What Charge Controller Size Do I Need For a 100W Solar Panel?

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A charge controller is necessary to keep batteries from overcharging when connected to solar panels. When a 100W solar panel stores energy in a battery, the controller ensures everything is working smoothly. For this to happen, you need the right charge controller size for your 100W solar panel.

A 100W solar panel with a 12V battery bank needs a 10 amp charge controller. Add the total watts of the solar panel then divide it by the battery voltage and add 25% for safety margin. 100W / 12 is 833., but add a safety margin and round it off to 10 amps.

How Big of a Charge Controller Do You Need?

The formula is total PV module watts x battery voltage + 25% = charge controller size.

If your battery is not 12V, replace it with whatever voltage you have (24V, 36V and 48V are now common).

  • 100W / 12V + 25% = 10.4 rounded off to 10 amps
  • 100W / 24V + 25% = 5.20 rounded off to 5 amps
  • 100W / 36V + 25% = 3.47 rounded off to 3.5 amps
  • 100W / 48V + 25% = 2.6 rounded off to 3 amps

A high quality 12V 100W solar panel like the Renogy Mono PV Module can produce up to 8.33 amps an hour, which is well within the range of the charge controller sizes above. But if you are going to use the solar panel all day, you need a bigger charge controller.

An efficient 100W solar panel can generate up to 41.5 amps a day with 5 sun hours (8.33 amps x 5 =41.5). A 10 amp controller will not be enough, so you need a 60 amp charge controller (remember to add a safety margin).

While solar panels do not produce peak output all day, your calculations should assume it does. This is for safety reasons to ensure the controller is the right size. During summer and in sunny states like Nevada and Texas, the panel output is close to peak levels on average.

As a general guideline:

  • 80 amp controller for 65 amps
  • 60 amp controller for 45 amps
  • 30 amp controller for 20 amps

These guidelines are for 12V 100W panels only. A 24V 100W module is equal to roughly 4 amps, not 8.3, so you have to make adjustments. The higher the battery voltage the lower the amps. However the total will depend on how much power your panel generates.

Why Have a Charge Controller Safety Margin?

The 25% margin is a general guideline only. You can make it higher or lower, but it is necessary. Overcharging a battery can be dangerous so do not take chances with the controller. It never hurts to have a safety margin, not just for charge controllers but also inverters and batteries.

If the controller is too small it won’t be able to keep the battery from overheating or completely failing. 100W may not be sufficient to damage the controller or the battery right away, but over time the components will falter.

If you use a 30 amp controller such as the EEEKit PWM with a solar panel that produces 30 amps, the system is pushed to the limit. During peak sun the current may go up higher than 30 amps, overloading the controller. It could damage not just the controller but also your battery.

Solar power systems are low maintenance, but the system can break down and accidents happen if components are mismatched. Solar panels have to be grounded, batteries and charge controllers have to be the right size too.

You can use a bigger charge controller, but never smaller. While 10 amps is enough for 100W as it already has a safety margin, you can use 20 amps or more. That is a good option if you ever decide to use more power in the near future. But if you have no plans to use more power, 10 amps is enough as the extra amp capacity will be wasted.

MPPT vs. PWM Charge Controllers

The biggest difference between the two is that a PWM charge controller can result in up to 15% solar energy loss. MPPT charge controllers are more efficient but are more expensive.

Which you choose is a personal preference, but usually MPPT controllers are used in large solar arrays. For small systems a PWM controller will often suffice. In this case we are dealing with 100W or 8.3 amps, so PWM should be enough.

But if you are going to connect several 100W solar panels in a series, consider using an MPPT controller. The more power you need, the lower the solar energy losses have to be. That 15% energy loss from PWM controllers will add up the bigger the system, so efficiency is a must.

A PWM charge controller must also have the same voltage as the solar panel, otherwise it will not run properly. There are now well-designed PWM controllers that reduce energy loss and have self diagnostic features built in.

A PWM controller cannot restrict its current output. If your 100W solar panel produces 40 amps a day and the controller is rated 30 amps, it could damage the controller, battery and the solar appliances you connected to it.

Use an MPPT charge controller if you want to use all of your solar panel’s power to charge a battery. What makes MPPT controllers effective is it pulls current using the panel’s maximum voltage. MPPT controllers also adjust the current to ensure optimum performance. Compared to PWM, the efficiency of an MPPT controller is anywhere from 90%-95%.

How to Size PWM Charge Controllers for 100W Solar Panels

There are five things to look for in a PWM charge controller spec sheet: the voltage rating, solar input capacity, battery type, terminal and the battery current rating.

Battery Current Rating. Also known as the amperage, this tells you how much amps the controller can handle. On paper a 30 amp charge controller will work with a 30 amp solar panel, but in real life you shouldn’t. Better add the safety margin discussed earlier because solar panel output will vary depending on the day and season. Sudden spikes could damage a controller already running at the limit.

System Voltage Rating. This tells you what battery bank voltage capacity the charge controller is compatible with. If the label says 12 volts, then do not use it for anything other than a 12V battery. If it says, 24V, you can use the controller for any battery no higher than 24V. 12V is acceptable, but not 36V or 48V.

Solar Input Capacity. This is the maximum amount of volts that can be placed in the controller. If you connect two 100W modules in a series and add the safety margin, the voltage would be 45. The charge controller must have a 50V solar input capacity to be safe.

Battery Compatibility. Some charge controllers only work with lead acid, while others are designed for lithium ion . Others are compatible with both, so it varies. Check the compatibility specs before buying. Never use a charge controller with a battery it is not made for, as that can be dangerous.

Terminal Gauge Size. Use the terminal gauge size as a guide to determine what solar wire to use in the installation.

One more thing needs to be considered: the efficiency of the controller. A good quality PWM controller will state its energy efficiency. The acceptable range is 85% and up. A loss of greater than 15% is too much, so look for another brand or buy an MPPT charge controller instead.

How to Size MPPT Charge Controllers for 100W Solar Panels

An MPPT charge controller can restrict its output, so your solar array can be any size. Even so there are some factors you have to consider to get the best possible results.

Amp Reading. This information tells you how many amps the MPPT controller will use. If the reading says 50 amps, the controller will operate with 50 amps. If your panel produces 60 amps, the MPPT controller will reduce the output to 50 amps.

Unlike a PWM controller, the high amps will not cause damage. However, you are wasting your panel’s potential, so the MPPT controller must be a match to the panel output. Because an MPPT can make full use of the panel output, the controller amp size should be commensurate to the panel power.

Voltage Rating. The rating is going to be higher than the battery it is designed for, but that is because MPPT controllers can lower or increase the voltage accordingly. You may never have to use the high voltage capacity, but it is there in case you do need it.

Solar Input Voltage. On some MPPT controllers this can be as high as 100V. If you connect a 100W solar panel to it, the controller will reduce the voltage to match the module. The same adjustment happens when you charge a battery.


Summarizing the main points, you just divide the solar panel watts by the voltage and add a safety margin percentage to the result. That tells you what charge controller to get. Thee calculations are easier than you think, and it makes a huge difference in performance and results.