How Many Watts Can a 20A Charge Controller Handle?

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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 charge controller’s capacity is determined by multiplying its amperage with the battery voltage. The conversion is amps x battery voltage = watts. 20A x 12V = 240W, so it can handle 200W solar panels. However the results can be affected by three factors: a different battery voltage, the charge controller type and solar panel energy loss / efficiency rating. This is why a lot of new solar power users make mistakes in their calculations.

Watts, Amps & Voltage Calculation Guide

One common solar myth – one of many you should know– is assuming the battery charges at its given voltage, i.e., a 12V battery charges at 12V. That’s incorrect. 12V batteries actually charge at 14.4V. A 20A controller can still handle that, but if it has lower amps it will take longer to charge if at all.

Load Voltage

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.

Can a 20A Charge Controller Handle 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 many (probably most) cases this isn’t so. 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

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

The quick answer is no, it can’t. You should get a 30A PWM or MPPT charge controller instead. The longer, detailed answer is below.

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 the difference isn’t enough to justify the extra cost.

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. In the scenario above, a PWM will do just fine.

When to Use an MPPT Controller?

It’s a different matter if the 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.

What if the Charge Controller Amp is Too Small for the Solar Panel?

Short answer: this is unlikely to happen because charge controllers specify (in the manual and website) the largest solar panel size it can work with.

But suppose you add more solar panels to your system and created an array. Now the charge controller is amp is too small. What will happen?

Contrary to popular belief, using an inadequate controller will not burn the controller or the panel. However the output will be limited to the controller’s amp capacity. You’ won’t be able to use your array’s potential as the controller limits production.

You’ll waste money and watt power that’s all. But don’t operate beyond the controller’s maximum voltage capacity as that can burn the system down. Just buy the right sized controller. Check its specs online or the box. There should be a datasheet stating the limits.

Can You Run a Charge Controller at the Amp Limit?

You’ve probably heard people you should never let batteries drop below 50% before charging. That’s good advice to make batteries last (there’s more here). But can you run charge controllers at the limit?

Most of the time you can use controllers to the maximum level. Majority have a high solar size limit anyway. MPPT controllers do a better job than PWM when it comes to excess panel watt size.

PWM cycle continuously to keep the voltage from reaching the limit. The system basically moves the amps around which won’t do the controller any good. An MPPT controller has a mechanism that keeps the amps from getting into the unit.

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.