Methodology
How these numbers are produced
Every constant we use, where it comes from, and what each calculator leaves out.
Constants
| Constant | Value | Basis |
|---|---|---|
| System derate factor | 0.77 | Inverter loss, panel temperature, soiling, wiring and mismatch combined |
| Annual panel degradation | 0.6% | Within the range most manufacturer performance warranties assume |
| Tariff escalation | 4% a year | Long-run trend in Indian retail electricity tariffs |
| Projection horizon | 25 years | Standard panel performance warranty period |
| Annual O&M | 1% of cost | Cleaning and periodic inverter servicing |
| Inverter efficiency | 88% | Typical domestic hybrid inverter |
| Usable roof fraction | 80% | Allowance for tanks, stairwells, parapets and access |
| Grid emission factor | 0.71 kg/kWh | Approximate Indian grid average, for the CO₂ figure |
Peak sun hours by region
| Region | Hours |
|---|---|
| Rajasthan, Gujarat, Ladakh | 5.5 |
| MP, Maharashtra, Telangana, AP, Karnataka | 5 |
| Delhi, UP, Punjab, Haryana, Bihar | 4.7 |
| Kerala, North-East, hill regions | 4.2 |
These are annual averages. Winter output in the north runs well below the average and summer well above, which is exactly why net metering — where surplus banks across months — matters so much in India.
Battery depth of discharge
| Chemistry | DoD | Cycle life |
|---|---|---|
| Flat plate lead-acid | 50% | 500 |
| Tubular lead-acid | 60% | 1,500 |
| Lithium (LFP) | 85% | 5,000 |
Where scheme and rate data comes from
Subsidy slabs, installed cost bands, state tariffs, metering regimes and feed-in rates all live in a database with an effective date rather than being hardcoded. Scheme parameters get revised by government and regulators on their own schedule, and keeping them as data means a revision is a correction, not a rebuild.
Data last reviewed August 2026.
The payback projection
This is where we differ most from other calculators. Rather than dividing net cost by a first-year saving, we project each of 25 years separately, applying panel degradation to generation and escalation to the tariff. Units you consume yourself are valued at the retail tariff; surplus is valued at the feed-in rate.
The flat result is shown alongside on every payback page, so you can see the gap rather than take our word for it.
What these calculators do not do
- They do not model shading. This is the single largest destroyer of real-world output and cannot be assessed without a site visit.
- They do not model roof orientation. Figures assume roughly south-facing. East or west typically loses 10–15%.
- They do not include inverter replacement. Budget for one between years 10 and 15.
- They do not discount future cash flows. A discounted analysis gives a longer, more conservative payback.
- They do not model time-of-day tariffs, which several states are introducing.
- They do not check structural capacity. An older roof may need strengthening.
- They are not a subsidy eligibility determination. Only the official portal and your DISCOM can confirm that.
Where to check these yourself
Solar rules change more often than anything else on this site, and we would rather you confirmed than trusted us. These are the official sources.
| What | Source |
|---|---|
| Subsidy amounts, vendor list, application status | PM Surya Ghar national portal — the only place that is authoritative on subsidy. Check before paying any vendor. |
| Scheme policy and renewable energy programmes | Ministry of New & Renewable Energy |
| Grid connectivity and metering regulations | Central Electricity Authority |
| Your tariff slabs, net metering rules, feed-in rate | Your own DISCOM and your State Electricity Regulatory Commission. These vary by state and change without much notice. |
| Solar resource and sun hours by location | National Institute of Solar Energy |
Corrections
Subsidy rules, feed-in rates and tariff slabs change, and our benchmarks will not fit every market. If a figure here is wrong, please tell us — we would rather correct it than defend it.