Choosing a suitable size and type of charge controller for your solar system is a great way to get the most capacity of your solar system panels and batteries while allowing for growth of the system later.

It also removes the risk of damage to the controller from excessive currents when it is undersized or sized too tight and the solar panels produce maximum power.

To size a controller, you need to decide on the controller type first: **PWM or MPPT**.

### Do you need a PWM or MPPT charge controller?

In general, you should go for an MPPT controller if budget is available. MPPT controllers though more expensive are more efficient and result in gains of 10-30% more stored energy compared to PWM controllers.

**How to size a PWM controller**

To size a PWM controller, you need to find out the following:

1. **Rated current of your solar panels**. If your solar panels have the same power rating and connected in series, the equivalent rated current is the same as that of one panel.

If connected in parallel, the rated current is the rated current of one panel x number of panels in parallel.

Then multiply the *current of your solar panels x 1.25* to allow for excessive sunshine that can happen when the panels are exposed to direct sunlight.

2. **Total combined voltage of your solar panels – current and planned**. If your solar panels have the same power rating and connected in series, the equivalent panel voltage is the same as that of one panel x the number of panels in series.

If connected in parallel, the rated voltage is the same as the rated current of one panel.

The size of your PWM charge controller voltage and current rating should be at least equal to the values in 1 and 2 above.

*For example, take 3 120W panels connected in series with a maximum power point voltage Vmpp of 18V and maximum power point current Impp of 6.7 A*

1. According to 1, above, **the rated current = 6.7 A; 6.7 x 1.25 = 8.4 A**.

2. According to 2, above, **the voltage = 18 x 3 = 54V**

*A 60V/ 20A or 60V/15 A or 60V/10A PWM controller works well as its maximum voltage-60V and current 20A or 15A or 10A all exceed the system voltage of 54V and current of 8.4A.*

**How to size an MPPT controller**

To size an MPPT controller, you need the following:

1. **Rated current of your solar panels**. If your solar panels are the same power (w) and connected in series, the equivalent rated current is the same as that of one panel.

If connected in parallel, the rated current is the rated current of one panel x number of panels in parallel.

Even though an MPPT can take higher currents and will limit to its rated value, the additional current will never be used- which is a waste.

Therefore, the rated current of your MPPT controller should be at least equal to the expected current of your solar system.

2. **Total combined voltage of your solar panels – current and planned**. If your solar panels are the same power rating (w) and connected in series, the equivalent panel voltage is the same as that of one panel x the number of panels in series.

If connected in parallel, the rated voltage is the same as the rated current of one panel.

The maximum voltage of the MPPT controller should be equal to or greater (preferably) than the equivalent voltage of your solar panels.

*For example, take 3 120W panels connected in series with a maximum power point voltage Vmpp of 18V and maximum power point current Impp of 6.7 A*

1. According to 1, above, **the rated current = 6.7 A; 6.7 x 1.25 = 8.4 A**.

2. According to 2, above, **the voltage = 18 x 3 = 54V**

*In this case, a 60V/10A or 60V/20A or higher MPPT controller works well.*

**Notes:**

To use the calculator, use the solar panel’s maximum operating voltage Vmp and panel’s maximum operating current Imp values.

The minimum solar controller voltage and current values provided by the calculator are a GUIDE only. Your charge controller rated Voltage and current values should be greater – use the next available standard values.

Finally, plan ahead as to how many panels you will connect and specify that in the no. of panels in parallel and number of panels in series section.

Otherwise, you will need to buy another controller if your sizing is too tight.

**Related Questions**

#### #1. What size of charge controller for a 500 watt panel?

The size of PWM charge controller to use with a 500W panel is** 20A** -this assumes 2 x 250W panels in parallel with an equivalent open circuit voltage of 36V and a peak current of 13.8A (2 x 6.9).

The size of MPPT controller to use with 500W panel is** 150V/30A MPPT controller**. This allows for up to 4 panels (4 x36V= 144V) and limits the charging current to 30A.

#### # 2. What size of charge controller for a 400 watt panel?

The size of PWM charge controller for use with a 400W panel is** 30A** -this assumes 2 x 200W panels in parallel with an equivalent open circuit voltage of 24V V and a peak current of 19.6A (2 x 9.8).

The size of MPPT controller to use with 400W panel is **150V/30A MPPT controller**. This allows for up to 4 panels (4 x24.3V= 97.2V) and limits the charging current to 30A.

#### # 3. What size of charge controller for a 300 watt panel?

The size of PWM charge controller for use with a 300W panel is** 10A** -this assumes that the panel has

an open circuit voltage of 38 V and a peak current of 7.9A.

The size of MPPT controller to use with 500W panel is** 100V/15A MPPT controller**. This allows for

2 panels (2 x38V= 76V) and limits the charging current to 15A.

#### # 4. What size of charge controller for a 200 watt panel?

The size of PWM charge controller for a 200W panel is** 15A** -this assumes that the panel has an open circuit voltage of 18 V and a peak current of 11.1A. If you plan to increase the size of your solar system, use a charge controller with a higher current rating.

#### # 5. What size of charge controller for a 100 watt panel?

The size of PWM charge controller for a 100W panel is** 10A** -this assumes that the panel has an open circuit voltage of 18 V and a peak current of 6A. If you plan to increase the size of your solar system, use a charge controller with a higher current rating.

#### # 6. What size of charge controller for a 50 watt panel

The size of PWM charge controller for a 50W panel is** 10A** -this assumes that the panel has an open circuit voltage of 17 V and a peak current of 2.9 A. If you plan to increase the size of your solar system, use a charge controller with a higher current rating.

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