Solar System Size Calculator

Determine the right size solar system for your home. Enter your monthly electricity usage, average daily peak sun hours for your location, and expected system losses to calculate the optimal system size, number of panels needed, and estimated installation cost.

Monthly Usage (kWh)

Peak Sun Hours (hrs/day)

System Losses (%)

How to Use

1 Find your monthly kWh usage on your most recent electricity bill.

2 Look up the average peak sun hours for your area (NREL or local solar maps).

3 Set system losses (15% is a good default for most installations).

4 Review the recommended system size and number of panels.

5 Use the estimated cost to start getting quotes from solar installers.

Formula

Daily Kwh monthly_kwh / 30

Effective Sun Hours peak_sun_hours × (1 - system_loss / 100)

System Size Kw daily_kwh / effective_sun_hours

Panels 400W ceil(system_size_kw / 0.4)

Estimated Cost system_size_kw × $2,500

Sizing a System for a US Home

A home using 900 kWh/month in an area with 5 peak sun hours and 15% system losses: Daily usage = 900/30 = 30 kWh, Effective sun hours = 5 × 0.85 = 4.25, System size = 30/4.25 = 7.06 kW, Panels needed = ceil(7.06/0.4) = 18 panels, Estimated cost = 7.06 × $2,500 = $17,647.

Why It Matters

Properly sizing your solar system ensures you generate enough electricity to meet your needs without overspending on unnecessary panels. An undersized system won't offset your bill sufficiently, while an oversized system may not be approved by your utility.

Who Uses This Calculator?

• Homeowners and renters estimating electricity costs, appliance usage, solar savings, or EV charging expenses.
• Solar shoppers, EV owners, and households comparing energy upgrades before requesting quotes or changing usage habits.
• Anyone who wants a quick estimate of home energy costs using clear assumptions.

Frequently Asked Questions

What are peak sun hours?

Peak sun hours refer to the number of hours per day when solar irradiance reaches 1,000 watts per square meter. This is different from daylight hours. The US averages 4-6 peak sun hours depending on location, with southern states receiving more.

What causes system losses in solar installations?

System losses come from inverter inefficiency (2-3%), wiring losses (2-3%), shading (variable), soiling/dust (2-5%), temperature effects (5-15%), and module mismatch (1-2%). A combined 15% loss factor is standard industry practice.

How many solar panels does the average home need?

The average US home using 900 kWh/month typically needs 17-21 panels rated at 400W each, creating a 7-8 kW system. This varies based on location, roof orientation, and local sun conditions.

What size are 400W solar panels?

A standard 400W residential solar panel is approximately 6.5 feet long by 3.5 feet wide (about 17.5 sq ft). An 18-panel system would require roughly 315 sq ft of unshaded roof space.

Related Guides

Solar Panel Guide

Understand solar sizing, savings, incentives, and payback before comparing quotes.

Solar Payback Guide

Understand solar payback period, net system cost, incentives, and savings assumptions.

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This calculator provides estimates for informational purposes only. Verify results with a qualified professional for important decisions.