About the Solar Plant Size
Solar plant size estimation determines the optimal rooftop or ground-mount system capacity (kWp) based on your monthly electricity consumption, available roof/land area, budget, and subsidy eligibility — balancing generation against load to maximise savings without over-investing.
Formula
System size (kWp) = Monthly consumption (kWh) ÷ (Specific yield × 0.85 × 30)
- Monthly consumption
- Average monthly electricity bill in kWh (from DISCOM bill)
- Specific yield
- City-specific kWh/kW/day (e.g. 4.0–5.5 for India)
- 0.85
- System efficiency / performance ratio (typical for grid-tied systems)
- 30
- Days per month
How to calculate it
- 1Read 3–6 months of electricity bills to find average monthly consumption in kWh.
- 2Look up your city's peak sun hours (PSH) or specific yield (kWh/kW/day): typically 4.0–5.5 in India.
- 3Apply the formula to get a first-pass system size.
- 4Cross-check against available roof area: 1 kWp requires roughly 8–10 m² of unshaded area.
- 5Adjust for subsidy: PM Surya Ghar is capped at 3 kW for the maximum ₹78,000 benefit — size up to 3 kW first to capture the full subsidy.
- 6Sanity-check: annual generation should not exceed 80–90% of annual consumption (excess export is often credited at a lower rate).
Worked example
A household using 400 kWh/month in Pune (specific yield 5.2 kWh/kW/day). Size = 400 ÷ (5.2 × 0.85 × 30) = 400 ÷ 132.6 = 3.02 kWp. Rounded to 3 kW — the exact size that captures the maximum PM Surya Ghar subsidy of ₹78,000.
What you can calculate
- Load-based sizing
- Area-based sizing
- PSH by city
- Derate factors
Key definitions
Definitions from the SolarPedia glossary
Frequently asked questions
How many solar panels do I need for a 3 kW system?
With modern 400–450 W monocrystalline panels, a 3 kW system requires 7–8 panels. Each panel needs roughly 1.8–2 m² of roof area, so 7 × 2 m² = 14 m² of unshaded south-facing roof is sufficient — a standard Indian terrace.
Should I size my solar system to cover 100% of my bill?
Not necessarily. Net metering export rates are typically lower than the retail import tariff, so over-generating means selling cheap. The optimal size usually covers 80–100% of daytime load. If you have an EV or battery, sizing to 110–120% of consumption can be worthwhile.
How much roof area is needed for rooftop solar in India?
A rule of thumb is 8–10 m² of unshaded roof per kWp of installed capacity (for 400–450 W panels). A 3 kW system needs ~25–30 m², and a 5 kW system ~40–50 m². Shading, parapet setback, and equipment space can reduce usable area by 15–25%.
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