What Size Charge Controller Is Needed For A 100w Solar Panel?

What Size Charge Controller - Featured Image

Who wants to know what size charge controller is needed for a 100w solar panel? As you know a solar panel gathers the energy from the sun using photovoltaic (PV) solar cells. The PV cells then convert the energy collected into direct current (DC) electricity. 

Fantastic, using the power of the sun along with a solar panel electricity is produced. This can then be used to power appliances like TV’s and charge electronics like smartphones. But hold on it’s not quite as simple as that. This is because you can’t plug a solar panel directly into an appliance like a TV. 

You can charge small electronics with a solar panel called a portable solar charger. But for larger appliances they need other components to make it possible to power them. A grid-tied residential installation needs a grid inverter. A portable or off-grid installation needs batteries and an inverter..

This means a grid tied solar installation doesn’t need a separate solar charge controller. But portable and off-grid installations do. This is because the power used to charge the battery needs to be regulated. This is done using a solar charge controller.

What Size Charge Controller Is Needed For A 100w Solar Panel?

Assuming a 12V battery with a single 100w solar panel a 10A solar charge controller is needed. The calculation for this is really easy to understand. It is total watts divided by volts and adding an extra 25% for environmental factors such as weather conditions.
So 100w (solar panel) ÷ 12V (battery) = 8.33 x 25% = 10.4. You then round 10.4 to the nearest whole number which in this case is 10A

If it were a 24V, 36V or 48V battery system you would use the same calculation but substitute the voltages. For example 100w (solar panel) divided by 24V (battery volts) x 25%

This is an informational video of the Renogy Rover Elite 20A MPPT Solar Charge Controller

Why Do I Need A Solar Charge Controller For A 100 Watt Solar Panel?

A solar charge controller is an absolute necessity. This is because it regulates the voltage and amps delivered to the battery. A 12V solar panel actually produces more than 12 volts. For instance a 100 watt solar panel might be able to produce 18.6 volts.

Voltage At Maximum Power (VMP)

This is known as the voltage at maximum power (VMP). This figure can normally be found on the rear of the solar panel or in the manufacturer’s specifications. The charge controller dampens down the VMP to the correct voltage. 

What Size Charge Controller Is Needed For A 100w Solar Panel? - VMP reading

This is really easy to do. Imagine you connect the solar panel to the battery to top it off and go off and do something else. It slips your mind that you have the battery charging. You are not using a solar charge controller and the battery is topped off after a few hours.

Instead of it now trickle charging because the battery is fully charged. Let’s say it’s 2pm in the afternoon and the sun is blazing. The solar panel is still pumping a potential VMP of 18.6V into the battery. The over-voltage can cause the battery cells to be permanently damaged or even ignite. 

Lithium Battery Solar Cells

A solar charge controller reduces the amount of volts going into the battery in the last phase of charging. It then reduces it down to a trickle charge so there is no chance of overcharging

Never try to charge a battery with a 100 watt solar panel without using a charge controller. I know there are 5w and 10w trickle charge solar panels that you do not need to use with a controller. But any higher wattage solar panel and you will damage your battery.

Related == >>>>>>>>>> Renogy Solar Panels 100w Versatile 12V Charging Power

What To Look For When Buying A Solar Charge Controller

There are several things you should look for when buying a solar charge controller. If the controller doesn’t have these things incorporated you need to think twice. A cheap build equals a cheap price equals having to spend more replacing it.

What should I Look For In The Controllers Build 

In the build you want it to have an aluminum shell because this helps with heat dissipation. You might not know that excessive heat in any solar system components equals reduced efficiency. It’s ok if the controller has a plastic facing but the rest of it should be metal.

Renogy Die Cast Aluminium Shell

Remember if it is made from entirely plastic components including the shell it will most likely not last very long. All this means is you will have to spend more money replacing it. Buying a quality controller in the first place would have saved you money and the time and effort of replacing it.

What Features Should I Look For In A Solar Charge Controller?

Make sure it is compatible with all battery types including if your battery is lithium. This is because not all controllers work with lithium.  Four stage charging is preferable to three so look for controllers that have this feature:

  • Bulk
  • Absorption
  • Float
  • Equalisation

A backlit LCD display is preferable so the various operation and error codes can be clearly seen on the screen. If it is being used on a boat or mounted outdoors you will want it to be waterproof.

A backlit LCD display is preferable
Ease Of Use

You will want a solar charge controller to be easy to use. As I mentioned before it is important to be able to see the display clearly.showing data such as;

  • Battery state of charge
  • Battery type being charged
  • PV voltage
  • Charging amps current
  • Battery Voltage
  • Temperature
  • Fault codes

A clear user manual provided with the controller is essential. Or one that can be downloaded from the manufacturers website. Also look for ease of installation especially if the controller is to be fitted in a tight space.

Related == >>>>>>>>>> Renogy Smart Lithium Iron Phosphate Battery Review

What Protections Should A Good Quality Solar Charge Controller Have?

It is no good buying a cheap solar charge controller. Because if it doesn’t incorporate the proper protections for your 100w solar panel, battery and system. You might have to spend your hard earned cash replacing elements of your solar system because it has failed.

This means your solar charge controller should have protections like;

  • Over-voltage
  • Over-discharge
  • Over-temperature
  • Over-current
  • Reverse polarity
Renogy Smart 100Ah Lithium Iron Phosphate Battery

Should I choose a MPPT or PWM Solar Charge Controller?

MPPT v’s PWM is a question most often asked when it comes to cost. Rather than efficiency and battery protection. I can tell you that although an MPPT controller is more expensive. It is up to 40% more efficient even in a smaller system. And as you know with any solar system achieving the best efficiency is the goal.

Should I choose a MPPT or PWM Solar Charge Controller?
So what is the difference? 

MPPT stands for Maximum Power Point Tracking and PWM stands for Pulse Wave Modulation.

An MPPT controller works by; 

Keeping the current transfer between the 100w solar panel and battery at its best efficiency at all times. This is achieved by continually matching and optimizing the current whatever the PV input.

An PWM controller works by;

Allowing packets of current to charge the battery like a switch. It’s internal circuitry divides the packets into pulses of the same voltage size.

The Bottom Line

The bottom line is that an MPPT controller will deliver the maximum amount of charging current to the battery. Because a solar panel delivers different voltages in different environmental conditions. Using an MPPT solar charge controller will ensure the best charging performance at all times. 

A PWM controller can’t be used with different voltages between the battery and 100 watt solar panel. In other words 12V must be used with 12V or 24V with 24V. In contrast an MPPT controller can be used with any PV/battery voltage combination.

Renogy 100w solar panel

Many people seem to think that the solar charge controller is not that important. Unfortunately they are wrong. They will find out how wrong they are when they have to replace their batteries.

Related == >>>>>>>>>> Portable Solar Briefcase Review ACOPower PLK 100w

My Closing Thoughts

We now know that a 100w solar panel needs a 10A solar charge controller. The calculation can be used for any wattage solar panel or solar array and battery voltage. It is particularly handy for arrays because the total PV voltage output needs to be matched up.

For example a 600w solar array with a 12v battery bank would need a 60A solar charge controller. 600 ÷ 12 = 50 x 25% = 62.5 rounded to the nearest whole number 60A

Not using a controller will overcharge a battery which you will want to avoid at all costs. A controller that has a metal shell, with good features and ease of use is important. And checking that it has the proper protections will save your battery from irreparable damage.

It is better to use an MPPT rather than a PWM charge controller. Because you will want the most efficient solar system possible.

For further reading take a look at my;

I hope you enjoyed this post and have found it helpful. If you have any questions about sizing a solar charge controller for a 100w solar panel. Or want to leave your own personal review, please feel free to leave a comment below.

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4 Comments on “What Size Charge Controller Is Needed For A 100w Solar Panel?”

  1. I’ve been thinking about going solar for the last year and have been doing a lot of research on the topic. I found this article super helpful, as I would not have thought about making sure I had the appropriate controller. For example, I didn’t realize there was a danger over overcharging. 

    1. Hi Alicia

      Yes lots of people think a solar charge controller isn’t necessary or a cheap one will do. Until they have to replace their batteries because they have been damaged by overcharging. Deep cycle batteries are not cheap so the mistake can prove costly



  2. This article makes sense to me. Maybe because I’m a roadie for a couple different production companies and I set up quarter million dollar audio stages and you have to know what you’re doing so that there isn’t a HUGE problem at any point in the show. And I’ve been into high dollar RC’s for over a decade and I know for a fact that you can easily ruin batteries with the wrong charger or even the wrong cable. 

    So I know how careful you have to be. And I assume the damage that can occur by not having the right control for your panels could be very costly.  

    This is a very informative article, Steve. 

    Are you completely solar now?

    1. Hi Caleb

      Ye you are right you need to be really careful with solar panels and batteries. No controller or the wrong controller can cost you. I am really pleased my article made sense to you. 



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