1. Solar System Size Calculation Formula
Example: a Nagpur home using 350 units a month, with 4.8 peak sun hours and a 0.8 performance ratio: 350 ÷ (4.8 × 30 × 0.8) = 350 ÷ 115.2 = 3.04, rounded up to 3.5 kW. Run yours in the solar system size calculator.
2. Number of Solar Panels Formula
Example: 3,500 W ÷ 550 W = 6.4, so 7 panels of 550W. Higher-wattage 620W modules bring it down to 6. The how many solar panels calculator handles the rounding and module options.
3. Solar Panel Output Calculation Formula
Monthly output = Daily × 30 · Annual output = Daily × 365
Example: 3 kW × 4.8 × 0.8 = 11.5 units a day, about 346 a month. Full walk-through in the panel output guide or the generation calculator.
4. Roof Area Calculation Formula
Example: a 3 kW system needs 240 to 300 sq. ft. of shadow-free roof, including row spacing and walkways. Check your roof with the roof area calculator.
5. Solar System Cost and Subsidy Formula
Net cost = Gross − Central subsidy (₹30,000 / ₹60,000 / ₹78,000 cap) − State top-up
Example: 3 kW × ₹58,000 = ₹1,74,000 gross; minus the ₹78,000 subsidy = ₹96,000 net. Price your own case in the solar panel cost calculator and subsidy calculator.
6. Savings and Payback Formula
Payback (years) = Net cost ÷ Annual savings
Example: 346 self-used units × ₹8.5 = ₹2,941 a month, ₹35,292 a year. Payback = 96,000 ÷ 35,292 ≈ 2.7 years. See the savings and payback calculator for escalation-adjusted 25-year figures.
7. Battery Size Calculation Formula
Capacity (Ah) = Battery kWh × 1000 ÷ Bank voltage
Example: 800W for 4 hours = 3.2 kWh; ÷ 0.85 ÷ 0.85 (lithium DoD) = 4.4 kWh, or 185 Ah at 24V. The solar battery calculator compares lithium and lead-acid instantly.
8. Solar Loan EMI Formula
Example: ₹96,000 at 7% for 5 years gives an EMI of ₹1,901. The solar loan EMI calculator also compares the EMI against your monthly savings.
9. Panel Efficiency and Tilt Formulas
Fixed tilt = Latitude · Winter = Latitude + 15° · Summer = Latitude − 15°
A 550W panel of 2.58 sq. m. is 21.3% efficient; a Nagpur roof (21.1°N) tilts at 21°. Use the efficiency calculator and tilt angle calculator.
Quick Reference: All Formulas at a Glance
| What You Want | Formula | Typical Result |
|---|---|---|
| System size (kW) | units ÷ (PSH × 30 × 0.8) | ~1 kW per 120 units |
| Panel count | watts ÷ panel W, round up | 2 panels per kW (550W) |
| Daily output (kWh) | kW × PSH × 0.8 | ~4 units per kW |
| Roof area | kW × 80–100 sq. ft. | ~90 sq. ft. per kW |
| Net cost | kW × ₹58k − subsidy | ₹96k for 3 kW |
| Payback | net cost ÷ annual savings | 3 – 6 years |
| Battery (kWh) | W × hrs ÷ 1000 ÷ 0.85 ÷ DoD | 4.4 kWh for 800W × 4h |
| Inverter (VA) | W ÷ 0.8 × 1.2 | 1.5 × load watts |
Print or screenshot this table; it condenses everything an installer's proposal should be able to justify.
The Constants Behind Every Formula
Standard planning constants (India, 2026)
- Performance ratio: 0.8 (dust, heat, wiring, inverter losses)
- Peak sun hours: 3.5 (Shillong) to 5.8 (Jodhpur); ~4.8 typical
- Module wattage: 540 – 620W; roof norm: 80 – 100 sq. ft./kW
- Costs: ₹58k/72k/85k per kW (on-grid/hybrid/off-grid) ±10%
- Tariff escalation: ~5%/yr; degradation: ~0.5%/yr
Prefer not to do the maths at all? The solar calculator for India on our homepage chains all of these formulas together from a single electricity bill.