Over 25 years a 3 kW system in a region with 5 peak sun hours will generate about 98,000 units. That is the number on offer. How much of it you actually collect is decided by four things, and three of them are yours to control.
The 23.3 is just the sum of 25 years of declining output. Everything below is about the difference between collecting 98,000 units and collecting 85,000.
Dust is the biggest lever you own
Our generation estimate uses a 0.77 derate factor, and inside it dust is carried at 3 to 8%. That band assumes you clean the panels now and then. If you never do, in a dry region you end up at the top of it and beyond.
Here is what each level of neglect costs on that 4,216-unit base at a Rs 8 tariff:
| Soiling loss | Units lost, year one | Year one | Over 25 years |
|---|---|---|---|
| 3% — cleaned, or rain-washed | 127 | Rs 1,010 | Rs 38,700 |
| 5% — cleaned occasionally | 211 | Rs 1,690 | Rs 64,700 |
| 8% — cleaned rarely | 337 | Rs 2,700 | Rs 1,03,400 |
| 12% — dusty area, never cleaned | 506 | Rs 4,050 | Rs 1,55,100 |
| 15% — dry season, no rain for weeks | 632 | Rs 5,060 | Rs 1,93,800 |
You can redo the last column with one multiplier:
So the gap between a cleaned array and a neglected one — roughly eight percentage points of output — is worth about Rs 1 lakh across the life of the system. For a job that takes twenty minutes a month.
How to clean, and how often
- Monthly is enough in most places. Every two weeks in dry, dusty regions, during pollen season, or near construction.
- Early morning or evening only. Never on hot panels — thermal shock can crack glass, and that is not a warranty claim you will win.
- Plain water and a soft brush or cloth. No detergent, no hard scrubbing, nothing abrasive. The anti-reflective coating is the thing you are protecting.
- Do not walk on panels. Micro-cracks are invisible and permanent.
- Monsoon does some of it for you, which is why post-monsoon output often jumps. It does not clear bird droppings or sticky pollution film.
If you pay someone, it is charged per visit and a monthly arrangement comes to a few thousand rupees a year — call it Rs 2,000 to Rs 4,000. Over 25 years that is Rs 50,000 to Rs 1,00,000, which is a real number and belongs in your arithmetic. Doing it yourself is free and takes about as long as watering plants.
Shading: the loss you cannot clean off
Dust is recoverable. A new shadow is not. A neighbour's extra floor, a tree that kept growing, a water tank moved to a convenient spot — these arrive after installation and stay.
Put the same multiplier on it:
Rs 1.29 lakh — more than the entire net cost of the system in the subsidised case. This is why an installer will look worried about a branch that seems harmless to you, and why checking shadows at 9 am, noon and 3 pm before installation matters more than any equipment choice. The shading guide covers how to survey it and what to do when you cannot design it out.
The inverter, around year 10 to 15
The inverter is the component most likely to fail first, and it is the one real capital cost across the life of the system. Budget Rs 25,000 to Rs 50,000 for a replacement somewhere between years 10 and 15.
Our payback projection does not include it. That is a deliberate omission we would rather state than bury: the projection subtracts an annual operations and maintenance allowance of 1% of system cost — about Rs 1,750 a year on a Rs 1,75,000 system — and that allowance covers cleaning and small servicing, not a new inverter.
Which is the honest conclusion: it matters, it should be budgeted, and it does not change the decision. It also falls long after break-even, so it does not move the payback year at all — the payback guide explains why the payback year is the less interesting of the two numbers.
Product warranty and performance warranty are different things
"25 year warranty" on a panel almost always means the performance warranty. The one that covers the panel actually breaking is much shorter, and sales pitches blur the two constantly.
| Warranty | Typical term | What it covers | What it does not |
|---|---|---|---|
| Panel product | 10 – 12 years | Manufacturing defects — delamination, junction box, frame | Gradual output decline |
| Panel performance | 25 years | Output staying above a stated curve | Physical damage, or anything you caused |
| Inverter | 5 – 10 years | The unit itself | Surge damage, usually |
| Workmanship | 1 – 5 years | The installation — mounting, cabling, sealing | Anything after it lapses |
Note the gap in the first row. Between year 12 and year 25 a panel that physically fails is usually outside its product warranty, while still nominally inside a 25-year performance warranty that does not cover failure. That gap is the real meaning of "25 year warranty".
What a performance claim is actually worth
Performance warranties guarantee a declining curve, typically landing around 80 to 85% of original output at year 25. Our projection applies 0.6% a year, which puts a healthy array at roughly 94% at year 10, 89% at year 20 and 86% at year 25 — comfortably inside the warranty line. The warranty only bites when you fall below it.
Rs 3,800 a year. Now the unwelcome part: to claim it you must measure your array's output under defined conditions, prove the shortfall, and find a manufacturer who is still trading twenty years later. The warranty is worth having. It is not worth paying a large premium for, and it is not a substitute for monitoring.
What 25 years of upkeep actually costs
| Item | Over 25 years | Notes |
|---|---|---|
| Cleaning, done yourself | nil | About 20 minutes a month |
| Cleaning, paid monthly | Rs 50,000 – 1,00,000 | At Rs 2,000 – 4,000 a year |
| Inverter replacement | Rs 25,000 – 50,000 | Double it if the first fails at year 10 |
| Minor electrical work | small, unpredictable | A connector, a fuse, a surge device |
| Structure anti-rust | nil if galvanised | A recurring cost on plain mild steel |
| If you pay for everything | Rs 75,000 – 2,00,000 | 6 – 16% of lifetime savings |
Against lifetime savings around Rs 12.7 lakh, that is the honest cost of ownership. It is not nothing, and nobody selling you a system will put it in writing. It also does not come close to changing the answer.
What fails, in what order
- The inverter. First, and by a wide margin. Electronics in heat.
- Connectors and DC isolators. Exposed to weather and UV. Cheap to fix if caught.
- The mounting structure. Only if it was plain mild steel. Galvanised or aluminium does not rust.
- Roof sealing at bolt points. Shows up in a monsoon, not on installation day.
- The panels. Last, and usually not at all within 25 years.
Notice that the panels — the thing you spent half the money on and all of the decision time on — are the least likely to give you trouble. The things that do are the ones buried in the cheapest lines of the quotation.
Run your own numbers through the payback calculator, which projects all 25 years one at a time with the degradation and escalation assumptions used above, then subtract one inverter from the lifetime figure yourself. And if you want to see where the 0.77 derate and the 4,216 units come from, that working is in how many units your solar actually makes.
Questions people ask
Do I need an annual maintenance contract?
Usually not for a domestic rooftop system. The work is cleaning, which you can do, and watching the monitoring, which you should do. An AMC is worth considering if the array is on a raised structure you cannot safely reach.
Can I use a pressure washer?
No. Pressure forces water past the frame seals and can damage the junction box. Low-pressure water and a soft brush is the whole method.
Will hard water leave marks?
It can leave a mineral film that itself reduces output. Wipe dry with a soft cloth, or clean early enough that the panels are not baking as the water evaporates.
My output dropped 30% overnight. Dust?
No. Dust is gradual. A sudden step change is a string down — a loose connection, a failed panel or an inverter fault. Call the installer and quote the inverter's error code rather than resetting it repeatedly.
Does the performance warranty cover output lost to dust or shade?
No, and this is where most claims fail. The warranty covers the panel's intrinsic output under test conditions. Soiling, shading and a badly designed layout are all excluded, and they are responsible for most real-world shortfalls.
Should I pay more for a longer inverter warranty?
Often yes. The inverter is the component most likely to need replacing, a 10-year term against a 5-year one covers the risk window, and the price difference is usually far less than Rs 25,000 to Rs 50,000.
Do panels stop working at 25 years?
No. They keep generating at reduced output. Twenty-five years is the performance warranty period, not an end date, and output continues to decline slowly after it.