People often assume a 3 kW system makes 3 units every hour the sun is up. It does not, and the gap between the rating and the reality confuses a lot of new solar owners into thinking something is wrong.
Nothing is wrong. Here is how the number actually works.
The formula
Three parts to understand:
System kW
The rated capacity. This is measured under laboratory test conditions that your roof will never match — 25°C panel temperature and perfect light. Real conditions are always below it.
Peak sun hours
Not the number of daylight hours. It is the equivalent hours of full-strength sunlight in a day. Your panels produce something from 7 am to 6 pm, but the total works out to about 5 hours of full output.
| Region | Peak sun hours |
|---|---|
| Rajasthan, Gujarat, Ladakh | 5.5 |
| MP, Maharashtra, Telangana, AP | 5.0 |
| Delhi, UP, Punjab, Haryana, Bihar | 4.5 – 4.8 |
| Kerala, North East, hill regions | 4.0 – 4.5 |
The 0.77 loss factor
Roughly a quarter of theoretical output is lost along the way:
| Loss | Typical | Why |
|---|---|---|
| Temperature | 8 – 12% | Panels lose output as they heat. Indian summers are hard on this. |
| Inverter conversion | 3 – 5% | DC to AC is never free |
| Dust and soiling | 3 – 8% | Depends entirely on how often you clean |
| Cables and wiring | 2 – 3% | Resistance over distance |
| Mismatch and tolerance | 2 – 3% | Panels are never perfectly identical |
A worked example
Quick rule of thumb: 1 kW makes about 4 units a day across most of India.
Month by month, it varies a lot
The yearly average hides a big swing. For a 3 kW system in a typical northern city:
| Period | Units per month | Why |
|---|---|---|
| March – May | 400 – 450 | Long clear days, best months |
| June – September | 220 – 300 | Monsoon cloud |
| October – November | 350 – 400 | Clear post-monsoon skies |
| December – February | 280 – 340 | Short days, low sun, fog in the north |
That is the whole argument for net metering. Your surplus in March covers your shortfall in July, and the grid does the storing for free.
Why your neighbour's system makes more than yours
Same size, same city, different output. Usually one of these:
- Shading. The biggest cause by far, and often something small you stopped noticing.
- Direction. East or west facing loses 10 to 15% against south.
- Dirty panels. Easily 5 to 15% in a dusty area.
- Inverter quality. A cheap inverter converts less and clips more.
- Cable runs. A long run from roof to meter loses more than a short one.
Cleaning matters more than people think
Dust is the one loss you can fix for free. In dry, dusty regions panels can lose 15% within a few weeks of no rain.
- Once a month is enough in most places
- Every two weeks in dusty areas or during heavy pollen
- Early morning or evening — never on hot panels, thermal shock can crack glass
- Plain water and a soft brush. No detergent, no hard scrubbing.
Panels get weaker every year
About 0.6% a year. This is normal, not a defect, and it is written into the performance warranty.
| Year | Output vs new |
|---|---|
| 1 | 100% |
| 10 | ~94% |
| 20 | ~89% |
| 25 | ~86% |
Panels do not stop at 25 years. They keep working at reduced output. Twenty five years is the warranty period, not the end of life.
How to check your own system
- Note the inverter's total generation reading on the 1st of each month
- Compare to the same month last year
- A drop of more than 10% year on year means something changed — new shade, dirty panels, or a fault
If your inverter has monitoring, use it. Most people install it and never look at it again, and then do not notice for a year when one string stops working.
What a normal day looks like
Generation is not flat across the day. For a 3 kW system on a clear day:
| Time | Output | Units in that hour |
|---|---|---|
| 7 – 8 am | ~0.3 kW | 0.3 |
| 9 – 10 am | ~1.4 kW | 1.4 |
| 11 am – 1 pm | ~2.4 kW | 2.4 each hour |
| 2 – 3 pm | ~2.0 kW | 2.0 |
| 4 – 5 pm | ~1.0 kW | 1.0 |
| After 6 pm | ~0 | 0 |
| Day total | ~12 units |
Notice it never reaches the full 3 kW rating. That rating is a laboratory figure at 25°C panel temperature. On a real roof at noon the panels are at 55 to 65°C, and output drops with heat. Hitting 75 to 85% of rated capacity at peak is normal and healthy.
Why this shape matters
Most of your generation happens between 10 am and 3 pm. Most household consumption happens early morning and evening. That mismatch is the whole reason net metering exists.
It also means one free lever is available to you: move flexible loads into the middle of the day. Washing machine, water pump, geyser, dishwasher, EV charging. Under net billing, where exported units are worth much less than what you buy, this is worth real money.
Reading your bill after solar
Your bill changes shape and confuses people the first month. A net metered bill shows:
| Line | What it means |
|---|---|
| Import units | Drawn from the grid, mostly at night |
| Export units | Sent back during the day |
| Net units | Import minus export. This is what you pay for. |
| Banked units | Credit carried forward if export exceeded import |
| Fixed charges | Unchanged. Solar does not reduce these. |
If your bill still shows a few hundred rupees despite good generation, that is usually fixed charges, meter rent and electricity duty. Those do not go away.
Warning signs worth acting on
| Symptom | Likely cause |
|---|---|
| Sudden drop of 30 – 50% | One string down — loose connection or failed panel |
| Gradual drop over weeks | Dust build-up. Clean and re-check. |
| Drop at the same time each day | New shading — something grew or got built |
| Inverter shows an error code | Note it and call the installer. Do not just reset repeatedly. |
| Output fine but bill unchanged | Net meter may not be recording export correctly |
That last one is worth checking in the first two months. A net meter installed but wired incorrectly will show generation on your inverter while your bill behaves as though nothing changed.
Questions people ask
Why does my system never reach its rated output?
Because the rating is a laboratory figure. Heat, dust, wiring losses and inverter conversion all take a share. Peaking at 75 to 85% of rating is normal and not a fault.
Will output be less on a cloudy day?
Yes, typically 20 to 40% of a clear day depending on cloud thickness. Panels work on diffuse light, so you still generate — just less.
Does rain damage panels?
No. Panels are built for it, and rain cleans them. What matters is that mounting and cable entries were sealed properly at installation.
How do I know if a panel has failed?
A single failed panel usually shows as a persistent drop in one string's output on the monitoring app. Without monitoring you are unlikely to notice until the annual bill looks wrong — which is the argument for insisting on monitoring in the first place.
Does hot weather help or hurt?
It hurts. Panels lose output as they heat, so a clear 28°C day in March often beats a hazy 42°C day in June. Sunlight helps, heat does not, and summer brings both.
How much does dust really cost me?
In a dry, dusty area, 10 to 15% within a few weeks of no rain. On a 3 kW system that is roughly 40 to 60 units a month — a few hundred rupees, every month, for a job that takes twenty minutes. It is the highest-return maintenance task available.
My generation dropped after monsoon. Is something wrong?
Probably not. Post-monsoon skies are usually clear and output recovers. If it stays low into October and November, check for dust, new shading, or a string that stopped reporting.