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.
Year by year, what it actually looks like
A 3 kW system, Rs 82,000 net cost, 4,200 units in year one, Rs 8 a unit:
| Year | Units | Rate | Saving | Cumulative |
|---|---|---|---|---|
| 1 | 4,200 | Rs 8.00 | Rs 33,600 | Rs 33,600 |
| 2 | 4,175 | Rs 8.32 | Rs 34,736 | Rs 68,336 |
| 3 | 4,150 | Rs 8.65 | Rs 35,898 | Rs 1,04,234 |
| 5 | 4,100 | Rs 9.36 | Rs 38,376 | Rs 1,79,000 |
| 10 | 3,978 | Rs 11.39 | Rs 45,309 | Rs 3,88,000 |
| 15 | 3,860 | Rs 13.85 | Rs 53,461 | Rs 6,34,000 |
| 20 | 3,745 | Rs 16.85 | Rs 63,103 | Rs 9,25,000 |
| 25 | 3,634 | Rs 20.50 | Rs 74,497 | Rs 12,70,000 |
Two things stand out. Payback lands in year 3, not year 5 — because savings rise while the cost stays fixed. And by year 25 the annual saving has more than doubled, purely because electricity got dearer.
Why the tariff assumption matters most
Panel degradation is well understood and barely moves the answer. The tariff assumption moves it enormously.
| Tariff rise | 25 year savings | vs 4% |
|---|---|---|
| 0% — tariffs never rise | Rs 8,05,000 | – 37% |
| 2% a year | Rs 10,20,000 | – 20% |
| 4% a year (our assumption) | Rs 12,70,000 | — |
| 6% a year | Rs 16,40,000 | + 29% |
Even at 0% — electricity never getting costlier again, which has never happened — solar still pays back comfortably. That is the reassuring part. The projection is not resting on an optimistic assumption.
What we still leave out
We would rather list these than have you discover them later.
Inverter replacement
Rs 25,000 to Rs 50,000, once, around year 12 to 15. Subtract it from the lifetime figure. It does not change the payback year, since it falls well after break-even.
Cleaning and maintenance
Small but not zero. If you pay someone to clean panels monthly, that is a few thousand a year. If you do it yourself, it is free. Dirty panels can cost 5 to 15% of output, so this is money either way.
The discount rate argument
A strict financial analysis would discount future savings — a rupee in year 20 is worth less than a rupee today. That gives a longer, more conservative payback. We do not do it, because the comparison most people actually want is against a rising electricity bill, not against a fixed deposit.
Policy change
If your state moves from net metering to net billing, your surplus becomes worth less and savings drop. Several states have made that change in recent years.
Why other calculators show smaller numbers
| What they do | Effect |
|---|---|
| Multiply year one by 25 | Ignores tariff rise — understates badly |
| Ignore degradation | Overstates slightly |
| Use today's tariff throughout | The single biggest error |
| Assume 100% self-consumption | Overstates, unless you have net metering |
The errors run in both directions, which is why a flat figure can look reasonable while being wrong in structure. Ours is not more optimistic — it is differently constructed, and we print the flat result beside it on every payback page so you can see the gap yourself.
Questions people ask
Is 4% a year a safe assumption for tariffs?
It reflects the long-run trend in Indian retail electricity prices. Nobody can promise the future. The table above exists so you can see what happens if we are wrong in either direction.
Do panels really last 25 years?
They last longer. Twenty five years is the performance warranty period, not the end of life. Panels installed in the 1990s are still producing. Output continues declining slowly after year 25.
What is a good payback period?
Anything under six years is good for a home system in India. Under four is excellent and usually means a high tariff, good sun, and proper net metering.
Why does my installer quote a shorter payback than you do?
Usually one of three reasons: they assume you use every unit at retail value, they use your highest tariff slab rather than your average, or they leave out the fact that fixed charges stay on your bill. None of these is dishonest exactly, but together they can turn a five year payback into a claimed three.
Does the payback change if I have a battery?
It gets longer, because the battery adds cost without adding generation. A battery is a backup purchase, not a savings one. Keep the two decisions separate.
Should I use my average tariff or my top slab rate?
Your average across the year is the honest figure. Solar reduces consumption from the top slab down, so the first units saved are worth your highest rate — but as your bill falls you drop into cheaper slabs and later units are worth less. Using the top slab throughout overstates the saving.
Why does the payback year matter less than people think?
Because nothing changes at that point. It is simply the year cumulative savings pass the cost. The system keeps generating for two decades afterwards, and the lifetime figure is the number that actually describes the investment.
Does a bigger system pay back faster?
Not necessarily. Above 3 kW the subsidy stops, so cost per kW jumps while savings per kW stay the same. A 3 kW system generally pays back faster than an 8 kW one.
What if I move house before payback?
A working solar system adds to what a buyer will pay, though rarely the full remaining value. If you expect to move within three years, the arithmetic is weaker. Beyond five years it stops being a concern, because you have already recovered the cost.
Should I wait for panel prices to fall further?
Panel prices have fallen for years and may keep falling. But every year you wait is a year of savings you did not get, and subsidy schemes are not permanent. Waiting for a better price on a purchase that pays back in four years usually costs more than it saves.
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.