diy solar

diy solar

Charging times

Off Gridin' It

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Dec 11, 2020
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How do i figure out how long it will take for a given solar array to charge a given battery bank. (What is the formula?)
For instance if i have a 1200 watt array how long will it take to charge a 400 ah lithium battery bank.
Or
how long for a 250 watt array to charge a 100 ah battery bank.
 
You need to tell us the battery voltage. 12V? 24? 48?

The trick is to remember that
1) Watts in to the Charge Controller is the same as Watts out of the charge controller.
and
2) Watts = Voltage * Current (With a little algebra this is the same as Current = Watts/Voltage)

So, lets assume a 12 V (nominal) 400AH battery bank and a 1200W solar array. (Note: A 12V(nominal) LiFePO4 battery will run at ~12.8V.)

There will be 1200W coming out of the controller so the the current coming out of the charge controller will be 1200W/~12.8V = ~93.75A

Since it is a 400Ah Battery bank it will take 400Ah/~93.75A=~4.27 hours (at full power) to charge from completely empty to completely full.
However there will be various system losses so it might be closer to 5 hours.
 
You need to tell us the battery voltage. 12V? 24? 48?

The trick is to remember that
1) Watts in to the Charge Controller is the same as Watts out of the charge controller.
and
2) Watts = Voltage * Current (With a little algebra this is the same as Current = Watts/Voltage)

So, lets assume a 12 V (nominal) 400AH battery bank and a 1200W solar array. (Note: A 12V(nominal) LiFePO4 battery will run at ~12.8V.)

There will be 1200W coming out of the controller so the the current coming out of the charge controller will be 1200W/~12.8V = ~93.75A

Since it is a 400Ah Battery bank it will take 400Ah/~93.75A=~4.27 hours (at full power) to charge from completely empty to completely full.
However there will be various system losses so it might be closer to 5 hours.
It’s a 12 volt battery bank.
 
You never get the rated power out of a solar panel. They are rated with a calibrated lamp that shines at 1000Watts over a surface area of 1 square meter. That only happens on perfect days, on the equator, sun overhead, no clouds, no dust, panels exactly perpendicular to the sun.


The earth tilts about 23 degrees every season. When your tilted away from the sun, it shines on you at a angle. This ends up spreading the sun power out (less concentration). Above 66 degrees is the Arctic circle. It can be dark all winter.

The angles of the sun move a lot. In my camp, I faced the panels solar south. If you watch the sun rise and set, the sun actually rises Northeast and sets Northwest. I never noticed that until I put the panels up. There's no sun if they are facing south and the sun rises in the north.

I found it's best to buy just 1 or 2 panels, hook up a solar controller, and see what I can pull out of the panel with my actual setup. In the winter, I'm just getting 10% output of my 600Watts of solar for just a few hours. It can take a long time to charge a battery.

Understand your batteries. If the sun is shining, some people see the battery voltage is full voltage, but 2 hours later, the battery is dead. They really never could charge the battery due to not enough solar for that time of year. They replaced the batteries and the same thing happened.
 
You need to tell us the battery voltage. 12V? 24? 48?

The trick is to remember that
1) Watts in to the Charge Controller is the same as Watts out of the charge controller.
and
2) Watts = Voltage * Current (With a little algebra this is the same as Current = Watts/Voltage)

So, lets assume a 12 V (nominal) 400AH battery bank and a 1200W solar array. (Note: A 12V(nominal) LiFePO4 battery will run at ~12.8V.)

There will be 1200W coming out of the controller so the the current coming out of the charge controller will be 1200W/~12.8V = ~93.75A

Since it is a 400Ah Battery bank it will take 400Ah/~93.75A=~4.27 hours (at full power) to charge from completely empty to completely full.
However there will be various system losses so it might be closer to 5 hours.
My response to him was a little simpler.

volts * ah = wh

wh/solar array watts = hours to charge (in perfect conditions).

However, it’s nice to understand the amps relation, too; though, it still boils down to solar conditions...
 
the 5 hour number is good if you live at the equator and a cloudless day.


in the real world it will take 10 plus hours most likely if you run the battery all the way down
 
You never get the rated power out of a solar panel. They are rated with a calibrated lamp that shines at 1000Watts over a surface area of 1 square meter. That only happens on perfect days, on the equator, sun overhead, no clouds, no dust, panels exactly perpendicular to the sun.

I routinely get > rated. 6700 ft elevation, excellent sun, fixed tilt. Here are my insolation numbers:

1617078689401.png

OP's battery: 400Ah * 12V = 4800Wh

1200W Array will produce a given total per day based on the insolation.

Right now, for me, I'm getting 7.13 hours/day, so:

1200W * 7.13 = 8556Wh

Thus, it would take my array about 4800Wh/8556Wh about 0.56 days to charge the battery.

At my location and panel orientation, 0.56 days of solar is about noon, so the battery would be charged around noon starting at sunrise with no loads being used.

As you can see, the answer isn't simple arithmetic.

First, you need to know your insolation for your location, panel orientation and tilt. Links #5 and #6 in my signature.

Then you take your daily Wh production and compute how many days it takes you to charge. If it's a fraction of a day, 0.5 days is around noon in most cases (mine isn't due to my panels facing 165° instead of 180°) and you estimate from there.

If you're using power while you're charging your batteries, it just gets that much uglier. If you run the numbers, then at least you can determine how much power you can use in a day and still get your batteries charged.

In the above example, I have 8556-4800Wh = 3756Wh of power I can use and still have my batteries fully charged at the end of the charging day.
 
First, you need to know your insolation for your location, panel orientation and tilt. Links #5 and #6 in my signature.
The US Government national labs have mapped solar Horizontal and Direct Normal Irradiance values at


They also have software and databases that you can use.

I still recommend spending for one panel and trying it out on your land, your trees, your weather.

I set my angles by looking at the weather forecast. They show sunrise and sunset. Exactly 1/2 the time between those will be solar noon. Set an alarm and run out there with your solar panel and a large plastic roofing square. Rotate and tilt the panel until you can slide a pencil up and down the 90 degree perpendicular square and not see the shadow move. That would be "Direct Normal Irradiance". The most you can get at that one perfect point in time. Everything earlier or later would be less.

You could measure the watt hours out of the panel for a day to see if you match up with NREL's average DNI KW/M2/Day charts for that month.

If you are using a 100W panel, that is 1/10th of the standard 1000W/M2/Day number.
 
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