To size a solar system, find your average daytime electricity use in kWh, divide it by about 5 peak sun hours to get the system size in kW, divide the system size by the panel rating to get the number of panels, and multiply by about 2 m² per panel to get the roof area. The examples below use simple round numbers; an on-site survey by the installer gives the accurate figure.
Method 1: Read Your Actual Meter
Record your meter for 3–7 days to find the average daytime use.
Step 1: Average Daytime Use
Read the meter at 08:00 and 17:00 each day. Example:
| Day | Daytime use |
|---|---|
| 1 | 20 kWh |
| 2 | 25 kWh |
| 3 | 20 kWh |
Average: (20 + 25 + 20) ÷ 3 ≈ 21.6 kWh per day
Step 2: System Size
A panel produces the equivalent of about 5 hours of full output per day, so 21.6 ÷ 5 = 4.32 kW. Round up to about 5 kW (5,000 W); the exact size depends on each installer’s packages.
Step 3: Number of Panels
One panel produces about 400–500 W. Using 450 W: 5,000 ÷ 450 = 11.1, so install 12 panels.
Step 4: Roof Area
One panel is about 1 × 2 m = 2 m², so 12 × 2 = 24 m² of roof.
Step 5: Savings
At 4 baht per unit, savings are 21.6 × 4 = 86.4 baht per day, or about 2,592 baht per month. This is based on reduced daytime use, not maximum output, because the system supplies what is actually used.
Step 6: Payback
Savings per year: 2,592 × 12 = 31,104 baht. A 5 kW system costing around 200,000–300,000 baht (mid-point 250,000) pays back in 250,000 ÷ 31,104 ≈ 8 years.
Method 2: Add Up Your Appliances
Use the power rating (watts) on each appliance’s label:
Daytime use = power (W) × number of units × daytime hours of use
| Appliance | Power | Units | Hours | Energy |
|---|---|---|---|---|
| 12,000 BTU air conditioner | 1,000 W | 1 | 5 | 5,000 Wh |
| 24,000 BTU air conditioner | 2,500 W | 1 | 3 | 7,500 Wh |
| 14 cu.ft. refrigerator | 200 W | 1 | 9 | 1,800 Wh |
| Television | 100 W | 1 | 3 | 300 Wh |
| Total | 14,600 Wh (14.6 kWh) |
System size: 14.6 ÷ 5 = 2.92 kW, so about 3 kW.
Important Note
These examples use approximate numbers, and each installer may use different factors. For factories, the same logic applies at a larger scale, but load profiles, roof structure and shading need a professional survey. See solar benefits for factories and designing a system for future needs.
Frequently Asked Questions
How much roof area does a 5 kW system need?
About 24 m² with twelve 450 W panels of roughly 1 × 2 m each.
Why are savings based on usage rather than maximum output?
Because an on-grid system supplies the electricity you actually use during the day; savings depend on that use.
Request a site survey or see ONVALLA’s solar solutions.
