How to choose solar panels

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 18 volts in full sunlight? “But that’s better”, you say. After all 18 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
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 at the sun overhead in a clear sky and where the temperatures of the solar cells is at 25°C. These are 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.

Solar panel temperature
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.

Solar panel output versus temperature

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)

The electrical characteristics graph looks like this:

Typical solar panel electrical characteristics
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.

Solar panel voltage regulation
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 will receive less photons resulting in less output. Let's say it only generates 800 watts per square meter of solar energy (equivalent to the middle graph) then 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.

A 12 volt Solar panel is designed to charge up 12 volt batteries
The best 12V solar modules are made of 36 solar cells connected in series and enclosed between a sheet of toughened glass and hard plastic and sealed within a high grade aluminium frame for protection and weatherproofing.
12V Solar components
To create a stand-alone 12v solar panel system you will also need a solar regulator to ensure the battery is not overcharged.
The purpose of the regulator is to act as charge controller, allowing the maximum charge from the solar panel when the battery charge is depleted but tapering off as it nears full charge, most solar regulators also control the power to the load, disconnecting it as the battery becomes depleted again after powering the load devices for some time without being charged from the solar panel.
Once the solar panel begins recharging the battery the regulator will recommence powering the load.
Deep cycle batteries are recommended for use with solar kits, these are made to withstand daily cycles of charging and deep discharging.
If you intend running AC appliances then you will also need a DC to AC power inverter.

To estimate your solar power requirement use this as an example:
A 20 watt globe will use 200 watts in 10 hours (the wattage rating of any appliance indicates how much power it uses in one hour).
Assuming 5 hours of peak sunlight per day means you will need a solar panel capable of producing 200 watts in 5 hours.
Dividing 200 by 5 to get the watts size for the solar panel gives 40, so you will need a 40 watt solar panel.
In an actual application the system will include wiring, a regulator and a battery to store the power. These components all consume energy, thus decreasing the overall inefficiency of the system, so you should allow for that.
Choose a battery large enough to provide power for about three days to allow for cloudy days (ALL solar panels need bright, full sunlight over the whole solar module to generate their rated output).
With an adequate 12V solar power supply you can use a 12 volt solar panel to power anything that can be run off a car battery.
If you prefer a complete solar kit that you can set up in minutes and folds into a flat carry bag for easy storage then check out our portable folding solar panels that include everything except the 12V battery.

12V 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.
  • A battery with 75% will read about 12.5V, 12.3V indicates about 50% and 12.1V is about 25% charge. Less than 12V would indicate almost no usable charge is left in the battery.

Questions about Solar Panels
Q: I want to set up a solar trickle charger for our 200 amp-hour caravan battery whilst the van is in storage, I can connect to the battery via an Anderson plug that charges from the car whilst travelling The battery is housed inside the van and is not directly accessible from outside. What sort of solar panel and regulator is required?
A: A 2 watt solar panel will be OK for trickle charging but not to recharge a flat battery. If you leave the battery connected to any powered circuits that constantly consume power then it will discharge more quickly but if it is electrically isolated when not in use then it will self-discharge at less than 1% of capacity per week. A typical 200Ah (amp-hour) battery with a self-discharge rate of about 1% per week would lose 2 Ah per week.  The 2 watt solar panel will generate a current of about 100mA (0.1Amps). Therefore in a typical day with an average of 5 hours of sun. you can expect it to recharge about 0.5 Ah back into the battery, that is enough to replace the self-discharge losses in a 200Ah battery that is still in good condition. You do not need a regulator for the 2 watt solar panel as the battery is larger than 50Ah. The main purpose of a solar regulator is to prevent overcharging a battery but the 2 watt solar panel will only generate a maximum charging current of 0.1 amps, which is too low to overcharge.

Q: How much cable comes with the 10 watt solar panel and does it have an inbuilt diode to prevent battery drainage at night?
A: The 10 watt Suntech solar panel has 3 meters of 18AWG twin cable fitted to the terminal of the junction box. ( Edit: The later models of solar panels with junction boxes do not have any cable attached.)
No, a blocking diode is not fitted, solar panels do not normally need any blocking diodes as the reverse current is negligible during normal use, in fact an installed SCHOTTKY blocking diode would cause a slight voltage drop which may cause more power loss than the reverse current at night. If the solar panels are to be kept covered or out of the sun for an extended time, then its better to connect a power isolator switch to prevent battery drainage.

Q: Do I require a regulator for a 10 watt 12 volt panel or can I connect it directly to a battery?
A: That depends on the size of the battery,for instance if it's large, say 100Ah battery,then the solar panel does not have much chance of overcharging it, which is the primary reason for using a solar regulator (also called a charger controller)A 10W solar panel will only generate about 3 Ah in one day of peak sunlight so you can connect it directly to the battery. For smaller batteries of less than 20Ah you might want to use a small solar regulator to prevent overcharging. if 3Ah per day is likely to overcharge your battery.

Q: I want to set up a 12V solar charging system that will charge and then maintain a 12 volt ride-on mower battery (24Ah). Would a 10 Watt 12 volt panel coupled to the Projecta regulator achieve this outcome? Is the necessary wiring and connectors between the panel and the regulator included and can I connect alligator clamps to the output wiring for the mower battery terminals?
A: The 10 watt solar panel has a maximum current of about 0.5 amps so in one full day of sunshine it will generate about 3Ah. If you use the mower once a week then you can probably expect about 4 days of good sunshine between mowing which would generate about 12Ah, probably enough to recharge the battery. A solar charger is not really necessary because the low charging current (0.5 amps) produced by the 10 watt solar panel is unlikely to overcharge the battery. But if you prefer to use one then any small solar regulator will be OK, connected between the solar panel and the battery. There is a small length of wiring included with the solar panel but you will need wiring for the regulator. We can supply additional wiring and battery clamps.

Q: (Regarding the 12V solar charging system discussed before) Would you recommend an in-line fuse of any sort or is it OK as is?
A: A fuse is not required on the solar panel circuit. Only if you are setting up a permanent power supply with the battery supplying power to a connected circuit then you would put an appropriate inline fuse in the battery's positive side to prevent a short causing the wiring to overheat and catch fire but I presume you are not taking power from the battery while it is being charged.

Q: (Regarding the 12V solar charging system discussed before) Also, there are no instructions on how to mount the solar panel - any special methods required? I was going to use pan head screws in the channel on the back of the panel with loose nuts securing these to 2 pieces of wood which will then be bolted to the shed roof.
A: Mounting the solar panel is a matter of personal preference. There are mounting kits on the market but your idea sounds fine. An alternative is to use a strong construction adhesive to fix it to the wooden rails. Keep an air gap under the panel to allow for a good airflow to aid cooling as cooler panels produce more electricity than hot ones. An angled support of about 30 to 40 degrees to the horizontal and facing north would maximise exposure to the sun, this would also stop dirt and debris collecting on the solar panel and obstructing the panels, clean panels produce more electricity than dirty panels. Also see our Solar Panel Mountings category for commercially available products.

Q: Will a 40 watt solar panel run our Engel fridge and some LED lights while we are camping?
A: The answer depends on the battery capacity in Amp hours that you use to run the fridge and lights and how long it lasts before it needs to be recharged. But generally speaking I think you would need 80 watt or larger solar panel. The solar panel will have to be big enough to charge up a battery that must run the fridge for a couple of days without recharging, in case you have cloudy days with little sun.

Q: I would like to use a 12 volt solar panel run a small 12 volt exhaust fan which uses a current of 2 amps during all daylight hours. Can you also tell me what size solar panel is needed to run it for any time, day or night.
A: If it consumes 2 amps then to find watts multiply volts by amps: 2 X 12 = 24 watts so if you wanted to run this fan directly from a solar panel only while the sun is shining then I would recommend a 30 watt solar panel. If you need the fan to run at any time then you also need a 12V battery to store the solar power. The size of battery depends on how long it needs to run every day. Storage battery sizes are measured in amp-hour (Ah) If the fan requires 2 amps to run then in 24 hours it will consume 24 x 2 = 48 Amp-hours from the battery. It is best to choose a battery with double the required capacity so I would recommend a 100Ah deep cycle battery. Now when choosing a solar panel to keep the battery charged up you need to remember that on average a solar panel will usually only generate peak power for 5 hours a day. Therefore the solar panel must generate 48 Ah in 5 hours. 48/5 = 9.6 amps ( let's round that off to 10 amps). When choosing a solar panel it's usually better to think in terms of the amps it will generate, and a useful general rule to remember is that a 20 watt solar panel generates about 1 amp. To keep the battery topped up every day so you can run the fan continually you need 10 amps so 10 x 20 watts= 200 watts, so you will need 200 watts of solar power. That's quite a large solar panel but that is only if you want to run it for 24 hours a day, if you only wanted to run it for 6 hours a day then the battery size and solar panel size can be 25Ah and 50 watts respectively.