Estimate generation
Model AC electricity output for the site, including temperature, shade, inverter and other system losses.
Article · Calculate value for money
How much can installing solar PV reduce your electricity bill per month? And how many years will the payback be?
See a simple calculation method. Complete with examples showing assumptions at every step. So that you can evaluate without misunderstanding that every house will be equally economical.
Short answer
How much can solar PV save?
It depends on how much of the electricity you use at home when it is produced. Self-consumed electricity reduces purchases from the grid. Surplus electricity has value only where an applicable sale contract exists. System size alone cannot establish savings or payback.
Estimate generation using site location, orientation, shade and system losses, then compare it with your usage by time of day and your actual tariff.
3 steps
How to calculate savings
Find self-consumption
Compare hourly generation with household usage. Self-consumed energy = generation × self-consumption share.
Convert to money
Bill reduction = self-consumed energy × value of avoided electricity purchases. Calculate surplus exports separately if a sale contract applies.
Hypothetical example
A 5 kWp system
Assume the system generates 500 kWh of AC electricity in a month, the home uses 70% (350 kWh) itself, and avoided electricity costs an average of 4 baht per kWh. The example bill reduction is 350 × 4 = 1,400 baht per month, or 16,800 baht per year. The remaining 150 kWh is not counted as income.
The 500 kWh output, 70% share and 4 baht rate are teaching assumptions, not measured production, a current tariff or a customer result. Replace them with site and bill data before deciding.
Payback period
How to calculate payback
Simple payback = net investment ÷ annual net benefit
If investment is hypothetically 150,000 baht and there are no other costs or revenues, 150,000 ÷ 16,800 ≈ 8.9 years. This is a mathematical example, not a quotation or promised payback.
A real project must include maintenance, replacements, degradation, tariff changes, taxes and financing costs. Count export revenue only when supported by an applicable contract.
Why are the real numbers different?
7 Factors that cause unequal savings.
- Does the house use a lot or a little of daytime electricity?
- Is the system size too large for daytime load?
- Direction, angle of inclination, and shade of the roof
- Weather and seasons
- panel performance Inverter and power cord
- The user's actual electricity rate
- Maintenance and cleanliness of the panel
Increase value without adding panels
Shift electricity use to sunny hours
Set the washing time
Move washing and drying to mid-morning to afternoon. When the system is producing power
Pump water and charge the car during the day.
Schedule flexible loads during sunny hours to reduce unused surplus electricity.
Turn on the air conditioner with a plan.
If there are people staying in the house during the day Using the air conditioner during power generation can greatly increase the proportion you use yourself.
Before believing the numbers in the quotation
Ask the installer to complete 5.
- Use electricity usage data for which month and for how many months?
- Assuming the output per kW, how much?
- Assume what percentage of your own electricity is used?
- How many baht per unit does the average electricity cost cost?
- total deterioration Maintenance cost And other expenses?
Credible bidders should be able to explain their assumptions. Don't just give numbers. “Maximum savings”
Want numbers from your actual bill?
Send the monthly electricity bill 6–12 to the PTH GREEN team to evaluate the system size, savings amount, and initial payback period for free.
Read more: Is your home suitable? · How much does each size system cost?