How many solar panels do I need?

Divide your daily kWh usage by (peak sun hours x panel wattage x ~0.8 system efficiency). A home using 900 kWh per month in a 5-sun-hour area needs roughly a 7.5 kW system — about 19 panels at 400 W each.

What are peak sun hours?

A peak sun hour is one hour of solar irradiance at full strength (1,000 W/m²). Most of the US gets the equivalent of 3.5 to 6.5 peak sun hours per day, which is why the same system produces more in Arizona than in Maine.

How much does a residential solar system cost?

Installed costs commonly run about $2.50-3.50 per watt before incentives, so a typical 7 kW system lands around $18,000-25,000. Prices vary with equipment, roof complexity, and local labor.

How do I calculate the solar payback period?

Divide net system cost by your annual electricity-bill savings. Payback commonly falls in the 8-12 year range, driven by local electricity rates, sunlight, and any state or utility incentives available to you.

What is net metering?

Net metering credits you for excess power your panels export to the grid, offsetting electricity you draw at night. Policies differ widely by state and utility — and they significantly change the savings math, so check your utility's current rules.

Solar Panel Calculator

Size a solar panel system from your monthly kWh usage. Estimate panels needed, system cost, payback period, and lifetime

System Details

System Size

System Cost (Before Incentives)

Daily Sun Hours

Roof Orientation

Roof Tilt Angle

Shading Factor

Electricity Details

Current Electricity Rate

Monthly Electric Bill

Annual Rate Escalation

System Degradation Rate

Financing & Incentives

Total Incentives

Financing Option

Payback Period

9.0 years

Time to break even

First Year Savings

$1,945

16210.6 kWh

25-Year Savings

$66,317

Total energy savings

Net System Cost

$17,085

After incentives

Return on Investment

288.2%

25-year ROI

System Performance

System Size:8.5 kW

Annual Generation:16210.6 kWh

Daily Production:44.4 kWh

Capacity Factor:21.8%

Financial Incentives

System Cost:$25,500

Federal Tax Credit (30%):-$7,650

State/Local Incentives:-$765

Net Cost:$17,085

Solar Investment Tips

• South-facing roofs with minimal shading are ideal
• Consider energy efficiency upgrades before installing solar
• Get multiple quotes from certified installers
• Factor in roof age and condition before installation
• Understand your utility's net metering policies

How it works

A solar panel calculator sizes a system to your electricity use. It divides your daily energy need by the average peak sun hours and a system efficiency factor to find the kilowatts of panels required, then the number of panels.

System size

System kW = daily kWh ÷ (peak sun hours × efficiency)        Panels = system watts ÷ panel watts

peak sun hours: equivalent full-sun hours per day for your location
efficiency: ~0.75–0.8 for real-world losses

Worked example

Use 30 kWh/day
5 peak sun hours, efficiency 0.8

System = 30 ÷ (5 × 0.8) = 7.5 kW
At 400 W panels: 7,500 ÷ 400

≈ 7.5 kW — about 19 panels.

Good to know

Peak sun hours vary by region and season — use a local average.
The efficiency factor accounts for inverter losses, heat, wiring, and dust.
Net metering and battery storage change how much generation you actually need to offset bills.