Practical PV System Designs – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Third Term)

A practical PV system design matches the size of a solar system to a household’s daily electricity needs.

Behavioural objectives

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

1. Calculate the total watts and the daily watt-hours from a load table.
2. Size an inverter using a 1.25 safety margin.
3. Work out battery capacity in amp-hours for a 12V system, using the 50% depth of discharge rule.
4. Calculate the solar panel watts needed from the daily energy demand, a 30% efficiency loss and 5 peak sun hours.
5. Check the charge controller current for a 600W panel array on a 12V system.
6. State the main parts of the final system for the Yusuf family.

What are the key terms in PV system design?

These are the words and formulae used in every PV design calculation.

• Load: the appliances that use electricity, such as bulbs, fans and a TV.
• Watt (W): the unit of electrical power. Watts = volts × amps.
• Watt-hour (Wh): the unit of energy. Wh = watts × hours of use.
• Kilowatt-hour (kWh): 1,000Wh.
• Peak sun hours: the hours in a day when sunlight is strong enough to give full panel output. This note uses 5 hours.
• Depth of discharge (DoD): the share of a battery’s capacity that is used. A lead-acid battery should use only 50%.
• Efficiency loss: energy lost in wires, heat, dust and conversion. This note adds 30%.
• kVA: the inverter rating in kilovolt-amperes. 1kVA is about 800W of real power.

What is the case study for this lesson?

The Yusuf family lives in a three-bedroom bungalow and wants solar power to replace their noisy petrol generator.

• Problem: the “I-pass-my-neighbour” generator is noisy and petrol is costly.
• Aim: a solar system for their basic needs at night.
• Running time: the items must run through the night after sunset (8 hours for lights and fans).

What is the Yusuf family’s load?

• 6 LED bulbs: 10W each, to light the house.
• 2 standing fans: 60W each, to cool the rooms at night.
• 1 LED TV: 100W, for news and football.
• 1 decoder: 20W, for the TV.
• 3 phone chargers: 10W each.

How do we size a PV system step by step?

We size a PV system in four steps: energy demand, inverter, batteries and solar panels.

Step 1: How do we calculate the daily energy demand?

The daily energy demand is the total watt-hours the appliances use in one night.

1. List each appliance and its quantity.
2. Multiply quantity by watts to get total watts.
3. Multiply total watts by hours of use to get watt-hours.
4. Add all the watt-hours.

• Total nightly need: 2,160Wh (about 2.2kWh).

Step 2: How do we size the inverter?

An inverter changes the battery’s DC power into the AC power that household appliances use.

• Total running watts: 330W, if everything is on together.
• Reason for a margin: fan motors draw extra power when they start.
• Safety margin: multiply by 1.25 to give spare capacity.

1. Total load = 330W.
2. 330W × 1.25 = 412.5W.
3. Choose an inverter rated above 412.5W.

• Decision: a 1kVA (800W) inverter. It carries the 330W load and has room to start the fans.

Step 3: How do we size the batteries?

Battery capacity is the stored energy, measured in amp-hours (Ah) at the system voltage.

• Battery type: tubular (lead-acid) batteries.
• 50% DoD rule: do not drain lead-acid batteries below half, so they last longer.

1. Energy needed = 2,160Wh.
2. Allow for 50% DoD: 2,160Wh × 2 = 4,320Wh.
3. Convert to amp-hours for a 12V system: 4,320Wh ÷ 12V = 360Ah.

• Decision: two 200Ah batteries connected in parallel, giving 400Ah. This is above the 360Ah needed.

What is the difference between series and parallel battery connection?

Series connection adds the voltages. Parallel connection adds the amp-hours.

• Yusuf design: two 12V 200Ah batteries in parallel give 12V and 400Ah.

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Step 4: How do we size the solar panels?

The panels must produce in one day the energy the batteries gave out at night, plus the losses.

1. Energy needed = 2,160Wh.
2. Add 30% for efficiency loss: 2,160Wh × 1.3 = 2,808Wh.
3. Divide by peak sun hours: 2,808Wh ÷ 5 hours = 561.6W.

• Decision: two 300W panels, giving 600W. This is above the 561.6W calculated, which gives a small extra allowance.

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What does the charge controller do?

A charge controller regulates the current from the panels to the batteries and stops overcharging.

• MPPT controller: tracks the panels’ maximum power point to collect more energy.
• Current check: 600W ÷ 12V = 50A.
• Decision: a 30A controller is too small. A 60A controller is above 50A and suits the panels.

What is the final system for the Yusuf family?

The final system has four main parts, each chosen from the calculations above.

Evaluation

1. Define depth of discharge and state the DoD used for tubular batteries.
2. If the Yusuf family buys a refrigerator next month, which parts of the system must be checked again? Give reasons.
3. Why is a 60A controller used instead of a 30A one? (Hint: 600W ÷ 12V.)
4. The TV is left on for 12 hours instead of 5. Calculate the new daily energy demand.
5. Why were the two 200Ah batteries connected in parallel?

Assignment

Write out the four steps used to design the Yusuf family’s solar system. For each step, state the calculation and the decision that was made.