How to size a solar system?
- Determine Your Energy Needs (kWh)
Check your electricity bills for average monthly or annual kilowatt-hour (kWh) usage.
Example: If your home uses 900 kWh/month, your daily usage is ~30 kWh (900 ÷ 30 days).
Future needs: Account for EV charging, HVAC changes, or expanded usage.
- Calculate Solar Production Potential
Solar output depends on:
Sunlight hours (peak sun hours per day in your location).
Use tools like NREL’s PVWatts or Global Solar Atlas.
Example: 4.5 peak sun hours/day (average in sunny regions).
System size formula:
System Size (kW)
Daily kWh Usage
Peak Sun Hours
×
1.15
(inefficiency buffer)
System Size (kW)=
Peak Sun Hours
Daily kWh Usage
×1.15(inefficiency buffer)
Example: 30 kWh ÷ 4.5 hours × 1.15 = 7.67 kW system.
- Adjust for Efficiency Losses
Factors: Shading, panel tilt, temperature, and inverter losses (~10–20% loss).
Solution: Oversize the system slightly (e.g., 8 kW instead of 7.67 kW).
- Panel Capacity and Quantity
Panel wattage: Typical panels are 300W–400W.
Example: 8 kW system ÷ 350W panels = 23 panels (8,000 ÷ 350 ≈ 23).
Roof space: Each panel needs ~18–25 sq. ft. (23 panels × 20 sq. ft. = 460 sq. ft.).
- Battery Storage (If Off-Grid or Backup)
Calculate battery bank size based on days of autonomy (backup days).
Example: 30 kWh/day × 2 days backup = 60 kWh battery capacity.
Depth of discharge (DoD): Lithium batteries allow ~80–90% DoD; lead-acid ~50%.
Adjusted capacity: 60 kWh ÷ 0.8 (DoD) = 75 kWh battery bank.
- Inverter Sizing
Match inverter capacity to system size (e.g., 8 kW system → 8 kW inverter).
Microinverters vs. String Inverters:
Microinverters (per-panel) handle shading better.
String inverters are cheaper but less flexible.
- Net Metering vs. Off-Grid
Grid-tied: No battery needed (net metering credits excess power).
Off-grid: Requires larger solar + battery systems for self-sufficiency.
- Professional Assessment
Get quotes from installers who analyze:
Roof orientation (south-facing ideal in the Northern Hemisphere).
Local weather, shading, and permitting requirements.