The first question almost everyone asks is how much their monthly bill needs to be for solar to be worth it. It is an understandable question, and it helps to know that two homes with exactly the same bill can get returns that differ by almost a factor of two.
The variable that matters more than the size of the bill is what time of day you use electricity.
Why timing matters more than the bill
Solar panels only generate while there is daylight — roughly 8 a.m. to 5 p.m. Electricity produced in that window that is used in the house straight away saves you the full retail price of around THB 4.20 per unit (kWh).
If nobody uses it, the power flows out to the utility grid, where the buy-back rate is around THB 2.20 per unit — almost half of what you pay to buy it. Selling back also requires enrolling in a program with a limited quota that only accepts residential meters.
This is why a home that is empty all day while everyone is at work gets less value than a shop that is open during the day, even with the same bill.
Work it out yourself in 4 steps
Follow along with your own home's numbers — an ordinary calculator is enough.
- Divide your monthly bill by 4.20 to get the units (kWh) you use per month, then multiply by 12 for units per year.
- Estimate what percentage of your electricity you use during the day: around 30% for a home that is empty during work hours, around 50% for a home with someone in, and around 75% for a shop.
- Multiply your yearly units by that percentage to get the amount of electricity you can use yourself during the day.
- Divide by 1,350 to get a suitable system size in kilowatts. 1,350 is the number of units one kilowatt of panels produces per year in Southern Thailand.
Example: a home with a THB 5,000 monthly bill where everyone is out during the day. 5,000 ÷ 4.20 = 1,190 units per month, or 14,280 units per year. Daytime use of 30% = 4,284 units. Divided by 1,350 = a system of about 3.2 kW.
For the same home, if someone is in during the day with the air-con on, the figure moves to 50% = 7,140 units. Divided by 1,350 = a system of about 5.3 kW, saving almost twice as much — with exactly the same bill.
How is Southern Thailand different from other regions?
The figure of 1,350 units per kilowatt per year used above is the value for Southern Thailand with the rainy season already included. The South receives the least sunshine of any region in the country.
When comparing estimates, check what sunshine figure each one uses. Calculations based on the national average, or on the sunnier Northeast (Isan), give results roughly 10 to 15% higher than what systems actually produce in the South.
Homes where we would say: don't install yet
These conditions should be resolved before installing.
- The bill is below roughly THB 2,500 a month and nobody is home during the day. Payback would be so long that the money is better used elsewhere.
- The roof is deteriorating or due for replacement. Replace the roof first; otherwise, in a few years the panels will have to come down and be reinstalled at extra cost.
- The sunny part of the roof is shaded by large trees or neighboring buildings for most of the day. Even partial shade can cut the output of the whole set significantly.
- You plan to sell the house within 2 to 3 years, because the added value in the sale price usually does not match the money invested.
Summary
Solar is most worthwhile for homes and businesses that use most of their electricity during the day — the more daytime use, the faster the payback. Homes that use most of their power at night can still install, but should choose a size that matches their real daytime use. An oversized system sends surplus power out to the grid at a low price.
Frequently asked questions
How high does my monthly bill need to be before solar is worth considering?
Generally, from around THB 2,500 to 3,000 a month it starts to become attractive — but only if you use a fair amount of power during the day. If the house is empty all day while you work, the bill needs to be higher than that for solar to pay off.
Where does the figure of 1,350 units per kilowatt per year come from?
It is the sunshine yield for Southern Thailand with the rainy season included. We use it because it matches real conditions here, rather than the national average, which is higher than what systems here actually produce.
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Keep reading
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- Monthly Bill of THB 3,000, 5,000 or 10,000 — What Size Solar System Do You Need?The best-value size is not the largest your roof can hold — it is the size that matches the electricity you use during the day.
- Thailand's 2026 Solar Tax Deduction: Up to THB 200,000 — Who Actually QualifiesThe measure runs until the end of 2028, but several conditions stop people who have already installed from using it. The most common mistakes are system size and the names on the documents.
- Thai Electricity Is THB 3.95 per Unit for Sep–Dec 2026 — How to Calculate Solar Payback CorrectlyThe tariff you see in the news and the rate you actually pay are not the same. Here's how to work out your own real cost per unit so your payback calculation is accurate.