Do solar panels only provide wattage required by load?

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matthewmiles asked Jan 8, '22

Do solar panels only provide wattage required by load?

I have two 350 watt 24 volt panels in parallel connected to two 140 amp hour agm batteries through a Victron smart solar 100/30 MPPT. Just put the system together. Main load is a 12 volt 45 watt fridge. In ten days of good sunshine the MPPT unit has never displayed solar power above 269 watts. It’s usually showng Pmax around 160 watts. Batteries don’t drop lower than 12.8 volts at night and quickly get up to 13 volts or more and fridge running fine so all seems good. Will the panels only produce close to their maximum when the load is larger? Thanks

matthewmiles

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Answer 1 · 2022-01-08T10:46:56Z

seb71 answered · Jan 08, 2022 at 10:46 AM

Yes.

But you are also limited by that 30A maximum current which your 100/30 charger can output.

Since you are using a 12V battery, this means maximum power output is about 12Vx30A = 360W (slightly higher in practice - up to about 430W or so, because the battery voltage can go higher than 12V; I used the nominal voltage for simplicity).

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matthewmiles commented · Jan 08, 2022 at 11:03 AM

Thanks. Maybe I should have chosen the 100/50 unit to get the most out of my panels.. But my load will never be very large - a few LEDs and the fridge is all I need.

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seb71 matthewmiles commented · Jan 08, 2022 at 11:07 AM

Although at first look it might seem as a complete waste, you still benefit from your oversized PV array during cloudy weather, when the PV panels can produce only a fraction of their rated power.

Also, if you have no shadow issues and if the two panels are placed in the same plane (oriented the same), you could wire them in series instead.

Your 100/30 probably can take that configuration (string voltage should be under the maximum limit of 100V for your charger, even in the coldest weather).

The benefit is higher PV array voltage (the charging will start sooner), lower current (less loses on the PV array wires).

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matthewmiles seb71 commented · Jan 08, 2022 at 11:17 AM

As you can probably see I’m a complete newbie. I was surprised how cheap the panels were (and they’re Australian made with good reviews) so bought two.
I had thought that the 30 amp figure on the Victron unit was referring to input from the panels

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seb71 matthewmiles commented · Jan 08, 2022 at 11:35 AM

Technically there is also a maximum allowed limit for the Isc from the PV array connected to the MPPT charger, but the parameter you must pay close attention to is the maximum voltage of your PV array.

If you will consider changing from panels in parallel to panels in series, better check the actual Voc and the Voc temperature coefficient (combined with the minimum temperature) for your panels and location.

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nickdb ♦♦ seb71 commented · Jan 08, 2022 at 11:41 AM

Oversized arrays to help during cloudy weather are such a myth and waste of money.

The reduction in power is so enormous, you'd need to erect 10X the panels to compensate.

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seb71 nickdb ♦♦ commented · Jan 08, 2022 at 12:04 PM

When there are very dark (thick) clouds or panels with a thick snow layer on them, yes.

But not all cloudy days are pitch black.

Sure, if you have stable grid power and/or just a few cloudy days each year, it's less tempting to oversize the PV array.

It also helps even in sunny winter days, if the panel angle is not optimal for the low winter Sun.

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matthewmiles seb71 commented · Jan 08, 2022 at 08:13 PM

Thanks again for your help. I will look at a series configuration. At present the panels get a bit of shade in the morning but I’ll deal with that soon. The data sheet states Voc as 45 at standard test conditions and 41.5 at nominal module operating temperature. My location is usually very sunny and never gets snow. Minimum temperature for last few years was around 4 degrees or 39 in Fahrenheit. It’s usually too hot, hence the new fridge. I’m twenty miles from the grid and there’s no mobile phone or internet or tv. Perfect!

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seb71 matthewmiles commented · Jan 08, 2022 at 08:31 PM

The data sheet states Voc as 45 at standard test conditions

In this case, DO NOT WIRE THEM IN SERIES.

At 4°C, panel Voc would probably be close to 48V. Two in series would be too close to the 100V maximum allowed for the MPPT charger.

Keep them as they are, in parallel.

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matthewmiles seb71 commented · Jan 08, 2022 at 08:38 PM

I had no idea that temperature is so critical. Thanks again. As I said it’s only a simple shack in the bush that I visit on weekends so no need for lots of power. I will take your advice and leave it as is

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seb71 matthewmiles commented · Jan 08, 2022 at 08:49 PM

In your case, most of the time the voltage will be much lower, because of the high temperatures. But one chilly day is enough to damage the charger.

If you want to take a closer look about how much the panel temperature influences the voltage, you can read this reply I recently wrote for a similar situation.

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matthewmiles seb71 commented · Jan 08, 2022 at 09:30 PM

You are most helpful! Using your calculations and factoring in the increasingly erratic weather in my part of the world- crazy hot droughts and wildfires followed immediately by cold conditions and gigantic floods- I can readily see that the 100 volt limit of my charger would be inadequate in a series configuration.

I appreciate your sharing of your knowledge!

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