Guide
Solar Payback: What Actually Decides It
Rooftop solar is sold on a payback period, and most of the advice about shortening it points at the wrong inputs. These figures come from sweeping the calculator on this site one input at a time, so each is the effect of that input with everything else held still.
Payback does not depend on system size
This is the first thing to get out of the way, because it is the question people open the calculator with.
Holding everything else steady, a 1 kW system and a 20 kW system on the same roof pay back in the same year. So does every size between. The reason is arithmetic rather than anything about solar: doubling the system doubles what it costs and doubles what it generates, and payback is one divided by the other.
What size changes is the money. Over twenty-five years the 1 kW system returned about Rs 3.1 lakh and the 20 kW system about Rs 61.7 lakh — twenty times as much, on the same payback year. Size is a decision about how much you earn, not how quickly you break even, and it is bounded by your roof, your sanctioned load and how much you actually consume.
A fixed subsidy is the one thing that makes size matter
Add a flat subsidy — one that pays a fixed amount rather than a percentage — and the picture inverts. With Rs 78,000 taken off the price regardless of size, the same sweep gives a payback of year 1 at 1 kW, year 4 at 5 kW and year 5 at 20 kW.
Nothing about the panels changed. A fixed sum is simply a larger fraction of a smaller system, so the smallest system that meets the subsidy's conditions has the shortest payback by definition.
This is worth knowing before comparing quotes: a payback figure computed with a subsidy in it is not comparable with one computed without, and two systems of different sizes under the same subsidy are not comparable with each other either.
What does move it
Swept across their plausible ranges with no subsidy, the inputs sort clearly. Installed cost from Rs 40,000 to Rs 90,000 per kW moves payback by four years. The tariff you are offsetting, from Rs 5 to Rs 12 a unit, moves it by four. The share you consume as it is generated, from 30% to 100%, moves it by four. Sunlight, from 3.5 to 6 peak hours a day, moves it by three.
System size moves it by zero. Annual tariff escalation, anywhere from 0% to 8%, moves it by one year. Panel degradation, from 0% to 3% a year, moves it by one.
Two of those deserve comment. Self-use is in the top group because a unit you consume is worth the full retail tariff while a unit you export earns whatever the export rate is — usually far less. Shifting consumption into daylight is free, and it is the only lever on that list you can still pull after the system is installed.
| Input | Range swept | Moves payback byyears |
|---|---|---|
| Installed cost | Rs 40,000 – 90,000 / kW | 4 |
| Tariff offset | Rs 5 – 12 / unit | 4 |
| Self-use share | 30 – 100 % | 4 |
| Sunlight | 3.5 – 6 h/day | 3 |
| Tariff rise | 0 – 8 %/yr | 1 |
| Panel degradation | 0 – 3 %/yr | 1 |
| System size | 1 – 20 kW | 0 |
The number people optimise is not the number holding the money
Degradation and tariff escalation barely touch the payback year — one year each across their whole range — because a payback that lands around year five arrives before compounding has had time to do much.
Over twenty-five years they dominate. With the tariff flat, the same system returned about Rs 9.8 lakh; at 3% a year, Rs 15.4 lakh; at 8% a year, Rs 33.3 lakh. That is a three-and-a-half-fold swing driven entirely by an assumption about future electricity prices — and it is an assumption, not an input you can verify. Degradation runs the other way: 0% a year returned Rs 16.7 lakh and 3% a year returned Rs 10.4 lakh.
So the panel warranty everyone compares matters mostly for the lifetime figure, and the lifetime figure rests on a guess about tariffs. Treat a twenty-five-year total as a scenario rather than a projection, and be suspicious of any quote that gives you one without saying what escalation it assumed.
What to do with this
Compare quotes on installed cost per kW. It is the input with the widest real spread and it is the one you can actually negotiate.
Size the system to your consumption and your roof, not to a payback target — the payback will not move.
Find out your export rate before you sign anything. It sets how much the self-use share is worth, and self-use is the only lever left afterwards.
Ask any quote what tariff escalation it assumed. If it will not say, its lifetime saving figure means nothing.
And compare payback figures only when they were computed the same way — with the subsidy or without, never one of each.
Common questions
Will a bigger system pay back faster?
No — not unless a fixed subsidy is involved. Sweeping system size from 1 kW to 20 kW with everything else held steady gave the same payback year at every size, because cost and generation both scale with the number of panels. A bigger system earns more in total; it does not break even sooner.
Solar Payback Calculator →What is self-use and why does it matter so much?
It is the share of what you generate that you consume as it is produced, rather than exporting. A unit you use is worth the full retail tariff; a unit you export earns the export rate, which is usually much lower. Moving that share from 30% to 100% shortened payback by four years in the sweep — as much as halving the installation cost — and running heavy appliances in daylight is the only lever on the list that costs nothing.
How much does panel degradation really matter?
For the payback year, almost not at all: sweeping it from 0% to 3% a year moved payback by one year, because payback arrives before the decline compounds. For the twenty-five-year total it matters a great deal — the same sweep ran from Rs 16.7 lakh down to Rs 10.4 lakh. Compare warranties for the lifetime figure, not the payback.
Why does the calculator ask for a tariff rise?
Because leaving it out is a choice too, and a wrong one — electricity prices have not been flat anywhere. It matters more than any other input to the lifetime figure: 0%, 3% and 8% a year gave Rs 9.8 lakh, Rs 15.4 lakh and Rs 33.3 lakh. It is also the input you can least justify, which is why the lifetime number should be read as a scenario.
Should I add a battery?
On a grid-connected roof, usually not on financial grounds. Its return is only the difference between the tariff avoided and the export rate given up, and on the calculator's defaults the pack reaches the end of its cycle life without having paid back. Batteries earn their keep where the grid is unreliable or where there is no export credit at all — which are reasons to buy one, but not payback ones.
Battery & Inverter Sizing →