Estimate transparency
Solar calculator methodology
This page documents the formulas and fixed assumptions used by Lumifly's public savings calculator. The output is a planning estimate, not a quotation, guaranteed saving or final bill forecast.
Methodology updated: 24 September 2026
Fixed assumptions
- Generation
- 110 units per kW per month (1320 units per kW per year)
- Solar capacity cost
- ₹65,000 per kW
- Maintenance and insurance
- ₹15,000 per kW, charged once for the 20-year planning period
- Planning horizon
- 20 years
- Financing assumption
- 0.5% per year on the outstanding “your share” balance for the payback calculation
- Emissions factor
- 0.82 kg CO₂ avoided per generated unit
Calculation sequence
- 1. Capacity: the entered monthly bill is reversed through a simplified slab tariff model, including a 4% uplift, to estimate monthly units. Estimated units are divided by 110, rounded to the nearest whole kW, and limited to 1–9 kW.
- 2. Generation: capacity × 110 gives monthly generation; monthly generation × 12 gives annual generation.
- 3. Savings: estimated generation is valued through the same simplified tariff slabs. Annual savings equal monthly savings × 12. The model does not separately deduct operating costs because its maintenance and insurance assumption is included in the upfront project investment.
- 4. Investment: capacity × ₹80,000, comprising ₹65,000 per kW for solar capacity and ₹15,000 per kW for maintenance and insurance.
- 5. Payback: the calculator subtracts the indicative subsidy from investment and estimates the time for annual savings to repay that balance while applying the 0.5% annual financing assumption.
- 6. Long-term values: the 20-year benefit is annual savings × 20. Avoided emissions are annual generation × 0.82 kg per unit × 20.
How the subsidy figure is treated
The calculator currently uses indicative Odisha planning amounts of ₹55,000 for 1 kW, ₹1,10,000 for 2 kW, and ₹1,38,000 for 3 kW and above. These are internal estimate inputs. Residential rooftop CFA does not automatically apply to managed, shared or Virtual Net Metering projects, and the calculator does not establish eligibility. Confirm the current scheme, project structure and assistance before relying on this deduction.
Read the subsidy eligibility guide →What the estimate does not model
It does not model a consumer's complete tariff order, fixed charges, taxes, seasonal generation, degradation, downtime, export settlement, banking, curtailment, changing tariffs, individual loan terms, project-specific charges or final VNM allocation rules. Actual results depend on the site, project, DISCOM, regulatory approval and customer agreement.