Almost every solar calculator online does the same thing. It works out your first year saving, multiplies by 25, and shows you a lifetime savings figure. That method is wrong, and it is wrong in two directions at once.
What it gets wrong
Panels get weaker every year
Solar panels lose about 0.6% of their output every year. That is not a defect, it is normal, and it is written into the manufacturer warranty. After 25 years a panel makes roughly 85% of what it made when new.
A flat calculation ignores this and overstates your savings.
Electricity gets costlier every year
Indian electricity tariffs have risen at roughly 4% a year over the long term. The unit you save in year 20 is worth much more than the unit you save this year.
A flat calculation ignores this too, and understates your savings.
Which one wins?
Tariff rise wins, comfortably. 4% growth beats 0.6% decay every year, and the gap compounds. So the honest number is higher than what most calculators show, usually by 15 to 20% over 25 years.
Payback year changes too
On payback the two effects nearly cancel out in the early years, so the difference is smaller. A flat method might say 5.2 years where the real answer is 5.0. Not dramatic.
The big difference shows up in lifetime savings, because that is where twenty years of compounding lives.
What our calculator still does not include
We would rather tell you this than have you find out later.
- Inverter replacement. You will need one around year 12 to 15. Budget Rs 25,000 to Rs 50,000 and subtract it from the lifetime figure.
- Cleaning and maintenance. Small, but not zero. Dusty areas need cleaning more often, and dirty panels can lose 5 to 15% of output.
- Future tariff policy. Nobody knows. 4% is based on history, not a promise.
- Net billing changes. If your state switches from net metering to net billing, your savings drop.
The honest summary
Solar in India pays back in roughly 4 to 6 years for a normal home with subsidy. After that you get around twenty more years of nearly free electricity, minus one inverter and some cleaning.
That is a good investment by any measure. It does not need exaggerating, which is exactly why we show you the flat number alongside ours. You can see both and decide which you believe.