Last updated: 9 October 2026 · Applies to every calculator on https://solarcalculatorspro.online
The values on this page are taken directly from the calculation code that runs the site. If a number here and a calculator ever disagree, that is a bug: please report it. All outputs are planning estimates, not quotes; see the disclaimer.
Key Assumptions at a Glance
| Assumption | Value used | Details |
|---|---|---|
| Peak sun hours | 3.5 – 5.8 hours/day, by city | Solar resource |
| Performance ratio | 0.80 (low–high range 0.75 – 0.85) | Generation |
| Module wattage | 540 – 620 W | Sizing |
| Roof area | 80 – 100 sq. ft. per kW | Sizing |
| System cost per kW | On-grid ₹58,000 · hybrid ₹72,000 · off-grid ₹85,000 (±10%) | Costs |
| Central subsidy | ₹30,000/kW up to 2 kW + ₹18,000 for the 3rd kW, capped at ₹78,000; off-grid ₹0 | Subsidy |
| Electricity tariffs | 29 state/DISCOM slab tables; city planning tariffs ₹4 – ₹10/unit | Tariffs |
| Exported units | ₹3.00 per unit | Tariffs |
| Tariff escalation | ~5% per year | Savings |
| Panel degradation | ~0.5% per year (25-year projections) | Savings |
| Battery | Inverter efficiency 85%; usable depth of discharge 50% (lead-acid), 85% (lithium) | Battery |
| Battery cost | Lead-acid ~₹9,000/kWh · lithium ~₹18,000/kWh | Costs |
| Loans (main calculator) | 5 years @ 9.5% and 7 years @ 10%, reducing balance | Loans |
1. Solar Resource: Peak Sun Hours
Peak sun hours express a day's solar energy as the equivalent number of hours of full 1,000 W/m² sunlight. Each city carries an annual planning average, from 3.5 hours (Shillong) to 5.8 hours (Jodhpur and Bikaner), with about 4.8 hours typical for central India. The main solar calculator covers 101 cities across all 36 states and union territories; the other calculators share a 93-city list across 28 states and union territories.
These are annual averages. Monsoon months can produce 30–40% less and clear summer months more. You can cross-check any location against NASA POWER, whose daily surface solar irradiance in kWh/m²/day is numerically equal to peak sun hours on a flat surface, or simulate a full system in NREL PVWatts.
2. Generation and Performance Ratio
Monthly = System kW × Peak sun hours × 30 × 0.8 · Annual = System kW × Peak sun hours × 365 × 0.8
The 0.8 performance ratio accounts for real-world losses from dust, heat, wiring, inverter conversion and mismatch. Generation ranges use 0.75 at the low end and 0.85 at the high end. Run it in the solar generation calculator.
3. System Size, Panels and Roof Area
Off-grid systems: × 1.25 buffer before rounding
Panels = System watts ÷ 620 W (fewest) to System watts ÷ 540 W (most), rounded up
Roof area = System kW × 80 to 100 sq. ft.
Appliance-based tools convert each appliance's typical wattage × hours × quantity into monthly units before sizing. Try the system size, panel count and roof area calculators.
4. Electricity Tariffs, Bills and Exported Units
To turn a bill amount into units (and back), the main calculator uses 29 state and DISCOM domestic slab tables with indicative fixed charges. Most are based on the FY 2024-25 domestic schedules. Four follow the tariff orders in force on 9 October 2026 and are shared, line for line, with our DISCOM bill calculators:
- Tamil Nadu (TNPDCL): TNERC Suo-motu Order No. 6 of 2025, in force from 1 July 2025, at the rates households pay after the Government of Tamil Nadu subsidy, including the 200 free units for two-month bills up to 500 units from 10 May 2026. See the TNEB bill calculator.
- Kerala (KSEB): KSERC tariff order of 5 December 2024, rates for 1 April 2025 to 31 March 2027. See the KSEB bill calculator.
- Maharashtra (MSEDCL): MERC orders in Case No. 217 of 2024 and Case No. 75 of 2025 (25 March 2026), FY 2026-27 rates from 1 April 2026, including the ₹1.60 wheeling charge. See the MSEDCL bill calculator.
- Uttar Pradesh (UPPCL, urban): UPERC tariff order for FY 2026-27 (July 2026), which kept the existing schedule. The main calculator assumes a 2 kW contracted load for the fixed charge. See the UPPCL bill calculator.
Tamil Nadu and Kerala bill every two months, so the main calculator works out the two-month bill and halves it to give a monthly figure. Fuel adjustment charges, electricity duty and temporary free-unit schemes (such as Delhi's 0–200 free units or Punjab's waivers) are not applied, so the estimate reflects the underlying tariff. Tamil Nadu's free units are the exception, because they are part of the subsidised schedule every domestic consumer is billed on.
Cities without a slab table use a city planning tariff between ₹4 and ₹10 per unit with a simple progressive model: the first 300 units at the base rate, the next 200 at 1.25× and anything above 500 at 1.5×. The other calculators use the city planning tariff, with full slab tables for Delhi and Tripura.
Exported (net-metered) units are valued at a conservative ₹3.00 per unit, because export compensation differs by state and DISCOM and we do not yet apply state-specific export rules.
Check your own tariff: tariffs are revised by your State Electricity Regulatory Commission, usually every year. Your latest bill and your commission's current tariff order (for example, the Delhi Electricity Regulatory Commission) are the authoritative source. If your state has issued a newer order, savings scale roughly with the new rate; send it to us and we will update the table. Try the electricity bill calculator.
5. PM Surya Ghar Subsidy
Off-grid systems: ₹0 (not eligible)
This mirrors the central financial assistance for residential rooftop solar under PM Surya Ghar: Muft Bijli Yojana: ₹30,000 for 1 kW, ₹60,000 for 2 kW and ₹78,000 for 3 kW and above. Three state top-ups are added where announced:
- Delhi: ₹10,000 per kW, up to ₹30,000.
- Uttar Pradesh: ₹15,000 per kW, up to ₹30,000.
- Assam: ₹15,000 below 2 kW, ₹30,000 from 2 kW to below 3 kW, ₹45,000 at 3 kW and above.
Group housing and common-facility subsidies are not modelled. Always confirm eligibility and current amounts on the official PM Surya Ghar national portal, the Ministry of New and Renewable Energy and your DISCOM. Try the solar subsidy calculator.
6. System and Battery Costs
Net cost = Gross cost − Central subsidy − State top-up
Rates are indicative 2026 turnkey price bands for residential systems, including panels, inverter, structure, wiring and installation. They are market planning bands, not an official benchmark price, and real quotes vary with brand, roof type, structure height, city and site conditions. Battery costs use about ₹9,000 per kWh for lead-acid and ₹18,000 per kWh for lithium. Try the solar panel cost calculator.
7. Battery, Inverter and Charging Time
Battery capacity = Energy needed ÷ 0.50 (lead-acid) or ÷ 0.85 (lithium)
Depth of discharge is capped at 50% for lead-acid and 85% for lithium to protect battery life. Try the solar battery calculator.
Inverter size
Solar inverter kW ≈ Panel array kW ÷ 1.1, rounded to the nearest 0.5 kW
Try the inverter size calculator.
Battery charging time
Charging hours = Energy needed ÷ (Panel W × 0.75) · Days = Hours ÷ Peak sun hours
The 0.75 factor reflects that a panel delivers about 75% of its rated wattage in real conditions. Try the battery charging time calculator.
8. DC Wire Size
Rounded up to the next standard copper size: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 or 70 mm²
The calculator sizes copper cable to keep voltage drop at or below 3% over the round trip, using copper resistivity of 0.0175 Ω·mm²/m. It does not check current-carrying capacity, temperature derating or protection devices. A licensed electrician must confirm the final design, and installations must comply with the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 and your DISCOM's rules. Try the wire size calculator.
9. Tilt Angle and Direction
Winter tilt = Latitude + 15° · Summer tilt = Latitude − 15° (minimum 5°) · Facing true south (azimuth 180°)
City latitudes are stored for all 93 cities in the shared list. Try the tilt angle calculator.
10. Panel Efficiency
This is the efficiency at standard test conditions, the same basis manufacturers print on datasheets. Try the panel efficiency calculator.
11. Savings, Payback and Loans
Payback (years) = Net cost ÷ Annual savings
EMI = P × r × (1 + r)n ÷ ((1 + r)n − 1), where r = annual rate ÷ 12 and n = months
25-year projections assume tariffs rise about 5% a year and panel output falls about 0.5% a year. The main calculator shows representative EMIs at 5 years @ 9.5% and 7 years @ 10%; the solar loan EMI calculator accepts your own rate and tenure. Try the savings and payback calculator.
Known Limitations
- Estimates use annual averages; they do not model your roof's shading, orientation or obstructions.
- Most tariff tables follow FY 2024-25 schedules and may lag newer tariff orders; Tamil Nadu, Kerala, Maharashtra and Uttar Pradesh follow the orders in force on 9 October 2026.
- State-specific net-metering or net-billing export rates are not applied; all exports use ₹3.00 per unit.
- Prices are planning bands, not quotes. Brand, structure and site conditions move real prices.
- Wire sizing checks voltage drop only. Electrical design must be done by a licensed professional.
- No calculator replaces a site survey by a certified installer.
Primary Sources and References
- PM Surya Ghar: Muft Bijli Yojana national portal: residential subsidy amounts, eligibility and applications.
- Ministry of New and Renewable Energy (MNRE): rooftop solar programme and guidelines.
- NASA POWER: surface solar irradiance data to cross-check peak sun hours for any location.
- NREL PVWatts: independent simulation of PV system generation worldwide.
- Your State Electricity Regulatory Commission's current tariff order, for example the Delhi Electricity Regulatory Commission.
- Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023: electrical safety requirements for installations.
Data Update Log
| Date | Change |
|---|---|
| 9 October 2026 | Published this methodology and source list. No change to calculations or data. |
| 4 August 2026 | Site renamed Solar Calculators Pro and moved to solarcalculatorspro.online. No change to calculations or data. |
| 19 July 2026 | Launched with the calculation engine and data sets described on this page. |
Found an outdated figure? Read our corrections policy and email contact@solarcalculatorspro.online with a link to the official source.