Solar Panel Output

[CapnCrunch]

I'm still not sure that I fully understand the electrical specifics of solar panels, so could someone tell me what kind of charge rate (if thats the correct term) I can reasonably expect from the solar panel described below (mine are the 305MS version) under the following conditions:

12v system (6 FLA batteries factory rated at 122Ah capacity) at 40% discharge
Victron 100/30 MPPT
Clear day near the equator (panel not shaded)

[sailorboy1]

Couldn’t read the spec sheet. I get 90-120ah from my 290w panel on a good day. It depends on the clouds and whether I get any shading from the mast. So if yours is 305w you will get about the same.

Expected you will now get a bunch of confusing detail from people who want to use the tech talk.

[CapnCrunch]

Originally Posted by sailorboy1

Couldn’t read the spec sheet. I get 90-120ah from my 290w panel on a good day. It depends on the clouds and whether I get any shading from the mast. So if yours is 305w you will get about the same.

Expected you will now get a bunch of confusing detail from people who want to use the tech talk.

Did you click on the pic?

For some reason, I just can't seem to get a good handle on electrical/charging issues, so virtually ALL details are confusing to me. HOWEVER, I seems like I pick up some little detail from each new thread, so I'll keep plugging away at it.

I will be most interested in seeing the "math" behind each response, though.

[sailorboy1]

The “math” that matters is the number of amp-hours (ah) you use each day and what the solar can put in normally. Other than that the “math” moves and varies.

I have a 43’ boat and live on it full time. If anchored we will use around 100-140ah a day (frig and a few hours tv/ video biggest uses), so my 290w panel will do most of my use. Other people are going to reply and say they have 600-1000w solar and need it all.

There are so many threads here on the subject that you will speed your ability to understand by reading them instead of starting yet another threAd on it. Your question probably gs skied t lest monthly.

[CapnCrunch]

Originally Posted by sailorboy1

The “math” that matters is the number of amp-hours (ah) you use each day and what the solar can put in normally. Other than that the “math” moves and varies.

I have a 43’ boat and live on it full time. If anchored we will use around 100-140ah a day (frig and a few hours tv/ video biggest uses), so my 290w panel will do most of my use. Other people are going to reply and say they have 600-1000w solar and need it all.

There are so many threads here on the subject that you will speed your ability to understand by reading them instead of starting yet another threAd on it. Your question probably gs skied t lest monthly.

My question wasn't how much power I will use or need, but rather how much OUTPUT I could reasonably expect from this panel under optimal conditions. My uncertainty stems from the fact that it is a 32v panel charging a 12v system.

[sailorboy1]

I told you the expected OUTPUT

Now I wouldn’t tell you anything else. Good luck

[Paul Elliott]

That's a 305W panel, with a Vmp (voltage at maximum power) of 32.7V. It puts out about 9A. This is comfortably within the range of your Victron controller.

The controller will convert the panel output to charge your 12V battery. The full-sun charge current will be around 20 - 25 A (when the battery is low). Over a 24-hour period this will give you about 100Ah of charging capacity (assuming 20A max current, 5 hours / day average sun, panel facing straight up). Once the batteries get beyond the "bulk charge" stage, the charge current will taper off, so it will take longer to fully charge your battery than these numbers might indicate.

You will probably get less than 100Ah / day, depending on your latitude, cloud cover, and any shadows that fall on the panel.

[a64pilot]

Regardless of panel voltage etc, a decent rule of thumb is you will get 1/3 of a panels rated capacity in Watts, in AH.
So for my 1000 W of panels, I’ll get on average about a little over 300 AH.
Now this assumes you have a place to put it. If your getting that much power daily from your panels your batteries are not being fully charged, this is due to charge acceptance rate of the batteries.

The caveat to that is if your diverting some Solar power for other things like heating water maybe and not just charging your bank etc., you could still get all the available amps and fully charge your bank.


All of that boils down to Sailorboy is correct

[Mike OReilly]

400 watts. Higher latitude (~50 degree). My best day this season is 150 amp-hrs. Haven't crunched the number, but probably average ~100 amp-hrs.

[a64pilot]

I think he is asking what his max output current could be.
300W panel, max is 300W, voltage is irrelevant.
If you want it in amps, then voltage is relevant, but is simple math.
300 divided by whatever voltage you chose will equal amps at that voltage.
So for my absorption voltage of 14.3 V the amps at 300W is 21.
You’ll likely never see rated output of course

[CapnCrunch]

Originally Posted by a64pilot

All of that boils down to Sailorboy is correct

I wasnt disputing Sailorboy and certainly had no intention of pissing him off. I guess I muddied the waters by including the battery info. What I was really looking for (and maybe didnt express the question succinctly enough) was whether there is a "formula" that says that -under optimal conditions - a 305w panel with a Vmp of 32.7v and Imp of 9A charging a 12v system through a controller like mine should output (charge) at a rate of "X" A to the batteries. Or is the question stated correctly?

[sailorboy1]

watts = Voltage x amps

move the terms around as needed to get whatever you are looking for

[CapnCrunch]

Originally Posted by a64pilot

I think he is asking what his max output current could be.
300W panel, max is 300W, voltage is irrelevant.
If you want it in amps, then voltage is relevant, but is simple math.
300 divided by whatever voltage you chose will equal amps at that voltage.
So for my absorption voltage of 14.3 V the amps at 300W is 21.
You’ll likely never see rated output of course

That is exactly what I was looking for. I thought that was the formula but was unsure whether to use the rated Vmp for the panel, my nominal system voltage, my absorption voltage (14.2) or what.

I understand that I'm not likely to see rated output from the panels or the rated capacity of my batteries, but at least now I have a figure to work downward from. Thank you.

[a64pilot]

That formula also assumes a 100% efficient Solar controller too, which isn’t possible of course.
But it is good though to ensure your not trying to overdrive your controller, cause assuming 100% gives you some lee way.

[Frankly]

Where people go off the tracks is a solar panel is closer to a current source than the usually encountered voltage source. You can connect various loads to a 12 volt battery and the current will vary but the voltage will remain near 12 volts (the power from the battery is 12 X current= watts). With a solar panel the current is relatively constant (once well illuminated ) and the voltage can vary all over the place. That 30 volt panel when connected directly to a discharged 12 volt battery will put out 12 volts at around 9 amps and your 300 watt solar panel will be providing just over 100 watts. This is where the MPPT solar controller magic comes in as it will allow the panel to operate near its designed output and match that to the 12 volt battery. The solar controllers also look after the battery with the correct charging profile.


A 300 watt panel with the sun directly overhead (both angles, clear day) and a good MPPT controller should deliver near 300 watts to a well discharged battery bank. This normally is only during the bulk phase of the charging profile so the voltage is around 13.5 volts if you are just looking for a rough number.