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Solar Panel Calculator

Estimate system size, savings, and payback period. Calculate solar panel system size (kW), annual kWh production, 25-year lifecycle savings, and payback period using real irradiance and efficiency coefficients.

System Inputs

$/mo
$/kWh
hrs
$/kW
$
Federal ITC of 30% is applied automatically. Efficiency factor: 80%.

System Size

9.78 kW

Panels (400 W)

25 panels

Annual Production

12,857 kWh

Gross Install Cost

$27,397

Federal ITC (30%)

$8,219

Net Install Cost

$19,178

Year 1 Savings

$1,800

25-Yr Total Savings

$65,627

Simple Payback

10.7 yrs

25-Year Net Return (savings minus net cost): $46,449

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Solution Methodology

01

Enter Monthly Bill & Rate

Input your average monthly electricity bill and your utility rate ($/kWh) to derive monthly kWh consumption.

02

Set Peak Sun Hours

Enter the daily peak sun hours for your location (e.g. 4.5 h for the US average) — this drives annual production.

03

Size the System

System size (kW) = Monthly kWh ÷ (Peak Sun Hours × 30 × 0.80 efficiency).

04

Apply Federal ITC

Multiply total install cost (kW × $/kW) by 0.70 to get net cost after the 30% tax credit.

05

Project 25-Year Savings

Sum annual savings compounded at 3% utility inflation, subtract net install cost to show total lifecycle return.

Common Questions

How do I calculate how many solar panels I need?
Divide your average monthly kWh usage by the average monthly sun hours for your location, then divide by the wattage per panel (typically 400 W). For example, 1,000 kWh/month in a region with 150 peak sun-hours per month requires about 1,000 ÷ 150 ÷ 0.4 = 16–17 panels at 400 W each, before applying a 15–20% efficiency correction.
What is a solar payback period?
The payback period is the number of years it takes for your cumulative energy savings to equal the upfront installation cost net of incentives. It is calculated as (Net Install Cost) ÷ (Annual kWh × Utility Rate). With the 30% federal ITC, most residential systems achieve payback in 6–10 years.
What efficiency factor should I use for my solar system?
A conservative overall system efficiency of 75–80% (also called the performance ratio) accounts for inverter conversion losses (~4%), temperature derating (~7%), wiring resistance (~2%), and typical shading or soiling (~5%). High-quality modern systems in optimal conditions can achieve 82–85%.
Does the 30% federal solar tax credit apply to all systems?
The Investment Tax Credit (ITC) allows you to deduct 30% of the total installed cost of a solar system from your federal income taxes through 2032 for both residential and commercial installations. You must have a federal tax liability to benefit; unused credit can be carried forward to subsequent tax years.