Sizing Charge Controllers and Inverters – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Third Term)

Sizing a charge controller or an inverter means choosing a rating that safely handles the current or power in the solar system.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Calculate the size of a charge controller from the panel current and a 25% safety margin.
2. Compare PWM and MPPT charge controllers and say when to use each.
3. Calculate the running load and surge for an inverter.
4. Explain how inverter efficiency affects the power drawn from the battery.
5. Match an inverter to a battery of the correct voltage.

Where do the charge controller and inverter fit in a solar system?

The charge controller sits between the panels and the battery, and the inverter sits between the battery and the AC load.

What is a charge controller?

A charge controller is a device that manages the current going from the solar panels into the battery.

• Main job: it stops the battery from overcharging.
• Simple picture: it works like a traffic warden for the current from the panels.
• Unit used for sizing: amps (A).

How do you size a charge controller?

Choose a charge controller whose amp rating is at least the total panel current plus 25%.

The golden rule: the controller must handle the maximum current the panels can produce.

Key terms

• Isc (short circuit current): the highest current one panel can give, printed on the panel label.
• Parallel connection: panels joined so that their currents add up and the voltage stays the same.
• Series connection: panels joined so that their voltages add up and the current stays the same.
• Safety margin: extra capacity of 25% allowed for very bright sun or cloud-edge reflection.

Steps to calculate

1. Read the Isc on the back of the panel. Example: 9 A.
2. Multiply by the number of panels in parallel. Example: 4 panels, so 9 A × 4 = 36 A.
3. Add the 25% safety margin by multiplying by 1.25. Example: 36 A × 1.25 = 45 A.
4. Choose the next standard size above the answer. Example: a 50 A or 60 A controller.

• Too small: a 30 A controller would be overloaded in this example and could overheat and fail.
• Also check voltage: the controller’s maximum PV input voltage must be higher than the voltage the panels can produce.

What is the difference between PWM and MPPT controllers?

PWM and MPPT are the two types of solar charge controller.

• PWM (Pulse Width Modulation): a cheaper controller that switches the panel current on and off, like a tap that is either open or shut.
• MPPT (Maximum Power Point Tracking): a smarter controller that converts extra panel voltage into extra charging current.

What is an inverter?

An inverter is a device that changes DC power from the battery into AC power for household appliances.

• Unit used for sizing: watts (W) or kilovolt-amperes (KVA).
• KVA: 1 KVA is 1,000 volt-amperes. A 1 KVA inverter is often taken as about 800 W of real power.

How do you size an inverter?

Choose an inverter whose rating handles all the loads running at the same time plus the surge of the biggest motor appliance.

Step 1: Find the running load

Add up the watts of everything used at once.

• 5 bulbs (10 W each): 50 W.
• 2 fans (60 W each): 120 W.
• 1 TV: 100 W.
• Total running load: 270 W.

Step 2: Allow for the surge

Surge is the extra power a motor appliance (fridge, pump, air conditioner) draws for a very short time when it starts.

• Example: a 200 W fridge may need 1,000 W for about one second to start.
• Rule: the inverter must handle the running load plus the surge of the biggest appliance.
• Worked example: other loads 270 W + fridge surge 1,000 W = 1,270 W at the moment the fridge starts.
• Check both ratings: read the continuous rating and the surge (peak) rating on the inverter label.

How do inverter efficiency and battery voltage affect sizing?

Inverter efficiency is the share of battery power that reaches the load, and the rest is lost as heat.

Efficiency

• Typical loss: inverters usually lose about 10-15% of the power when changing DC to AC.
• Formula: power from battery = load ÷ efficiency.

1. Load = 500 W. Efficiency = 85% = 0.85.
2. Power from battery = 500 ÷ 0.85 = about 588 W.
3. Plan the battery and panels for this larger figure.

Matching battery voltage

Inverters come in 12 V, 24 V and 48 V versions, and the battery voltage must match the inverter.

• 12 V inverter: use with a 12 V battery.
• 24 V inverter: use with a 24 V battery.
• Wrong match: a 12 V inverter connected to a 24 V battery can be damaged at once.

Why use higher voltage for big systems?

• Rule of thumb: above 2 KVA, use 24 V or 48 V.
• Reason: current = power ÷ voltage, so higher voltage means lower current.
• Example: 588 W gives about 49 A at 12 V but about 24.5 A at 24 V.
• Result: cables heat less and lose less voltage.

What are the key sizing rules?

• Charge controller: panel amps (Isc × panels in parallel) + 25%.
• Inverter: running watts + surge of the biggest motor appliance.
• Controller type: prefer MPPT for all but very small systems.
• Inverter type: prefer pure sine wave, which gives clean power that is safe for electronics and motors.
• Efficiency: divide the load by the inverter efficiency to get battery power.
• Voltage: inverter and battery voltage must be the same.

Evaluation

A principal’s office has one small fridge (150 W running, 800 W surge), two laptops (60 W each) and four bulbs (10 W each).

1. What is the total running wattage?
2. Two panels of 8 A each are connected in parallel. What size charge controller is needed?
3. Is a 1 KVA (800 W) inverter enough to start the fridge? Give a reason.
4. State two differences between a PWM and an MPPT charge controller.

Assignment

Answer both parts, showing every step.

1. Three panels are connected in parallel, each with an Isc of 10 A. Calculate the charge controller size needed, including the 25% safety margin.
2. A 400 W load is connected to an inverter that is 85% efficient. Calculate the power the inverter pulls from the battery.