Solar Watt Calculator
Calculate the total solar panel wattage needed to meet your daily energy consumption based on location, peak sun hours, and system efficiency.
Size Your Solar Array
Enter daily energy needs, peak sun hours, and panel wattage to calculate your required array size.
How to Calculate Solar Panel Wattage
To size a solar array, divide your daily energy consumption (Wh) by peak sun hours and system efficiency:
For example, if you use 30 kWh/day with 5 peak sun hours and 75% system efficiency: Array = 30,000 ÷ (5 × 0.75) = 8,000W (8 kW). With 400W panels, you need 20 panels.
Peak Sun Hours by Region
| Region | Peak Sun Hours | Solar Potential |
|---|---|---|
| Arizona, Nevada, SoCal | 6.0–7.5 hrs | Excellent |
| Texas, Florida, Hawaii | 5.0–6.5 hrs | Very Good |
| Midwest, Southeast US | 4.0–5.0 hrs | Good |
| Northeast, Pacific NW | 3.0–4.5 hrs | Moderate |
| UK, Germany, Canada | 2.5–4.0 hrs | Fair |
| Australia, India | 5.0–7.0 hrs | Excellent |
System Loss Factors
A solar system typically produces 70-80% of its theoretical maximum due to various losses:
- Temperature losses: 5-15% (panels lose efficiency in heat)
- Inverter efficiency: 3-5% (DC to AC conversion)
- Wiring/mismatch losses: 2-5%
- Soiling (dust/dirt): 2-5%
- Shading: 0-25% (depends on site)
- Aging degradation: 0.5%/year
A combined derating factor of 0.75 (75%) is a good conservative estimate for most residential installations.
Frequently Asked Questions About Solar Panel Sizing
How many solar panels do I need for my home?
The average US home using 30 kWh/day needs approximately 20 panels (400W each) with 5 peak sun hours. The exact number depends on your location, roof orientation, shading, and panel efficiency. Use this calculator with your actual daily kWh from your electricity bill.
What are peak sun hours?
Peak sun hours is the number of hours per day when solar irradiance averages 1,000 W/m² (1 kW/m²). It is NOT the same as hours of daylight. Phoenix gets ~6.5 peak sun hours; Seattle gets ~3.5. This directly determines how much energy your panels produce daily.
What size solar system offsets my electric bill?
Divide your monthly kWh usage by 30 to get daily kWh, then use the formula: System kW = Daily kWh ÷ (Sun Hours × 0.75). For 900 kWh/month: 30 kWh/day ÷ (5 × 0.75) = 8 kW system.
Do solar panels work on cloudy days?
Yes, solar panels produce electricity on cloudy days but at reduced output — typically 10-25% of rated capacity. Panels respond to light, not direct sunshine. Overcast days still provide diffuse solar radiation that generates some power.
How much roof space do solar panels need?
A standard 400W residential solar panel is approximately 21 sq ft (6.5 ft × 3.25 ft). A 20-panel 8kW system requires approximately 420 sq ft of unshaded south-facing roof space. Factor in setbacks, obstructions, and walkways for maintenance access.