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# How to Calculate Solar Panel & Battery

## How to Calculate Solar Panel & Battery

(OP)
Hi Gurus,

I plan to power Three 30W LED Lights with Solar Panel for 10 hrs at most per night. Least average sunlight hours at location is 3 hrs (in Dec). This is my first Solar project and I would like to confirm if my Solar Panel and Battery selection calculations are fine:

Total Load : 3 x 30W = 90W
Watt Hour : 90W x 10hrs = 900 WHr
Required Solar Power output (in Dec) = 900 WHr/ 3Hr = 300W
Hence, I would need a 300W 24V solar Panel to power THREE 30W LED lights during low sunlight periods.

12V Battery Bank:
90W/12v = 7.5A x 10Hrs = 75AHr
I shall choose 100AHr c/20 Deep cycle Battery.

I would appreciate your feedback. Thanks

### RE: How to Calculate Solar Panel & Battery

Expect that during the winter far less sunlight energy will make it through the atmosphere - I would expect you need a nominal solar panel of 3kW to still produce 300 W come winter. You will also have conversion losses - figure only 75% of the electricity that is generated by the panels will be available to the LEDs; the efficiency may be better than that, but the solar panels will age as will the LEDs, lowering their output over the next decade. The battery will be compromised if it is allowed to be cold. Check to see what reduction will occur for the expected temperatures where it will be located.

### RE: How to Calculate Solar Panel & Battery

Even ignoring the fact that the OP shows as Alberta located, you can only get maximum sunlight for maybe an hour during the best day of the year AT THE EQUATOR. The rest of the sunlight hours get progressively worse. See https://www.energyhub.org/solar-energy-maps-canada... for insolation vs. month

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### RE: How to Calculate Solar Panel & Battery

Do you want the lights to work after a cloudy day? After a week of cloudy days?

### RE: How to Calculate Solar Panel & Battery

You have also chosen deep cycle battery. Lead acid batteries are damaged by deep cycling so you'll need to at least double the battery capacity relative to your need in order to keep the state of charge above 50% at all times. Or buy NiCad / Lithium packages that tolerate full capacity cycling.

### RE: How to Calculate Solar Panel & Battery

The very first question is: Why specifically do you need three 30W lights running 10 hours a night?

Keith Cress
kcress - http://www.flaminsystems.com

### RE: How to Calculate Solar Panel & Battery

(OP)
Hi Keith,

Its to light up a large bill board. 10Hrs is a worst case scenario (during dark winter months). It may not run for 10 Hrs, as I intend to incorporate a controller to turn-off the lights possibly after 3AM.

### RE: How to Calculate Solar Panel & Battery

Ah. Okay. That's probably the only good reason to run them continuously as security wise it's a lame idea to run them continuously.

As mentioned above you need to greatly over provision to make this work acceptably. There are many points to hit to prevent expensive failure, immediate or eventual.

To make this work you need to:
1) Size the batteries to run the lights without much solar input for a week.
2) Size the solar to fully charge the batteries in cloudy weather to be 80% more than the solar required in sunny weather.
3) Include a solar charger that will not over-charge the batteries.
4) Include a battery manager that will accurately prevent over-discharging.
5) Use an ambient sensor to turn the lights on ONLY when needed at night.
6) Manage the battery temperatures to temps as required by the chosen battery chemistry.

It would also help if you can prune off any unneeded running time. For instance, do you really need the billboard lit between 3AM and 5AM? Is the billboard in a location that cars of interest could actually trigger it to turn ON lighting as they pass which would additionally make the sign more effective?

Keith Cress
kcress - http://www.flaminsystems.com

### RE: How to Calculate Solar Panel & Battery

I was looking at Solar panels for somewhere near the eqautor and basically you got about 4.5 kWh per KW of nominal power based on 12 hours of sunshine.

So with only 3 hours sunlight you're looking at maybe 1 kwh/kw. Which is about what you need.

but then add on the impact of latitude and you're probably down to about 5-600Whr/kW of panel.

So for worst case you probably need about 2kW nominal power rating to power your lights in winter. Your issue then is how do you control and not damage the panels or lights in the summer when you probably have 4-5 times the electrical power you need or maybe even more given the nights are so short.

some covers for the panels you don't need might be a good idea....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: How to Calculate Solar Panel & Battery

Hello Maple2,

I have done solar and inverter design and installation for more than 10years.

Your design calculation are fairly much in order. You provided a 25% oversize battery during the worst sunlight weather condition.

Corrections you need to make:

1. Choose the same voltage rating for your battery and solar panels. You cannot have a 24W solar panel charging a 12V battery. It will damage the battery. You can of course use two batteries of 12V in series for a 24V solar panel.
2. Your LED lights have to be the same voltage rating as your solar panel and battery(unless you have some converters in between)
3. Be sure you have a very good charge controller to regulate power input from your solar panel, to deliver optimal power output to run those LED lamps and charge the batteries. A bad charge controller will keep supplying unregulated power to the batteries and over charge them, during periods of greater light intensity.

So with 24V battery, load current=90/24=3.75A. Required AH=3.75*10=37.5AH. So you need one 24V Deep cycle Battery of about 50AH, or two deep cycle batteries of 12V each of 50AH.

Good Luck.

Kind Regards

### RE: How to Calculate Solar Panel & Battery

Red Deer Alberta is about 16 degrees further north than California.
In the Winter the sum may be less than 20 degrees above the horizon.
California calculations and estimates for standby reserves may not be valid.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: How to Calculate Solar Panel & Battery

#### Quote (Ethnan)

You cannot have a 24W solar panel charging a 12V battery.
Not sure I agree with that... a SMPS here is a very useful addition at a minimal efficiency loss, and in many cases is preferred to limit the number of cells required. It may make sense here, but not necessarily.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: How to Calculate Solar Panel & Battery

#### Quote (MacGyverS2000)

Not sure I agree with that... a SMPS here is a very useful addition at a minimal efficiency loss, and in many cases is preferred to limit the number of cells required. It may make sense here, but not necessarily.
Yes, you can practically use a 24V MPPT solar panel to charge a 12V deep cycle battery, but why do that ?
The inherent operating problem is that the solar panel voltage of 24V is pulled down to match the battery voltage of 12V.
This in turn pulls the panel voltage away from its optimum operating voltage , reducing the panel power output and operating efficiency.
Doing this is not best-practice. For cost reasons, I agree with you, especially if you cannot easily get a 12V solar panel of same power rating.

### RE: How to Calculate Solar Panel & Battery

#### Quote (Ethnan)

The inherent operating problem is that the solar panel voltage of 24V is pulled down to match the battery voltage of 12V.
This in turn pulls the panel voltage away from its optimum operating voltage , reducing the panel power output and operating efficiency.
It does nothing to the panel voltage, it merely loses a bit of efficiency in the 24V->12V conversion through the SMPS... <5% loss with today's quality of converter chips. When you consider the output voltage of a set of PV cells varies quite a bit (not just a locked-in 24V), it makes sense to include an SMPS with a lower output voltage to ensure a valid charging output at a range of sun conditions... tough to charge a 24V battery when PV cell output dips down to 18V, unless your SMPS is already converter to a lower voltage. When PV output dips, you're still charging batteries, which in many cases the length of charge time (at slightly reduced efficiency) can outweigh a shorter charge time at greater efficiency. It's application (geographic?) specific.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: How to Calculate Solar Panel & Battery

#### Quote (MacGyverS2000)

When you consider the output voltage of a set of PV cells varies quite a bit (not just a locked-in 24V), it makes sense to include an SMPS with a lower output voltage to ensure a valid charging output at a range of sun conditions... tough to charge a 24V battery when PV cell output dips down to 18V, unless your SMPS is already converter to a lower voltage.

I totally agree with you. Was thinking through my tropics lens. For regions with varying sunlight intensity, this indeed is a valid and required consideration. Thanks very much !

### RE: How to Calculate Solar Panel & Battery

This is one of those applications where the best solution is do everything possible to reduce the expected load, including spending money to reduce the load.

The OP seems to have left the room anyway, since he doesn't seem engaged in any way.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: How to Calculate Solar Panel & Battery

(OP)
Thanks Guys for all the feedbacks. I initially chose a 24V battery to match the PV voltage and subsequently decided to go with 12V with as per the solar designs in my region of Alberta where it snows ALOT, but now I see understand. So, I have decided to use 3x330W LG Solar Panels, 12V battery (AGM), an MPPT charge controller, and a sensor(night) light switch.

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