Inverter Sizing and Safety Factors for Solar PV Systems for JSS 1

System Sizing (inverter)

Performance Objectives

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

1. explain the function and types of inverters;
2. use the inverter safety factor to determine the corrected inverter size;
3. determine the size of solar modules needed for the system (See Load Sizing Worksheet);
4. determine the correct solar module capacity considering the locations Peak Sunshine Hours (PSH).

Understanding Inverters in Solar PV Systems

A solar PV system uses sunlight to generate electricity. This electricity is first produced as Direct Current (DC). Most appliances in homes and schools use Alternating Current (AC) electricity.

An inverter is an electronic device that converts Direct Current (DC) electricity from solar panels or batteries into Alternating Current (AC) electricity that can be used by household appliances.

Without an inverter, the DC electricity from solar panels cannot power common AC appliances like televisions, fans, or refrigerators.

Types of Inverters

Inverters come in different types, each suitable for specific solar PV system setups. The main types of inverters are:

1. Stand-alone Inverters
2. Grid-tie Inverters
3. Hybrid Inverters

Stand-alone Inverters

Stand-alone inverters are used in off-grid solar systems. These systems are not connected to the national electricity grid. They rely entirely on solar power and batteries to store energy.

Stand-alone inverters convert DC power from solar panels and batteries into AC power for appliances. They are common in remote areas or places with unreliable grid electricity.

Grid-tie Inverters

Grid-tie inverters are used in solar systems that are connected to the national electricity grid. These systems allow homes or businesses to use solar power and also draw electricity from the grid when needed.

Grid-tie inverters convert DC power from solar panels into AC power that matches the grid’s electricity. Any excess solar power can sometimes be sent back to the grid.

Hybrid Inverters

Hybrid inverters combine the features of both stand-alone and grid-tie inverters. They can work with both solar panels, batteries, and the national electricity grid.

Hybrid inverters allow a solar system to store excess solar energy in batteries for use when the sun is not shining, and also connect to the grid. This provides flexibility and ensures a continuous power supply.

Inverter Sizing and Safety Factor

Choosing the correct inverter size is very important for a solar PV system to work well. If the inverter is too small, it cannot power all the appliances. If it is too large, it might be more expensive than necessary.

The size of an inverter is usually measured in Watts (W) or kiloWatts (kW). To determine the correct inverter size, the total power needed by all the appliances must be calculated.

Calculating Total Appliance Power

Before selecting an inverter, the total power consumption of all appliances that will be connected to the solar system must be known. This is done by adding up the power ratings of each appliance.

The power rating of an appliance is usually found on its nameplate or in its user manual. It is typically given in Watts (W).

The procedure for calculating total appliance power involves the following steps:

1. List all the appliances that will be powered by the solar system.
2. Find the power rating (in Watts) for each appliance.
3. Add up the power ratings of all the appliances to get the total power (in Watts).

For example, if a home has a fan (75 W), a television (100 W), and five light bulbs (10 W each), the total power needed is calculated as follows:

1. Fan: 75 W
2. Television: 100 W
3. Five light bulbs: 5 x 10 W = 50 W

Total power = 75 W + 100 W + 50 W = 225 W.

Inverter Safety Factor

It is important not to size an inverter exactly to the total power of the appliances. A safety factor is used to ensure the inverter can handle temporary power surges and operate efficiently without being overloaded.

The inverter safety factor is a multiplier, usually between 1.25 and 1.5. This means the calculated total power needed is multiplied by this factor to get the corrected inverter size.

A common safety factor used is 1.25 (or 125%). This means the inverter should be 25% larger than the total power of the appliances.

The formula for determining the corrected inverter size is:

Corrected Inverter Size = Total Appliance Power × Inverter Safety Factor

Using the previous example where the total appliance power is 225 W, and using a safety factor of 1.25:

Corrected Inverter Size = 225 W × 1.25 = 281.25 W

Therefore, an inverter with a capacity of at least 281.25 W would be needed. In practice, an inverter with the next available standard size, such as 300 W or 500 W, would be chosen.

Determining Solar Module Size

After determining the inverter size, the next step is to determine the size of the solar modules (solar panels) needed. Solar modules convert sunlight into DC electricity.

The size of solar modules is measured in Watts (Wp), which stands for Watt-peak. This indicates the maximum power a module can produce under ideal conditions.

Load Sizing Worksheet

A load sizing worksheet is a tool used to calculate the total energy consumption of all appliances over a day. This helps in determining how much energy the solar panels need to generate.

The worksheet typically includes:

1. Appliance name
2. Power rating (Watts)
3. Number of hours used per day
4. Total daily energy consumption (Watt-hours)

The daily energy consumption for each appliance is calculated by multiplying its power rating by the number of hours it is used per day.

Daily Energy (Wh) = Power (W) × Hours Used per Day (h)

The total daily energy consumption for the system is the sum of the daily energy consumption of all appliances.

Example of a Load Sizing Worksheet:

From this example, the total daily energy consumption is 950 Watt-hours (Wh).

Peak Sunshine Hours (PSH)

Solar panels produce electricity only when there is sunlight. The amount of sunlight varies throughout the day and year. To account for this, the concept of Peak Sunshine Hours (PSH) is used.

Peak Sunshine Hours (PSH) represent the average number of hours per day when the sun’s intensity is equivalent to 1000 Watts per square metre (W/m²). This value helps in estimating the actual energy a solar panel can produce in a day.

PSH values vary by location and time of year. For example, a location in Nigeria might have an average PSH of 4 to 6 hours per day.

Information on PSH for different states in Nigeria can be found in tables showing the monthly mean of daily global solar radiation on a horizontal surface (kWh/m²/day). This value is directly used as the PSH in hours.

Calculating Solar Module Capacity

Once the total daily energy consumption and the PSH for the location are known, the required solar module capacity can be determined.

The formula for calculating the required solar module capacity is:

Required Solar Module Capacity (Wp) = Total Daily Energy Consumption (Wh) / Peak Sunshine Hours (h)

Using the example where Total Daily Energy Consumption is 950 Wh, and assuming a PSH of 5 hours for the location:

Required Solar Module Capacity = 950 Wh / 5 h = 190 Wp

This means the solar system needs solar panels that can collectively produce at least 190 Wp. If individual solar panels are 100 Wp each, then two 100 Wp panels would be needed (2 x 100 Wp = 200 Wp), as this is the next available size above 190 Wp.

Safety Considerations for Solar PV Systems

Working with solar PV systems involves electricity, so safety is very important. Even at JSS1 level, it is good to know basic safety rules.

1. Always work with adult supervision: Never attempt to install or repair solar PV components without an adult or qualified technician present.
2. Handle components carefully: Solar panels and inverters can be heavy and fragile. Handle them with care to avoid damage or injury.
3. Avoid water: Water and electricity do not mix. Keep all electrical components dry to prevent electric shock.
4. Read labels: Always check the nameplates of appliances, inverters, and solar modules for their power ratings and other important information.
5. Use correct tools: Ensure the right tools are used for any task, and that they are in good condition.