The amount of solar power needed to run a house depends on several key factors, and here is a detailed breakdown to help you understand and calculate it:

1. Energy Consumption

• Average Household Usage: In the United States, the average household consumes about 877 kWh per month (or approximately 29 kWh per day). However, this varies widely based on location, household size, and energy habits.
• Personal Calculation: To determine your specific needs, review your electricity bills to find your monthly or daily kWh usage. This is the most accurate starting point.

2. Solar Panel Efficiency and Output

• Panel Wattage: Most residential solar panels have a power rating between 250W to 400W per panel. Higher-wattage panels generate more electricity but may cost more.
• Daily Output per Panel:
  • A 300W panel receiving 5 hours of peak sunlight per day (a common estimate) produces about 1.5 kWh per day (300W × 5 hours ÷ 1,000 = 1.5 kWh).
  • Adjust for your local sunlight hours (e.g., 4 hours in some regions, 6 in others).

3. Sunlight Availability (Solar Insolation)

• Peak Sun Hours (PSH): This varies by location. For example:
  • Arizona: ~6–7 PSH/day.
  • New York: ~4 PSH/day.
  • Germany: ~3 PSH/day.
• Use online tools like the National Renewable Energy Laboratory (NREL) PVWatts Calculator to find PSH for your area.

4. System Losses (Derating Factor)

• Solar systems rarely operate at 100% efficiency due to:
  • Inverter losses (~5–10%).
  • Wiring losses (~2–3%).
  • Shading, dirt, or temperature effects (~10–15%).
• Total estimated losses: ~15–25%. To compensate, multiply your required energy by 1.25 to 1.35.

5. Battery Storage (Optional)

• If you want to run your house entirely on solar (including at night), you’ll need batteries to store excess energy.
• Battery Capacity: Calculate based on your nighttime/cloudy-day usage. For example, storing 20 kWh for overnight use would require a battery system sized accordingly (e.g., Tesla Powerwall 2 stores 13.5 kWh).

6. Net Metering and Grid Connection

• If your utility offers net metering, you can sell excess solar energy back to the grid during the day and draw from it at night, reducing the need for batteries.
• Without net metering, you’ll need larger batteries or a hybrid system.

Calculation Example

Let’s say your household uses 30 kWh/day, and you live in an area with 5 PSH/day:

1. Adjust for losses: 30 kWh/day × 1.3 (to account for 23% losses) = 39 kWh/day needed from panels.
2. Panel output: A 350W panel produces ~1.75 kWh/day (350W × 5 hours ÷ 1,000).
3. Number of panels: 39 kWh ÷ 1.75 kWh/panel ≈ 22 panels.

Final Estimate

• Typical residential system size: 5–10 kW (enough for most homes).
  • A 5 kW system (16–20 panels) might produce 20–30 kWh/day in a sunny region.
  • A 10 kW system (32–40 panels) could produce 40–60 kWh/day.

Key Takeaways

• Start with your energy bills to find daily/monthly usage.
• Factor in sunlight hours (use PVWatts for precision).
• Account for losses (~20–25% extra capacity).
• Consider batteries if going off-grid or lacking net metering.
• Consult a solar installer for a site-specific assessment.

Example Scenarios

By tailoring these calculations to your specific situation, you can determine the ideal solar power system size for your home.