the SC current) at a panel output of 19 volts, the numbers suggest its a 300 watt panel.

A MPPT controller, which is a power converter, with 15A input at 19 volts should produce around 20 amps at 13 volts.

In practice unless its a cold and sunny location, the panel temperature will reduce the panel power output over 25 deg C, approximately 4% per 10 deg C. Its not unusial to get a loss of over 10%.

Your supplier needs to read up on MPPT controller performance, with a 24v battery system, a single 300 watt 19v solar panel would not have enough voltage to run the controller. A MPPT controller needs a voltage input higher than the battery voltage to function.

Mike

 

[Substrate]

Sounds about right. Understand that "STC" or standard test conditions measurements are made in a lab where the panels are flashed by a test rig. You on the other hand out in the real world may be different depending ...

The math is simple - for a "nominal 12v" panel, the actual voltage measured with no load on it will be somewhere between 16 - 21v. (You need to be at least 2v higher than a battery terminal voltage to do actual charging!) So using 18v as the reference is pretty common.

300 watts / 18v = 16.666 amps.

This is your standard P/I*E formula. (Ever see those electrical pie-charts?) where:

P = watts / power
I = current
E = voltage

But you may not be under lab-perfect test conditions. If you were to test it, the best way to approximate it is to test with:

1) Sun directly overhead at around noon.
2) No haze, overcast, or clouds.

And what has thrown some off during testing is that unless you are doing a panel short-circuit test (don't, unless you know how), is that the battery needs to be discharged somewhat. In other words, it can't be nearly fully charged, (say make it less than 80% full), where the natural affects of the CC algo limits current input.

There's more, but since you are not in summer like I am, perhaps wait until summer arrives, put the panel on a table facing straight up at noon on a very clear day, and use a battery that has had a good discharge on it so that it will accept full current.

I'll bet you that you'll see close to the "rated" stc sticker values.

 

[gfmucci]

I have to remember that not everyone who posts a solar panel test review on Youtube knows what they're doing.

I recently saw one review that claimed he could not get more than 50% of rated wattage out of the Ecoflow 160 watt panel under "ideal" conditions. I need to take another look at his review, but I wonder if his input device was discharged enough so that it didn't throttle back the input wattage charge rate.

 

[Substrate]

Yep - pretty common mistake first time out the gate by testing with nearly full batteries.

Or the classic of trying to measure the output of an SCC with no battery attached and getting no output.

The others are small things like using 100 feet of speaker wire, using SAE automotive connectors, putting the SCC on the back of the panel instead of putting it no more than 6 feet away from the batteries, things like that. Vendors will tell them stuff like that is ok, and like Fox Mulder, they "want to believe" and are taken aback when it just ain't so.

 

[Drago]

Very Simple question.. About wattage rating of solar panels,,,,,

I’ve bought numerous solar panels on eBay over the years...
None ever produce anything like the rated wattage....
I’m from Australia I bought a more expensive one recently from an Australian provider.
The provider claims his 300w pane will only produce max 16.6 amps into my 12v battery system...
and I should be happy with my tested 15amps (200w) results I get.
i connected a dc watt meter and the best I can get prior to (and after) going through an mppt solar controller or charger is 200w (maybe about 13v * 15 amps )....
They claim it’s about the Pmax which is 19v and how everyone rates there panels...
This means that only can produce max 16.6 amps and i only get that at my batteries voltage which means that in perfect conditions I’ll only get maybe 200 - 215 watts max from my panel....
. (and if a 19v battery existed and I had a charger for it it would get the full 300w) he also states charging a 24 v battery (as my mppt can charge a 24v battery) it would not be any better performance as It wouldn’t get the 300w as it has to step up the voltage...

Am I stupid is it really correct a 300w panel with a pmax of 19 will only put up to around 200w (13v 15 amps). Into my 12v battery??

Also is it correct when Will is referring to 100w panels connecting to 12v systems only put in say 70-80w at say 12-13v. ?????

A many manufacturers declare a false solar panel power - that means theoretical values: max Voltage without load x max shortcut Current. When we talk about usable solar output power we expect max voltage at full possible current at solar panel output. But in practice, as the load increasing the output voltage decreasing what means that 100 W panel output gives for example 14,5 V at current 4 Amps under ideal solar conditions. That is 58 W only at controller input. If you use an MPPT controller you will be able get about 52 W at battery input, but if you use PWM you will get about 44 W.

 

[wattmatters]

A 300W solar panel that absorbs 8 hours of sunlight per day will generate nearly 2.5 kilowatt hours per day. Hence, if we multiply this by 365 days annually, we acquire a solar output of roughly 900 kWh per year. Put simply, each solar panel will supply 900 kWh yearly.

In Australia (the OP's location) the average energy yield from each W(peak) of north facing fixed solar PV is ~4Wh/day, provided of course there is always a sufficient load for the panel to supply. Yield is a bit higher on average the further north and inland you go, and a bit lower the further south and coastal you get. But as a ROT that's about what you can expect in Australia.

Single/dual axis tracker systems can of course yield even more energy.

From 1 x 300W panel, provided there are no current restrictions (i.e. there is always a sufficient load to supply), then you should expect an annual yield of around 0.3kW x 4kWh/kW.day x 365days/year = 438kWh/year.

But if the battery gets filled by middle of the day and there are no other substantial loads, well the PV energy production will be significantly curtailed.