How Many Watts Can a 20A Charge Controller Handle?

You may have noticed that a lot of solar panel kits – and even DIY solar systems – use 20A charge controllers. It’s a popular choice because it can work with different types of solar panels. But if you’re new to solar power, how do you find out how many watts a 20A charge controller can handle? Will it be enough for a 12V, 200W solar panel for instance?

A 20A charge controller can handle 240 watts on a 12V solar system and 480 watts if the system is 24V. More advanced charge controllers support 12V and 24V solar panels and can adjust its settings to match the voltage requirements.

How to Calculate Charge Controller Watt Capacity

20A Charge controllers are designed to run 12V or 24V solar systems. This voltage limit determines how many watts the controller can run. The formula is charge controller voltage x amps = maximum watt capacity.

12V x 20A = 240W

24V x 20A = 480W

Larger charge controllers have support for 48V systems as well. Today you can find controller amp sizes ranging from 10A to 100A, though 20, 30 and 60A are the most widely used.

Keep in mind that 240 watts is the capacity limit for a 12V 20A charge controller, and 480 watts for 24V. We do not recommend running the device at its limit as it could overload the system.

For safety, 200 watts should be the maximum load for a 20A controller. That leaves about 20% available power in case of a spike. It is probably safe to load 240 watts a few times. But if you need 240 watts on a regular basis, better get a 30A charge controller to give you more flexibility.

If you don’t want to mess around with 12V and 24V systems, you can buy a charge controller that supports 12V and 24V. For example, the Binen PWM 20A Solar Controller supports 12V and 24V system. It adjusts the settings automatically depending on what system is is running. if you prefer an MPPT charge controller, we recommend the Renogy 20A Rover as it also provides support for 12V and 24V configurations.

Can a 20A Charge Controller Run a 24V 500W Solar Panel?

If we do the math (24V x 20A = 480) it’s going to come up a bit short. However this calculation assumes the solar panel is generating maximum power. In most cases the panel output will be lower than its capacity.. A 250W 24V panel produces 7.25-7.75 amps, so a 20A controller can get it done. But we don’t recommend you do this because it puts a lot of strain on the controller.

This applies only for 12V and 24V batteries. With 36V and 48V batteries, 500W is not a problem at all. Going through the conversion again:

20A x 36V = 720W
20A x 48V = 960W

If you want to run a 500 watt solar array, a 20A controller is not enough. As we pointed out the limit for 12V 20A controllers should be 200 watts, or 240 if you want to go the limit. but anything beyond that and you need a bigger controller.

Can a 12V 20A Charge Controller Run a 300W Solar Panel?

12V 20A controllers have a maximum capacity of 240 watts. The only way to run a 300W solar panel is to use a 24V 20A controller or higher.

it is true that solar panels do not always run at their stated output. A 300W panel might only reach 200 watts during cloudy days for instance. But output rises during sunny days so why take the risk? if your panel reaches its peak and generates 300 watts, it will overload the controller.

Most 300W 12V panels have a voltage capacity of 18V (as we mentioned earlier, 12V batteries charge at 14.4V and can handle more depending on design). The current for these panels tops out at 20A. This is at the absolute limit for a 20A PWM controller. If the panel over performs in fine weather you won’t be able to harness that extra power.

If you have a 30A controller you can use that power. And with peak sun the battery should receive up to 20A. A 200ah battery capacity will do nicely here. You can also use an MPPT controller, but you have to decide the additional cost is worth it.

How to Figure Charge Controller Watt and Amp Limits

The load voltage indicates the highest possible amps for your solar panel. For 12V batteries you can use 15 load volts (30 segment panel), 16 load volts (32 segment panel) or 18 load volts (36 segment panel). The load voltage is lower in all of them and each has an open circuit voltage.

Your 20A controller can run all these. Using our 200W panel example again:

• 200 watts / 15 volts = 13.3 amps
• 200 watts / 16 volts = 12.5 amps
• 200 watts / 18 volts = 11.1 amps

With a PWM controller, the 15V system generates the highest amps at peak sunlight. However it will go under the 14.4.V needed to charge a 12V battery when the sun’s angle changes (i.e. drops). The 16V and 18V panels will always be above 14.4V.

If you have an MPPT controller, it’s going to charge at the highest possible voltage (14.4V). For this to work the panel needs to generate more than 14.4V. In this case the best performer will be the 18V system. Some controllers can operate even higher capacities (200 watts, 36 volts, 72 segments).

All this calculating might be cumbersome, but if you’re going to install a solar panel, it’s essential you know the figures. This is the only way to avoid making costly mistakes. And once you know the conversion formula you can apply it to other solar panel and battery sizes.

Should I Use a 20A PWM or MPPT Charge Controller?

There is a lot of debate on which to use, PWM or MPPT. Everyone agrees that MPPT is better technically, but the question is if it is worth the cost. With small solar systems a PWM controller is enough.

It’s a different matter if you have 300W panel has a higher voltage. If it’s at 36V to 40V (typical of 72 cell panels), you should get an MPPT controller. Here’s why.

A PWM controller can only work up to 14.4V. If the panel voltage maxes out at 17V or 18V, the loss is minimal. But with 36V or higher, you’re going to lose more than half the panel’s power capacity. A PWM controller just can’t handle that kind of voltage range.

An MPPT controller operates the panel at maximum voltage so you get the entire 300W. An MPPT controller also puts up to 30A in the battery, so a 30A MPPT controller is ideal in this case. You can also change the setting so the controller runs optimally on a smaller battery.

MPPT controllers also work great if you have two 150W panels in a parallel or series configuration. In parallel you can get up to 16A/18V, and in a series 8A/36V. With an MPPT controller you can configure it to match the setup you’re using without wasting the power.

Tips for Using a 20A Charge Controller

• A lot of solar panel kits have charge controllers bundled, so that makes things easier. You don’t have to worry if the controller is the right size or not. If you are building a solar system and buying components separately, keep the following in mind.
• Know how much power you need. If you need 240 watts or less, a 12V 20A controller is enough. For 480W and below, a 24V controller will do. Anything more than those and you need large unit.
• Use the controller only as directed. Follow the instructions to the letter. If the controller won’t run, check the wiring as that is usually the cause.
• Place the controller as close to the battery as possible. The nearer they are, the shorter the cable you can use. Short, thick cables minimize power losses.
• Always leave room for an upgrade. Set up a flexible system so you can add more charge controllers later if necessary.

Conclusion

If you don’t fancy doing all this math, that’s okay as most charge controllers specify the maximum watts they can can work with. However, knowing the facts and figures behind it all helps if you run into problems with your panel. And this knowledge also makes it easier to find the right controller for you.