Solar Panel Calculator
Size a rooftop solar system from your electricity bill.
Monthly Electricity Usage
Cost per Unit
Average Peak Sunlight
Installed Cost per kW
Panel Wattage
System Size Required
2.50 kW
Panels Required
7 × 400W panels
Roof Area Needed
16 m²
Estimated System Cost
₹1,37,500
Monthly Savings
₹2,400
Payback Period
4.8 years
CO₂ Offset / Year
2.95 tonnes
How this calculator works
System size is worked out from how much energy you actually use: your monthly usage is converted to a daily average, then divided by your local peak sunlight hours and an 80% system efficiency factor (accounting for inverter and wiring losses) to find the kW capacity needed to cover it. Panel count follows from dividing that capacity by your chosen panel wattage.
Worked example
Using 300 units a month at ₹8/unit, with 5 peak sun hours and 400W panels costing ₹55,000/kW installed, works out to a 2.5kW system (7 panels), costing about ₹1,37,500, with a payback period of roughly 4.8 years from electricity bill savings alone.
Why the 80% efficiency factor matters
Solar panels rarely deliver their full rated output in real conditions — DC-to-AC inverter conversion, wiring resistance, panel temperature, and dust all reduce actual output below the panel's nameplate rating. Sizing the system to your efficiency-adjusted need, rather than the theoretical maximum, avoids under-sizing a system that then can't actually cover your usage.
What payback period doesn't include
This is a simple payback calculation: installed cost divided by annual savings. It doesn't account for financing costs if you take a loan, available government subsidies (which can meaningfully shorten real payback), or gradual panel degradation — most panels lose roughly 0.5-0.8% of output per year, so year-20 output is somewhat lower than year-1 output. Enter your net cost after any subsidy for a more accurate payback estimate.
Frequently Asked Questions
How is solar system size calculated from my electricity bill?
This calculator converts your monthly electricity usage into daily units, then divides by your location's average sun hours and a system efficiency factor (accounting for inverter and wiring losses) to estimate the system size in kW needed to cover that usage.
What is payback period and how is it calculated?
Payback period is how many years of electricity bill savings it takes to recover the system's upfront cost — calculated as total system cost divided by estimated annual savings.
Does this estimate include government subsidies?
No — it calculates cost and payback based on the cost-per-kW figure you enter. If subsidies apply in your area, enter your net cost after subsidy for a more accurate payback estimate.
Why does the calculator use 80% system efficiency instead of 100%?
Panels rarely deliver their full rated output in practice — inverter conversion losses, wiring resistance, heat, and dust all reduce actual output below the nameplate rating. Sizing to the efficiency-adjusted need avoids under-sizing a system that then can't actually cover your usage.
Do solar panels lose efficiency over time?
Yes — most panels degrade roughly 0.5-0.8% per year, so output in year 20 is somewhat lower than in year 1. This calculator's payback estimate doesn't account for that gradual decline, so real long-term savings are slightly lower than a simple year-1 estimate suggests.