Solar Panel Calculator — System Size and Savings
Estimate the solar panel system size needed to offset your electricity bill. Calculate number of panels, annual savings, and roof space required. See also our Electricity Cost Calculator and LED Savings Calculator.
How to Size a Solar Panel System
- Find your monthly electricity bill and rate from your utility statement.
- Look up peak sun hours for your location (typically 4-6 hours in the US).
- Choose a panel wattage (modern panels are 350-450W).
- Measure available south-facing roof area.
- Click Calculate to determine system size and expected savings.
Formula
Monthly kWh = Monthly Bill / Electricity Rate
System Size (kW) = Monthly kWh / (30 days × Sun Hours × 0.8 efficiency)
Number of Panels = System kW × 1000 / Panel Wattage
Annual Savings = Monthly kWh × 12 × Rate
Roof Area Needed = Panels × 17.5 sq ft per panelExample
Monthly bill $150 at $0.12/kWh, 5 sun hours, 400W panels:
Monthly kWh = $150 / $0.12 = 1,250 kWh
System Size = 1,250 / (30 × 5 × 0.8) = 10.42 kW
Panels = 10.42 × 1000 / 400 = 27 panels
Annual Savings = 1,250 × 12 × $0.12 = $1,800/year
25-Year Savings = $45,000
Roof Area = 27 × 17.5 = 473 sq ft
Peak Sun Hours by Region
| Region | Peak Sun Hours | Example States |
|---|---|---|
| Southwest | 6-7 | AZ, NV, NM |
| Southeast | 5-6 | FL, TX, GA |
| Midwest | 4-5 | IL, OH, MO |
| Northeast | 3.5-4.5 | NY, MA, PA |
| Northwest | 3-4 | WA, OR, MT |
| Mountain | 5-6 | CO, UT, ID |
Frequently Asked Questions
What does the 0.8 efficiency factor represent?
The 0.8 factor accounts for real-world losses including inverter efficiency, wiring losses, temperature effects, shading, and panel degradation. Actual production is about 80% of theoretical maximum.
How long do solar panels last?
Most solar panels are warranted for 25 years and can produce electricity for 30-40 years. They typically degrade about 0.5% per year in output.
What is the average cost of a solar system?
As of 2024, residential solar costs $2.50-3.50 per watt before incentives. A 10 kW system costs $25,000-35,000 before the 30% federal tax credit.
Do solar panels work on cloudy days?
Yes, but at reduced output (10-25% of rated capacity). The peak sun hours metric already accounts for average cloud cover in your area.
What roof direction is best for solar?
South-facing roofs produce the most energy in the Northern Hemisphere. West-facing is second best, producing about 80% of south-facing output.