How many watts of solar panels?

[PaulLad]

Will's generic formula is divide your total battery watts by 5 and this is how many watts of panels you should have. The idea being that you should be able to charge your batts in one full sunny day. So if I have 5000 watts of batteries, then 1000 watts of panels should do it right? But in late fall here, I'm only getting about 50 watts out of a 100 watt panel in full sun at the ideal angle. On top of that, the days are only 10 hrs long. That means that with 1000 watts of panels it would take 10 hours of full sun at the ideal angle to charge my batts. Sounds like I need 2000 watts of panels. Am I thinking straight? Thanks.

 

[timselectric]

Winter is terrible for solar.
We all need more panels.

 

[Warpspeed]

It depends very heavily on where you are, and the local climate.
And relative day/night consumption, you probably don't need much battery power during the day, most days.

Also your lifestyle.
Do you have a really understanding wife that realizes that doing six loads of washing on a cloudy day might strain the system.
Or multiple teenage daughters that have absolutely no comprehension about anything, and never switch anything off
Too many variables.....

I am in Melbourne Australia (38 degrees south)
I have 5Kw of panels, 5Kwh battery, and daily (24 hour) consumption is about 3.5 Kwh to 4 Kwh.
Most days the battery is fully recharged before 10Am.

I can usually get through two days in a row of total solid cloud in winter, just....
About two or three times per year the system falls on its face, and I have to revert to battery charging from the grid.

 

[Warpspeed]

Just to expand a bit on the above.
If I doubled up on panels, during really bad sustained cloud, I would get twice as much.
Twice almost nothing is still almost nothing.
So installing more panels here would not make much of a difference.

A better solution for me here, would be a much larger battery, as the climate here is highly variable, sunny days then cloud, then more sunny days.
A much larger battery would see me through the two or three times per year when the system fails.
If you have six or ten cloudy days in a row, the biggest battery in the world is not going to be big enough.
The solution to that is either a LOT more panels, or maybe a gasoline or diesel put put.

 

[Steve777]

Another variable is the angle of your panels. Here, off-grid folks will commonly angle their panels to maximize winter power collection, foregoing some summer production. Again, a lot depends upon your climate. If you are somewhere that winters are cloudy and overcast much of the time, the solutions become harder.

 

[PaulLad]

Another variable is the angle of your panels.

As I said, I'm only getting about 50 watts out of a 100 watt panel in full sun at the IDEAL ANGLE

 

[BigDoug]

That doesn't sound like things are working right. In full sun, at the right angle your 100 watt panel should produce ~100 watts.

 

[Warpspeed]

No.
A 100 watt rated panel will never generate 100 watts at sea level on Earth from sunlight.

The industry standard for testing solar panels is under laboratory conditions with 1Kw per square metre irradiation at 25 degrees Celsius.
That is the measured power output printed on the rating plate under those conditions.
Some ratings plates state that, some do not. But they are all tested and calibrated that way.

Under full sun in a clear blue sky, you might expect about 80% of the rating plate power at the very best.
Under less than ideal conditions, a whole lot less.

 

[BigDoug]

I must just have the best luck in the world then? Where I live isn't at sea level, but I'm also all of 550 feet above sea level, and consistently get 90+ % of the rated output on my 8-panel array when I get it at the correct angle and on a perfectly sunny day (to be clear - 90%+ only lasts for the few hours that everything is in alignment with the suns positioning and the angle).

Yes, under less than ideal conditions you're going to get less. But the OP said in full sun at the ideal angle.

 

[LydMekk]

75-80% of rated is standard.

 

[Brucey]

It depends very heavily on where you are, and the local climate.
And relative day/night consumption, you probably don't need much battery power during the day, most days.

Also your lifestyle.
Do you have a really understanding wife that realizes that doing six loads of washing on a cloudy day might strain the system.
Or multiple teenage daughters that have absolutely no comprehension about anything, and never switch anything off
Too many variables.....

I am in Melbourne Australia (38 degrees south)
I have 5Kw of panels, 5Kwh battery, and daily (24 hour) consumption is about 3.5 Kwh to 4 Kwh.
Most days the battery is fully recharged before 10Am.

I can usually get through two days in a row of total solid cloud in winter, just....
About two or three times per year the system falls on its face, and I have to revert to battery charging from the grid.

4kWh per day? Heating/hot water/ cooking with natural gas? What do you do for A/C?

 

[MichaelK]

But the OP said in full sun at the ideal angle.

That's because he's new and doesn't have a full understanding of what "full sun" means. A South facing solar panel at 8am is not going to get "full sun". Same at 4pm.

If you have rotating mounts like I do, and can point the panels directly at the sun at any point in the day, then you can approach "full sun". Even then, "full sun" in December" is a bit less than "full sun" in June because the sun is lower in the sky and has to pass through more atmosphere to get to the surface of the earth.

To better answer the OP's question, I'd add that the charging rate is better determined by the battery chemistry. Here are some broad generalities....

Traditional flooded lead-acid: 1/8th of C
Sealed AGM: 1/5th of C
Li: 1/4th of C

Since "C" is usually documented in Amphours, I like to do the math based on amps, not watts. Since the OP did not mention his system voltage his battery might be three different Ahr capacities....
5000Wh/12V = 416Ah
5000Wh/24V = 208Ah
5000Wh/48V = 104Ah

I think working with amps in this case is better than watts, because it tells you what the limits of your electronics needs to be. Doesn't help much to install 2000W of panels if you only have a 20Amp 12V controller.

So, to better answer the OP's question, let's assume he has an traditional lead acid 12V battery that's 416Ah in size. The math works out to be (416Ah/8) X 12.5V = 650W.

As WS mentions in post #8, I like to de-rate my panels, but I typically use 85%, so that 650W of panels turns into 650W/85% = 765W of panels.

Assuming an Li battery, the math becomes.... (416Ah/4) X 12.5V = 1300W. 1300W/85% = 1530W of panels.

Doing the math this way tells you that you need a 60A controller to successfully charge the lead-acid battery, but you need to implement a 100A controller if you want to charge the Li battery at 1/4th C.

 

[Warpspeed]

4kWh per day? Heating/hot water/ cooking with natural gas? What do you do for A/C?

Yes, natural gas for heating, cooking, hot hot water, and drying clothes when necessary.
Well insulated home, deciduous trees for summer shading, wide eaves let the sun shine in during winter months.

Climate here is very mild, no snow or frost ever, and it gets over 40 Celsius (104F) only for about five days per year.
I can comfortably survive one day at 40C+, but two or more very hot days in a row does require air conditioning.
I use only grid power for air conditioning, but its only necessary for a very few times each year.

I nice mild climate with a gentle sea breeze most of the time, the temperatures here never get extreme either way.

 

[PaulLad]

That's because he's new and doesn't have a full understanding of what "full sun" means. A South facing solar panel at 8am is not going to get "full sun". Same at 4pm.

I'm no expert but I'm not an idiot either. I know how to point a solar panel directly at the sun. The test was done around noon.

 

[42OhmsPA]

I'm no expert but I'm not an idiot either. I know how to point a solar panel directly at the sun. The test was done around noon.

I'm consistently seeing 3550w or more for around an hour on clear sunny days from my 12 305w used panels that are angled appropriately for winter. They produce more than my 18 315w panels that are angled poorly.

 

[Warpspeed]

As I said, I'm only getting about 50 watts out of a 100 watt panel in full sun at the IDEAL ANGLE

There is rather a lot to consider.

Dust and pollution in the air, a low sun angle travels through more of the atmosphere, your altitude, ambient temperature, and accuracy of measurement all matter.
Panels are also supposed to gradually degrade over time. And are the solar panels being optimally loaded ?
Either overloading or under loading the panes will reduce measured power.

I have two Turnigy power meters here that read about 4% different.
Its not uncommon for voltmeters and amp meters to be out by 1% or 2%.

There is too much uncertainty about the whole thing to know for sure exactly what is happening, without doing some back to back testing with some alternative hardware and instrumentation to compare.

 

[MichaelK]

I'm no expert but I'm not an idiot either. I know how to point a solar panel directly at the sun. The test was done around noon.

But you can't explain why you are only getting 50W out of a 100W panel? I'd be willing to help if you were willing to listen.

 

[z_zk_z]

The "Will's generic formula" is probably referring to TIME (number of Sun hours) and saying that it will take 5 hours to charge a "5000 watts of batteries" with "1000 watts of panels" (under prefect conditions). Therefore, if your panels are generating half the watts (i.e., only getting about 50 watts out of a 100 watt panel in full sun at the ideal angle), yes you would need "2000 watts of panels" to charge the "5000 watts of batteries" in 5 hours. PS. "5000 watts of batteries" is not much and it is desirable to feed the loads AND RE-charge batteries while the Sun is up (during those 10 hours).


"Will's generic formula is divide your total battery watts by 5 and this is how many watts of panels you should have. The idea being that you should be able to charge your batts in one full sunny day. So if I have 5000 watts of batteries, then 1000 watts of panels should do it right? But in late fall here, I'm only getting about 50 watts out of a 100 watt panel in full sun at the ideal angle. On top of that, the days are only 10 hrs long. That means that with 1000 watts of panels it would take 10 hours of full sun at the ideal angle to charge my batts. Sounds like I need 2000 watts of panels. Am I thinking straight? Thanks."

 

[SunDave]

Can confirm that closer to panel rating is correct for a new panel. I have a small setup in a northern climate (low sun angle) and can get very near nameplate rating in November. Slightly past the rated wattage on cold days in short bursts. I also know that you should consider using the power during the day too, since if you have some usage, you don't have 100% going to battery. Bad weather, etc.

In the big picture panels are one of the cheapest parts of the system. And going light on panels is just no good. It's like having a good machine, but no fuel for it. Makes no go.

 

[Kenny_]

Can confirm that closer to panel rating is correct for a new panel. I have a small setup in a northern climate (low sun angle) and can get very near nameplate rating in November. Slightly past the rated wattage on cold days in short bursts. I also know that you should consider using the power during the day too, since if you have some usage, you don't have 100% going to battery. Bad weather, etc.

In the big picture panels are one of the cheapest parts of the system. And going light on panels is just no good. It's like having a good machine, but no fuel for it. Makes no go.

IMO the inverter is the cheapest part of the system. Batteries are the most expensive followed by panels, but maybe that all depends on quantity of each.