How to Select a Solar Charge Controller

A solar charge controller is the brains of your PV based off grid system. It tells your solar array when and how to charge your battery bank. Its goal is to ensure that you get the maximum power possible from the solar panels and that you don’t overcharge your battery bank.

Is a Solar Charge Controller Necessary?

Some people skip the charge controller and hook up their panels straight to the battery. The problems with this cheap set up are:

  • The batteries will pull down the solar panels from their efficient voltage level to the battery’s voltage level hence limit the watts you receive
  • Solar panels are designed to work at higher voltages than the batteries they charge. A direct hookup will definitely overcharge batteries over time
how to choose a solar charge controller

Is Overcharging Batteries Really That Bad

You might be thinking that you can compensate for the reduced wattage by throwing in extra panels. After all, solar panels are considerably cheaper than a good MPPT charge controller.

But what about the overcharging. Well. Ignoring it will damage your batteries. Some chemistries like Flooded Lead Acid can take overcharging for a while but others like gel and sealed Lead Acid will deteriorate fast.

Overcharging Lithium based batteries will not only shorten their lifespan but could lead to catastrophic failure or a fire.

Choosing the Right Charge Controller

In a nutshell:

Only go for a PWM if you are on a very tight budget, want to deploy a below 400W system, live in an area with the ideal weather, and don’t plan to upgrade soon. Otherwise, MPPT is the best option.

The Amp rating depends on how much power you want to produce. Use the P=IV formula.

You can fill out the questionnaire below to figure out which solar charge controller you should use for your system. (Hitting submit at the end will let you see what other users had in mind when designing their systems)

Now that we’ve established that a charge controller is a must, it’s time to look at the top things you should consider when choosing a charge controller.

The main considerations should be on the charge controller operating technology. So far, there are only two types in the world

  • Maximum Power Point Tracking controllers (MPPT)
  • Pulse Width Modulation controllers (PWM)

MPPT vs PWM Charge Controller: Understanding the Differences

mppt vs pwm charge controller

There is a lot of debates out there as to which is better between MPPT and PWM charge controllers. In most occasions, the MPPT working concept is better if you are looking for more efficiency.

The main difference between the two is in how they work.

  • PWM are fast on-off switches and operate at a voltage just above the battery’s voltage
  • MPPT  controllers have a DC-DC converter and can draw power from the panel from its maximum power voltage regardless of the battery bank’s operating voltage

How Does a PWM Charge Controller Work?

A PWM controller is an on-off switch that charges your battery in pulses (hence the Pulse Width Modulation.) It will keep on connecting and disconnecting your battery to the solar array depending on the battery bank’s voltage.

When the voltage drops below a predetermined point, the PWM charge controller connects the batteries to the panels at a voltage that is slight higher. The batteries will absorb some charge and the voltage will increase.

The charger will severe the connection once the battery voltage hits a predetermined higher limit letting it sit until the voltage drops again.

The PWM Pitfall

Since this type of a charge controller is just a fast switch, it mimics what you would do by manually connecting and disconnecting a solar array to your battery to avoid overcharging.

The charge controller will always draw power from the array at a voltage that is slightly higher than the battery’s voltage.

Let’s imagine we  are working with a 100 Watts solar panel. The specifications on most 100W panels are:

  • Imp (Maximum power current) = 5.56A
  • Vmp (Maximum power voltage) = 18V
Pro Tip: The calculations here will use  the formula Power (P)=Current (I) x Voltage (V)

While this panel can theoretically produce (5.56 x 18) = 100 Watts, it will never hit this performance with a PWM since the charger will always pull down the voltage to the battery’s level.

Let’s plug in some values depending on different charge states the battery might be at and see how much power we end up with.

Battery VPWM Charge VI (Remains Constant*)Power (Watts)
12v13v5.56A72.28W
13.5v14v5.56A77.84W
14v14.8v5.56A82.29W

The power output is nowhere near the panel’s maximum performance. Furthermore, the actual figure will vary drastically as clouds and shade race by since the PWM controller doesn’t adapt to these very well.

Since PWMs can’t regulate voltage well, you will always have to get a solar panel or array whose voltage is as close to your battery bank’s voltage as possible. For instance 12V banks will go with the 18V panel which is designed for 12V systems.

How Does an MPPT Charge Controller Work?

MPPT controllers use a more complicated circuitry to track a panel’s maximum power point and then convert that voltage to something the battery bank can ‘consume.’

This means that the charge controller will try to keep our example panel at 18V as much as possible. In an ideal setting, this will give us access to around (18 x 5.56) = 100 Watts from the panels compared to the top figure of 82.29 Watts we got from the PWM.

To ensure that it doesn’t feed higher than needed voltage to the charge battery, the MPPT uses DC to DC (buck converters) to step down the voltage to the required level.

The charge controller will step down the voltage while boosting the current to keep the power delivered the same (power can’t be created or destroyed, remember?)

For instance, if it needs to charge at 13V, the controller will have to drop 18V to 13V while maintaining the 100 Watts.

The new current for this to happen must shoot to 7.69 Amps (7.69 x 13 = 100W.)

Naturally, the MPPT will deliver more power to the batteries regardless of the sun conditions. You can also tie up more panels in series without worrying about overcharging your battery bank.

For instance, you can use three 18V panels in series and the controller will still step down the (18 x3 = 54V) 54V produced to the 13V your battery bank needs.

The benefit here is your panels can produce the 14V plus threshold needed to charge even on gloomy days and you will need lower gauge cable to connect the panels to the charge controller.

MPPT vs PWM:  Which is Better

On paper, the MPPT is a way better charge controller than the PWM.  The MPPT also holds its own in real world applications where it delivers more power than the MPPT.

The MPPT is hands down better than the PWM

But WAIT and MINUTE!

An MPPT charge controller is way more expensive than an MPPT. You can get a 40 Amp PWM for as little as $30 while a 40 Amp MPPT will cost you around $130!

Choosing between the two often boils down to a couple of factors.

What is Your Budget?

If you are on a very tight budget, a PWM charge controller is better than nothing. Getting an extra panel instead of the MPPT in a small budget system will offset the power gains you could have reaped from using the MPPT.

The $130 in our scenario can get us an extra 100W panel at around $105 to offset the power difference.

Verdict: Use PWM if you are on a very tight budget and don’t think you will upgrade any time soon

How Much Power Do You Need?

Your power needs will determine how many solar panels you should invest in. Check this post I did on figuring out how many panels you need for an RV to learn more about this (the concept is the same even if your solar system isn’t going on an RV).

The maximum power you can push through a PWM is limited by current specifications upstream. Solar panels, cables, and connectors have their own maximum circuit current limits.

The most notable limit would be on popularly used MC4 connectors that have a ceiling of 40 Amps. Since you don’t want to push them to the edge, the safe maximum power you can run on a 12V system would be 400 Watts. A 24V system can run 800 Watts.

Most PWM charge controller designers know this and often max out at 40 Amps.

While you could run 800 Watts on a PWM, you will need more than a single 100 Watt panel to compensate for the charge controllers inefficiency.

Assuming an average 20% lower production compared to an MPPT, you will lose (800 x 0.2= 160) 160 Watts in a 800 Watt array.

A 40 Amp MPPT charge controller can easily handle 400 Watts in a 12V system or even 1900 Watts if you opt for a more efficient 48V system.

Verdict: Only use a PWM if you want a 12V or 24V system with a solar array producing less than 400W. Go with MPPT if you dream of more power

How Much Installation Space Do You Have?

The extra panels needed to meet your energy needs on a PWM controller won’t help much if you can’t mount them. If you have limited space like on a camper van, an RV or a tiny home, you will be better off with an MPPT so that you can harvest as much power as possible from the small space.

If you have room for an extra panel and are still under 400 Watts, you can go the PWM way.

The Weather Conditions

best solar charge controller for winter

Since MPPT charge controllers can track the optimum voltage point from a panel, they are very efficient in climates that push panels off the ideal running voltage.

An MPPT is a good deal if you live in a cold, cloudy place with well-defined winters. It will give you more power in a day than a PWM. PWMs are perfect for people who live in the tropics.

Distance from the Panels to Your Battery Bank

Power loss due to wire resistance increases with an increase in distance. To compensate for this, you have to use thicker wire or increase transmission voltage.

If your solar array is for instance 80 feet from your battery bank, you will have to use low gauge wire for the 400 watts array. The cost of the cable alone (around $280 to $400) is enough to get you an MPPT charge controller.

With the MPPT controller, you can connect the four 100W panels in series to get higher voltage and use cheaper 10 gauge wire that will cost around 99 bucks.

People installing a small system on the roof of their tiny home, RV or shed don’t have to worry much about this as the distance will be shorter. You still have to buy thicker wire but it won’t be that expensive for the shorter distance.

Verdict:  Go for an MPPT if the panels can’t be installed in under 30 feet from your battery bank.

Your Battery Bank Voltage

PWM are best for 12V or sometimes 24V systems. MPPT gives you the flexibility of choosing between 12, 24 and 48V.

The higher voltages supported by MPPT means you can push more power using less amps. Higher voltage systems are also more efficient due to lower resistances and are perfect if you want to run a powerful full-house solar system.

flow chart to choose between mppt and pwm

Are the Chinese Charge Controllers Worth it?

Since we have established that budget is the first and biggest determinant in the MPPT vs PWM battle, you might be wondering if there are some cheap MPPT alternatives out her.

The answer is yes. They do exist. Some off-brand Chinese charge controllers are true MPPT.  They will give you good value for your money.

However, you should avoid going for ridiculously cheap MPPT options as it is easy for sellers on eBay or Aliexpress to repackage and label PWM controllers as MPPT.

My off-the-bat test is in size, weight and price. I don’t expect an MPPT controller to be as tiny and light as a PWM and also would shun those 30A or 40A controllers going for under $50.

I will be testing a couple of the Chinese controllers to find out which are worth your time. You can also get very good tests from Adam Welch’s YouTube channel. He does a good job at identifying true controllers from phonies.

How May Watts Can My Charge Controller Handle?

The total wattage your charge controller can handle is a factor of your battery’s voltage and the controller’s amps rating.

From the Power = Current x Voltage formula, a 40 Amp charge controller will max out at:

Current RatingSystem VoltageTheoretical Max Watts
40A12V480W
40A24V960W
40A48V1920W

Since most solar panels rarely operate at their maximum power output and there will always be power losses on the cable between the panel and the controller, most manufacturers over quote the Wattage capability of their controller to compensate for this.

While this can work, I prefer overpowering my charge controller just in case the ideal hits one day. Giving your charge controller a gap of around 3 amps to 5 amps will put you on the safer side.

After all, it is better to forego that extra panel than end up frying your inverter on that single day when the panels produce their maximum rated power.

Throwing in a properly rated circuit breaker between the solar array and your charge controller will ensure that it never sees the maximum current it is rated for at any point hence keeping it safe throughout.

What about Solar Charge Controller With Load Output?

Small charge controllers under 30 Amps always have a load output terminal that can control your DC loads. These are perfect for DC-based systems like lights and other DC electronics.

A charge controller with a load output means you don’t have to buy a different battery protect to cut off the load when your battery is depleted. They are a compact and convenient way to protect your battery from over discharge.

However, most will have a limited output that rarely goes above 25 Amps. This is sufficient for most DC loads but you should never disconnect anything that exceeds this (for instance a power inverter) to this port as it will overload the inverter.

You can protect the inverter by adding an appropriate fuse between this output and your load terminal block for extra protection.

Verdict: Solar charge controllers with a load output are a good idea on small systems built on low budgets. You can save some money on an independent battery protect and program a low voltage disconnect straight into the charge controller

Battery Type Setting

There is a wide range of solar power batteries each of which has different charge parameters. Manufacturers recommend different voltage values for:

  • Low voltage disconnect
  • Bulk charge voltage
  • Float voltages
  • Overcharge voltage disconnect
  • Charge and discharge current protection

Finding a charge controller that lets you customize these figures is a great way to customize the charge controller for your battery bank.

Some controller will have predetermined settings for flooded lead acid, sealed lead acid, lithium and other chemistries while others will let you set custom values.

This is a crucial feature you must consider in your controller.

Further Reading: Can a Solar Generator Power a House?

Who Needs a Boost Charge Controller?

Sometimes, you might want to charge a higher voltage battery using a low voltage array. In this case, a boost charge controller is necessary. It steps up the voltage to something the battery can charge with.

These are rarely needed unless you leave far from the tropics or in a place that experiences very severe winter. You can have an additional boost charge controller that you wire in on those gloomy months to squeeze out some juice from your solar panels without rewiring the solar array or your battery pack.

Bottom Line

A charge controller will be the brains of your solar system. Settling for the right one will ensure that you get the best performance from your system at all times. You can also opt for all-in-one systems that have the charge controller bundled up with an inverter but you still have to choose if your all in one will have an MPPT or a PWM charge controller.

Oscar

In his spare time, Oscar loves tinkering with electronics. Solar panels, wiring, old TVs and sometimes DIY powerwalls. When he is not busy trying not to electrocute himself, you can find him in the garden tending to his vegetables and chickens.

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