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A charge controller ensures optimum current goes into the battery without overloading it. This will only work if the controller is properly sized, so if you have a 10 amp charge controller, it must be used within limits.
A 10A charge controller can handle 130 to 150 watts of solar power. 12V system often use 20A charge controllers, but if it is less than 150 watts, a 10A controller is enough.
Is a 10A Charge Controller Large Enough For My System?
Charge controllers are sized based on the solar system voltage and current or amps. The controller must be large enough to deal with the power generated by the solar panel.
If your solar panel is less than 150 watts, a 10 amp charge controller is sufficient. If it is higher than 150 watts, you will need a bigger controller,
But if your solar panel output is exactly 150 watts, that will push the charge controller to the limit. It might be able to handle that power, but over the long term you are better off getting a larger unit or connecting multiple charge controllers.
If you have no plans to use a large solar panel or battery, 10 amps will be sufficient. So if you only go hiking for a few hours during summer and travel light, a 100W solar panel, the Renogy Wanderer 10A charge controller and a small battery is enough.
But if you need more power now – or in the future – you have to know how to size charge controllers so the system does not overload.
The most important charge controller specifications are amps and voltages. These will tell you how many solar power watts it can work with.
How to Calculate Charge Controller Size
Charge controller amp ratings range from 1 to 60. The most widely used are 10A, 20A, 30A, 40A, 50A and 60A. Voltage ratings for charge controllers are 12V, 24V and 48V.
Solar panel watts x battery voltage = charge controller amp size
You may also add 10%-25% to the amps for reserve power. Doing this affects the charge controller size. Here is an example:
You have a 100 watt solar panel and a 12V battery. Following the steps given earlier.
100 / 12 = 8.3
That is 8.3 amps, so a 10A charge controller will be just right.
Even if you add a 20% cushion the result would be 10.3 amps, still within range. But if you add 25% the amp required goes up to 11. A 10A controller can still work but it would be under your preferred reserve power.
Those percentages are somewhat negligible, but become more significant when dealing with large solar panels. Let’s try a 120W solar panel with a 12V battery.
120 / 12 = 10
It is exactly at 10 amps. If you tried a 150W solar panel that would be:
150 / 12 = 12.5
It is over 10 amps. But solar controllers charge at a higher voltage than their nominal output. A 12V controller can reach 14.4 volts, so we can use 14.4 instead of 12V.
150 / 14.4 = 10.4
10.4 amps, which is slightly over the limit but still usable.
And solar panels do not always produce at their rated output either so the controller should still run within its limits. In other words, make sure your charge controller is not too small or there will be problems.
Solar Panel Output in Relation to Charge Controllers
Solar panel output does not always match its rating. Because of how solar power works, the output on average will be lower than its rating.
A 150W solar panel in theory generates 750 watts with 5 hours of sunlight. But due to the weather, location, efficiency and other factors, the output will be less than 150 watts an hour.
On a perfect day the panel might reach something close to 750 watts. But on a typical day it will be close to 720 watts or even lower.
Which brings us back to 10 amp charge controllers. If the solar panel produces 150 watts / 12.5 amps an hour consistently, it is still acceptable. Remember that these controllers can reach 14.4 volts.
And since solar panel output varies, the amps probably doesn’t even reach 12.5 amps most of the time. It might peak at 12.5 or higher at noon but drop off in the afternoon.
MPPT or PWM Solar Controllers
MPPT charge controllers cost more than PWM because they are more efficient. But for a 10A charge controller, a PWM is sufficient. The following will illustrate the difference between an MPPT and PWM and why the latter is enough for small solar power systems.
Earlier we mentioned that a 12V solar controller charges up to 14.4 volts. A 12V solar system on the other hand, charges up to 18 volts.
In this case, a PWM controller will pull the 18 volts down to 14.4 volts. There is some energy loss here, but given the solar panel size it is acceptable.
But if you have a 300W solar panel the losses increase. The larger the solar system the more significant the power losses become. With an MPPT controller this can be avoided as it automatically optimizes the system tor the best results. Our pick is the EPEVER MPPT 10A Controller.
How Much Reserve Controller Capacity Do I Need?
The other thing you need to consider though is the reserve power. If you add a 10% to 25% to the calculations, a 10A solar controller will be insufficient for most systems.
So this brings up the question, do you really need that much reserve power? If you look up charge controller calculations online, some people have up to 25% reserve, but others do not even bother with it. So what should you do?
Well it depends on how you use solar power. Here are some guidelines to keep in mind.
If you want to play it safe, add a 25% reserve power to the calculations. You will probably end up needing more than a 10A solar controller. But it doesn’t hurt to use a bigger charge controller. In fact it gives you flexibility to upgrade your system later on.
If you are going to use a small solar panel – 120 watts or less – a 10A solar controller will be enough. There is no reason to buy a bigger controller if you won’t be using its power.
If you typically use 150 watt solar panels, it depends on the output. If it is often close to 150 watts, get a 20A solar controller to avoid overloading risks.
But if the output is closer to 120 than 150, a 10A solar controller should be fine. Solar panel outputs will vary depending on the season so keep that in mind.
Do I Even Need a Charge Controller?
A charge controller is required for almost all solar systems equipped with a battery bank. The only exceptions are portable solar chargers which are less than 5 watts.
A charge controller performs many functions, but the most important is it protects the batteries from overloading, overheating and overcharging.
As long as a solar panel is exposed to the sun, it will convert solar energy into direct current (DC). This current flows into the battery.
Without a charge controller the current will just keep flowing into the battery and overload it. This will damage the battery and other devices you loaded onto the system.
Make sure your solar controller is large enough for the PV array you are building. Use the same procedure earlier, just replace the 150 watts with whatever solar array you install.
While 10 amp controllers are suitable for 100W solar panels, a 24V or 48V PV array needs a 24V or 48V charge controller. The ideal amp capacity would be 30 to 60 amps too.
If you buy a solar panel kit, it will come with a charge controller. This removes the need for any manual calculation as you can be certain the controller will work with the solar panels. However you still need to do the calculations if you decide to upgrade the system.
A 10A charge controller works with small solar panels pretty well. If that is all you need this device will do fine. But if you intend to upgrade to a larger solar system, consider a larger charge controller.