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12V Solar Panel Guide
Pmax 40 W
Vmp 17.4 V
Imp 2.3 A
Voc 21.8 V
Isc 2.58 A
What does this tell us?
The electrical characteristics graph looks like this:
Suntech 40 watt solar panel electrical characteristics
How to choose a solar panel to charge 12 volt systems
Did you know a 100 watt solar panel will probably only deliver about 70 watts to your battery? “That’s bad” I hear you say…
And a 12 volt solar panel will actually put out approximately 15 volts in full sunlight? “But that’s better”, you say. After all 15 volts is better than 12 volts. Yes, but… let’s delve a bit deeper.
Let’s set up a small solar panel to make up a solar charger for a 12 volt battery. Lets say you choose a Suntech 40 watt solar panel.
Before you begin assembly, lets check the technical data for a 40W Suntech solar panel, it includes the following important information:
Output 40 W And a 12 volt solar panel will actually put out approximately 15 volts in full sunlight? “But that’s better”, you say. After all 15 volts is better than 12 volts. Yes, but… let’s delve a bit deeper.
Let’s set up a small solar panel to make up a solar charger for a 12 volt battery. Lets say you choose a Suntech 40 watt solar panel.
Before you begin assembly, lets check the technical data for a 40W Suntech solar panel, it includes the following important information:
Pmax 40 W
Vmp 17.4 V
Imp 2.3 A
Voc 21.8 V
Isc 2.58 A
What does this tell us?
Pmax is the maximum power generated by the solar module in full sunlight with the panel facing directly into the sun and where the temperatures of the solar cells is at 25°C. These the standard test conditions (STC)
Vmp is the voltage at STC.
Imp is the amps at STC.
Voc is the open circuit voltage, the voltage at the terminals of the solar panel in full sun but not connected up.
Isc is the short circuit current, the current that would flow through the solar panel in full sun if the panel terminals were shorted out. This is the current to consider when sizing the required wiring for the panel, to be safe add 25%. For instance this solar panel would need at least 2.58 plus 25% = 3.225 amp cable.
Now let’s discuss the effect temperature has on solar cells.
Solar cells need to be kept as cool as possible because the solar cell’s efficiency drops by about 10% for every 20°C rise in temperature, so at 45°C you can expect 10% less power output. On the plus side, because solar panels love to be cool, at 5°C you can expect 10% more power than its rated output.
The power loss is caused by a decrease in cell voltage at higher temperatures. Let’s look at the diagram below. Ignore the dotted lines showing the current flow (amps) at various light levels, we’ll discuss that later, just look at the top line showing how maximum voltage decreases at higher temperatures of the solar cells.
At 50°C the optimum voltage is about 15 volts and at 75°C it’s only about 11 volts. Now you might think those temperatures are higher than you would get on a normal sunny day but solar panels are designed to absorb as much sunlight as possible and more sun equals more heat. Even on a relatively cool day when air temperature is about 10°C the solar cell will be at about 30°C, so on a typical warm sunny day the temperatures of the solar cells reach about 60°C.
That’s why 12v solar panels are designed to supply about 18 volts. A lot of the time the solar panel is hot and is only generating about 15 volts or less.
The actual temperature sensitivity depends on the type of solar panel but the bottom line is you can’t expect to get maximum power from a hot solar panel.
Vmp is the voltage at STC.
Imp is the amps at STC.
Voc is the open circuit voltage, the voltage at the terminals of the solar panel in full sun but not connected up.
Isc is the short circuit current, the current that would flow through the solar panel in full sun if the panel terminals were shorted out. This is the current to consider when sizing the required wiring for the panel, to be safe add 25%. For instance this solar panel would need at least 2.58 plus 25% = 3.225 amp cable.
Now let’s discuss the effect temperature has on solar cells.
Solar cells need to be kept as cool as possible because the solar cell’s efficiency drops by about 10% for every 20°C rise in temperature, so at 45°C you can expect 10% less power output. On the plus side, because solar panels love to be cool, at 5°C you can expect 10% more power than its rated output.
The power loss is caused by a decrease in cell voltage at higher temperatures. Let’s look at the diagram below. Ignore the dotted lines showing the current flow (amps) at various light levels, we’ll discuss that later, just look at the top line showing how maximum voltage decreases at higher temperatures of the solar cells.
At 50°C the optimum voltage is about 15 volts and at 75°C it’s only about 11 volts. Now you might think those temperatures are higher than you would get on a normal sunny day but solar panels are designed to absorb as much sunlight as possible and more sun equals more heat. Even on a relatively cool day when air temperature is about 10°C the solar cell will be at about 30°C, so on a typical warm sunny day the temperatures of the solar cells reach about 60°C.
That’s why 12v solar panels are designed to supply about 18 volts. A lot of the time the solar panel is hot and is only generating about 15 volts or less.
The actual temperature sensitivity depends on the type of solar panel but the bottom line is you can’t expect to get maximum power from a hot solar panel.
Typical solar panel output at various light levels and temperatures
OK back to our Suntech 40 watt solar panel, according to the data plate it will deliver a current of 2.3 Amps at 17.4 Volts in full sunlight at 25°C.
Lets check those figures: Volts times Amps equals Watts so 17.4 x 2.3 = 40 Watts.
From Ohms law V = IR so the load (resistance) required to achieve this output would be 17.4V divided by 2.3A = 7.56Ω (ohms)
Lets check those figures: Volts times Amps equals Watts so 17.4 x 2.3 = 40 Watts.
From Ohms law V = IR so the load (resistance) required to achieve this output would be 17.4V divided by 2.3A = 7.56Ω (ohms)
The electrical characteristics graph looks like this:
Suntech 40 watt solar panel electrical characteristics
On the chart I have highlighted the maximum power point (MPP) for standard test conditions (STC) where the 17.4 volts line intersects the 2.3 amps line. If you follow the vertical red line to where it intersects the PV curve for STC (top curve) you can see it is at the 40 watts level on the power scale. The PV curve is at its highest point here, which verifies it's the optimum operating point for this solar panel.
You will also need a solar regulator to ensure you don’t overcharge the battery, let's say you select a Sealite 10A solar regulator to ensure the battery is charged properly.
let’s check the specs for the Sealite 10A solar regulator
Output: 10 A the maximum current that it can regulate without overheating
Boost Charge: 14.2 V when a depleted battery is being recharged
Float Charge: 13.8 V when a battery is trickle charged up to keep it fully charged
Solar regulators are voltage regulators, they adjust the output volts to suit the battery. Most 12 volt batteries need to be charged at about 14.2 volts so that is what this solar regulator does. It will allow the maximum current to flow at all times, because higher current flow will recharge the battery faster. See the battery notes below for when you are connecting solar panels directly to batteries.
During the boost charge the Sealite solar regulator will supply 14.2 volts. The maximum current flow we can expect is 2.3 amps according to the solar panel’s electrical specifications. Watts = Volts x Amps so at 14.2 volts it will supply 32.7 watts maximum, any excess power provided by the solar panel is wasted.
Why not connect the solar output directly to the battery to obtain the best power output? Sure, that would charge your battery faster, but it might overcharge it so it’s wise to use a solar regulator or charge controller as it’s sometimes called.
If the light falling on the solar panel decreases because of the sun’s position or a cloudy day then the solar panel may only deliver 800 watts per square meter (equivalent to the middle graph) which means the regulator will only be supplying about 1.8 amps to the battery. Remember too that this amperage is only provided at the maximum power point, due to other electrical conditions it will probably about 10 to 15% less than this.
So now you know why a 12 volt solar panel might actually be providing 15 volts and a 40 watt solar panel will probably not put 40 watts into your battery.
Battery Notes:
You will also need a solar regulator to ensure you don’t overcharge the battery, let's say you select a Sealite 10A solar regulator to ensure the battery is charged properly.
let’s check the specs for the Sealite 10A solar regulator
Output: 10 A the maximum current that it can regulate without overheating
Boost Charge: 14.2 V when a depleted battery is being recharged
Float Charge: 13.8 V when a battery is trickle charged up to keep it fully charged
Solar regulators are voltage regulators, they adjust the output volts to suit the battery. Most 12 volt batteries need to be charged at about 14.2 volts so that is what this solar regulator does. It will allow the maximum current to flow at all times, because higher current flow will recharge the battery faster. See the battery notes below for when you are connecting solar panels directly to batteries.
During the boost charge the Sealite solar regulator will supply 14.2 volts. The maximum current flow we can expect is 2.3 amps according to the solar panel’s electrical specifications. Watts = Volts x Amps so at 14.2 volts it will supply 32.7 watts maximum, any excess power provided by the solar panel is wasted.
Why not connect the solar output directly to the battery to obtain the best power output? Sure, that would charge your battery faster, but it might overcharge it so it’s wise to use a solar regulator or charge controller as it’s sometimes called.
If the light falling on the solar panel decreases because of the sun’s position or a cloudy day then the solar panel may only deliver 800 watts per square meter (equivalent to the middle graph) which means the regulator will only be supplying about 1.8 amps to the battery. Remember too that this amperage is only provided at the maximum power point, due to other electrical conditions it will probably about 10 to 15% less than this.
So now you know why a 12 volt solar panel might actually be providing 15 volts and a 40 watt solar panel will probably not put 40 watts into your battery.
Battery Notes:
- Flooded (wet cell) Lead-Acid and AGM batteries should be boost charged at 14.5 volts with a charge current no more than 10% of the battery amp-hour capacity, so for a 100 AH battery this would equal 10 Amps.
- Gel type Lead-Acid batteries should be boost charged at 14.5 volts with a charge current no more than 5% of the battery amp-hour capacity, so for a 100 AH battery this would equal 5 Amps.
- The battery is fully charged when the current has dropped to 3% of the charge current.
- All Lead-Acid batteries can be float charged at 13.5 volts with a charge current of about 3% of boost charge, so for a boost charge of 30 Amps this would equal 0.9 Amps and for a boost charge of 20 Amps this would equal 0.6 Amps.
- Test the battery charge after it been standing for at least 4 hours. A fully charged flooded battery voltage should read 12.7V, AGM and Gel batteries should read 12.8V.
Great service. 10W Panel and regulator received this morning straight after the Easter break. All in good order. fitted the solar panel today and working a treat. Charges the battery quicker than I expected really for such a small panel. Good on you. Regards Peter D
I was using the wrong connectors for testing initially. Using the correct ones the panel has exceeded my expectations. Thank you for your help. Regards Megan W
Thanks for your quick response. I have fully charged the solar charger now and the message is gone. Thanks for the added info which I will take on board. I love the product.Best Regards Michelle H
The 120 watt folding solar panel from you earlier in the year has worked very well, I am now interested in a 10 watt 12 volt panel
Regards Ron
Thank you so much for this info and link. It's just what I needed. I will let you know how this all works out, but suffice to say for now, having received our solar garden lights, project #1 is going very well!
Cheers, Sue & Bruce
Thanks heaps for your help. I will be sure to recommend you to my friends. The wind turbine has sparked interest from the other farmers in the area so I will pass on your details to them. Thanks again
Regards Chris C
I have received the topray 28W Folding Solar Panel and its really nice, the led worklights lit up when I connected it and it was indoors at night! got to tell you I'm rapt
Thanks Jim M
Confirming the box of lights has arrived in good condition. Thank you for the feedback and service
Regards Peter S
Your products are amazing - congratulations!
Thanks, Maria B