A simple way to calculate the size of solar cells. Using this amount of power, how many panels should be installed to be cost-effective?

A simple way to calculate the size of solar cells: How many panels to install for the most cost-effective power supply?
In an era of steadily rising electricity costs, installing solar rooftops has become an increasingly popular option for homeowners. However, many homeowners are still confused about where to start. How many kilowatts of power are needed and how many panels are needed to truly reduce their electricity bills and achieve a reasonable return on investment?

This article will guide you through the step-by-step calculations. You don't need to be an engineer to choose the right solar cell size for your home.

Solar cells are a way to "produce your own electricity" and reduce your electricity purchases from the grid.
Instead of purchasing electricity from the grid, a solar cell system allows you to generate electricity during the day, immediately reducing your electricity bills. This is especially true for homes that use the air conditioner, refrigerator, computer, water pump, security cameras, and other appliances throughout the day.

In a simpler way, solar cells are like purchasing "power in advance" for the next 20 years. After the break-even point, you'll have free electricity.

Things to consider before calculating system size:
1. Average monthly electricity bill
2. Is daytime electricity usage high?
3. Roof type and sun exposure
4. Estimated budget
5. Goals, such as reducing electricity bills by 50% or 100%

Knowing these numbers helps to make accurate calculations and reduce errors.

Easy-to-understand system size calculation formula
There is a basic formula already used in industry:

Required system size (kW)
equals average monthly electricity bill divided by 1,000 and multiplied by 0.6

Or, in simpler terms:

System size in kW ≈ (monthly electricity bill ÷ 1,000) x 0.6

Why multiply by 0.6? Generally, solar PV systems can reduce electricity bills by approximately 60 percent if installed on-grid without batteries. and primarily use electricity during the daytime.

Example 1
Electricity bill: 3,000 baht per month
Calculated as 3,000 ÷ 1,000 × 0.6 = 1.8 kW
Therefore, a system of approximately 2 kW should be installed to meet the needs.

Example 2
Electricity bill: 5,000 baht per month
Calculated as 5 kW × 0.6 = 3 kW
However, to support actual usage, a system of 4 to 5 kW should be installed.

To reduce electricity bills by 80 to 100 percent, the system size must be increased proportionally.

How many solar panels are required to produce 1 kW?
Currently, most solar panels on the Thai market are between 450 and 560 watts per panel.
On average, approximately 2 panels are used for every 1 kW.

Therefore, a 3 kW system requires approximately 6 panels. A 5 kW system requires approximately 10 panels. A 10 kW system requires approximately 20 panels.

This also depends on the efficiency of the panels chosen. If you choose high-powered panels, you'll use fewer panels, saving more space.

Analyzing actual home appliances
Another accurate assessment is to look at the electrical load of appliances during the daytime. For example:

A 1200-watt air conditioner is used 8 hours a day.
An additional 900-watt unit is used 6 hours a day.
A 150-watt refrigerator is used 24 hours a day.
A 300-watt computer is used for 10 hours.
Other appliances, such as a TV and water pump, total 300 watts.

When you add up the numbers of appliances using electricity during the daytime, you'll see an average power consumption of at least 2,500 to 3,500 watts between 9:00 AM and 5:00 PM.

In this case, the ideal system size is 3 to 5 kW to cover actual daily usage.

Take into account Thailand's average sunlight.
Thailand has an advantage in solar energy. The average solar radiation level is high globally, making solar PV systems cost-effective.

In Thailand, a 1 kW solar system can typically produce approximately 4 to 5 units of electricity per day, or 120 to 150 units per month.

If you want to reduce your monthly electricity bill by approximately 3,000 baht, you should have a system that can produce 300 to 350 units per month.
Therefore, you should install at least 2.5 to 3 kW or more.

What happens if you install solar panels and produce excess electricity?
1. First, you can use it yourself first.
2. Any excess electricity can be sent to the grid if your home is permitted under the "Pulling the Grid" program.
3. If there is no electricity purchase policy, you may need to adjust your daytime electricity usage to reduce losses.

Therefore, the design of your solar system should primarily reflect your home's electricity usage patterns.

Choosing the Best Roof Installation Location
The roof should receive full sunlight and be shaded.

Facing south, it should receive consistent light throughout the day.
East: receives abundant morning light.
West: receives abundant afternoon light.

If your home has large trees or adjacent buildings blocking the light, you should adjust the location or increase the number of panels accordingly.

Estimated Budget for Solar Roof Installation for Homes
Installation costs depend on the equipment brand, panel quality, installation method, and warranty.
In general, in 2025, prices are as follows:

A 3 kW system will cost approximately 150,000 to 200,000 baht.
A 5 kW system will cost approximately 220,000 to 260,000 baht.
A 10 kW system will cost approximately 380,000 to 450,000 baht.

Larger systems are more cost-effective due to the reduced monthly electricity bills.

The payback period for solar panels is:
On average, 3 to 5 years if the home uses electricity heavily during the day.
If electricity is used less frequently or primarily at night, the payback period may be 6 to 8 years.

After the break-even point, you will have free electricity for more than 15 years.
This is because solar panels have an average lifespan of over 25 years.

Checklist before deciding to install solar panels:
Assess your home's electricity usage habits.
Assess your budget and energy reduction goals.
Check the roof structure to ensure it can support the weight.
Choose standard equipment with a genuine warranty. Choose an installation team with a proven track record.
Test the system before delivery and use an app to track electricity production.

Easy-to-understand summary:
If your average monthly electricity bill is 3,000 baht or more, a solar panel system can significantly reduce it.
A medium-sized home should typically use a 3- to 5-kW system, or approximately 6-12 panels.
This can be easily calculated based on your monthly electricity bill or the electrical appliances used during the day.
The average payback period is 3- to 5-years, with free electricity for a long period after that.

Therefore,