Plan solar panel count and generation
Start with a monthly electricity bill, kWh usage or a system size to estimate whole panels and possible generation. This is a screening calculation, not a site-specific design or a bill-savings forecast.
Back to calculator ↑Start with a bill, usage or desired capacity
Bill mode estimates kWh from an average effective rate: THB 1,350 ÷ THB 4.50/kWh = 300 kWh. This editable rate is an example; use bill kWh directly when available because fixed charges and tiers can distort the estimate.
Usage mode takes kWh over your chosen period and a generation target percentage. For 450 kWh in 30 days at 50%, the target is 225 kWh. Capacity mode takes a requested DC system size in kWp. Panel, sun and loss inputs have starting assumptions, so a panel model is not required to begin; update them when installer figures are available. The target percentage describes generated energy, not bill savings.
An optional roof area is compared with the simple sum of panel footprints; it does not lay out rows, setbacks, access paths or shading.
How capacity becomes generation
Bill mode first estimates kWh = bill ÷ assumed average effective rate. Bill and usage modes then use requested kWp = target kWh ÷ (days × assumed peak sun hours/day × output-retained fraction). The engine divides this capacity by panel watts and rounds panel count up. It uses installed kWp from those whole panels to estimate daily and period generation.
The starting 4.5 peak-sun-hours and 80% retained-output values are assumptions, not a site forecast or equipment rating. The 80% factor multiplies theoretical generation by 0.8.
Worked example: a 225 kWh target
With a 225 kWh target over 30 days, 4.5 peak sun hours, 80% retained output and 450 W panels, requested capacity is 225 ÷ (30 × 4.5 × 0.8) = about 2.0833 kWp. Panel count is ceil(2.0833 × 1,000 ÷ 450) = five panels.
Rounded installed capacity is 5 × 450 ÷ 1,000 = 2.25 kWp. Estimated output is 2.25 × 4.5 × 0.8 = 8.1 kWh/day, or 243 kWh over 30 days. If each panel occupies 2 m², simple total panel footprint is 10 m², excluding row spacing, safe access and setbacks.
Take the estimate to a roof survey
Actual output changes with shading, orientation, tilt, temperature, weather and equipment. Explore location-specific solar resources or PVWatts, then ask an installer to assess roof structure, usable layout, electrical connection and applicable local requirements.
Generated kWh does not equal bill savings: daytime use, export treatment and tariffs determine value. This tool does not estimate avoided cost, payback or export income. Match production with your load profile before making a financial decision.
Common questions
Will matching monthly kWh eliminate my bill?
Not necessarily. Generation and demand can occur at different times, and bills have other components.
Are 4.5 hours and 80% site-specific values?
No. They are editable illustrative assumptions; check local resource and the actual roof.
Does a 10 m² panel footprint mean a 10 m² roof is sufficient?
No. Setbacks, access, shading, geometry and mounting requirements need a layout assessment.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.