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Residential Solar System Size & Panel Calculator

Compute required residential solar system size (kW), panel count, and annual savings.

Quick Answer:Divide your daily kilowatt-hour usage by your local peak daily sun hours (adjusted for 80% system efficiency) to find system size in kW.

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How to Use the Solar Panel Calculator: System Size, Panels & Savings

1Input your average monthly electric bill in dollars.
2Specify your local utility electricity rate ($/kWh, national average is ~$0.16/kWh).
3Enter your regional average daily peak sun hours (typically 3.5 to 5.5 hours across the US).
4Review your recommended system capacity, required 400W solar panel count, and annual savings.

Formula & How It Works

Solar Array Sizing Equation

System kW = (Monthly Bill / Rate / 30) / (Peak Sun Hours × 0.80 Efficiency)

Calculates necessary kilowatt capacity to offset monthly electric consumption.

Worked Examples

$160 Monthly Electric Bill at $0.16/kWh

Input: Bill: $160/mo, Rate: $0.16/kWh (1,000 kWh/mo), Sun: 4.5 Peak Hours/Day

Explanation: Generating 1,000 kWh per month in a 4.5-sun-hour region requires a 9.3 kW array comprising 24 modern 400-watt monocrystalline panels.

Result: System Size: 9.26 kW | 24 Panels (400W) | ~$1,824 Annual Savings

Frequently Asked Questions

What is the 30% Federal Solar Investment Tax Credit (ITC)?▼

The US federal Section 25D tax credit allows homeowners to deduct 30% of the total cost of installing solar panels and battery storage directly from federal income taxes owed.

How much roof space is required for a solar installation?▼

Each modern residential 400-watt solar panel occupies approximately 18 to 20 square feet; a standard 20-panel system requires roughly 400 square feet of unobstructed roof space.

What happens on cloudy or rainy days?▼

Solar panels continue generating power during overcast weather using ambient diffuse daylight, typically producing 10% to 25% of their peak rated output.

Do solar panels work during a grid power outage?▼

Standard grid-tied solar systems shut down automatically during blackouts for utility line worker safety unless paired with a solar battery backup system (like Tesla Powerwall or Enphase).

Quick Reference & Conversion Table

Monthly Electric Bill	Avg Monthly kWh	System Size Required	400W Panel Count
$100/mo	625 kWh	5.79 kW	15 Panels
$150/mo	938 kWh	8.68 kW	22 Panels
$200/mo	1,250 kWh	11.57 kW	29 Panels
$300/mo	1,875 kWh	17.36 kW	44 Panels

⚠️ Common Pitfalls to Avoid

•Confusing daylight hours with peak sun hours (peak sun hours represent hours where solar irradiance reaches 1,000 watts per square meter).
•Ignoring roof azimuth (south-facing roofs generate significantly more power than north-facing roofs in the northern hemisphere).

💡 Practical Real-World Uses

•Evaluating solar installer quotes and sizing proposals before signing residential leases or purchase contracts.
•Calculating federal solar tax credit eligibility (Residential Clean Energy Credit 30%).

Last reviewed & updated: 2026-11-11

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