How to Calculate Solar Plant Size for Your Home?
Choosing the correct size for your solar installation is the most critical step in your solar journey. An undersized system won't eliminate your electricity bill completely, while an oversized system is a waste of capital because most Indian states limit the buy-back rate for excess power.
The Science Behind Sizing
In India, we are blessed with abundant sunlight. A standard rooftop solar power plant generally produces 4 units (kWh) of electricity per kW of capacity per day. This is an annual average—generation will be higher in May and lower in December.
Step-by-Step Calculation Formula:
- Check Your Usage: Look at your electricity bill. Find the average monthly units consumed (e.g., 360 units).
- Divide by 30: Find your daily consumption. (360 / 30 = 12 units/day).
- Divide by Generation Factor (4): Since 1kW generates 4 units/day, divide your daily need by 4. (12 / 4 = 3).
- Result: You need a 3kW system.
Understanding Sanctioned Load
Your local DISCOM (like BESCOM, MSEDCL, TPDDL) has a rule: Your Solar Plant Capacity cannot exceed your Sanctioned Load.
If your electricity bill shows a "Sanctioned Load" or "Connected Load" of 2kW, but your calculation shows you need a 3kW solar system, you must first apply to the DISCOM to increase your load to 3kW. Only then will your Net Metering application be approved.
Shadow Analysis & Roof Orientation
The calculation assumes a South-facing roof with zero shadow. In reality, you might need to oversize your system slightly (by 5-10%) to compensate for:
- Orientation Losses: If your roof faces East or West, you lose about 5-10% generation.
- Dust & Heat: Dust accumulation and extreme heat (above 25°C) reduce panel efficiency.
- Wiring Losses: DC to AC conversion involves about 2-3% loss.
Common Appliance Loads
To give you an idea of what powers what:
- 1 kW System: Fans, Lights, TV, Laptop, Fridge (No AC).
- 3 kW System: All above + 1.5 Ton AC (run during day) + Washing Machine.
- 5 kW System: Multiple ACs, Geysers, Water Pump (1 HP).
Conclusion
Use this calculator as a starting point. Always consult with a site engineer who will visit your property to access the roof structure and cable routing before finalizing the exact capacity.