The Solar Panel Size Calculator tells you exactly what size solar system you need, how many panels, how much roof space, and when the system pays for itself. Enter your monthly electricity usage in kWh from your bill, select your location's sun zone or enter custom peak sun hours, choose your panel wattage and system e..
Find on your electricity bill. US avg: ~900 kWh/mo · UK avg: ~300 kWh/mo · India avg: ~200 kWh/mo
= 30.00 kWh/day average
Medium Sun — SE USA, S Europe, India — 4.5 peak sun hours/day
Common modern panels: 400–500W. Higher wattage = fewer panels needed.
Accounts for inverter losses, wiring, temperature derating, and shading.
US avg: $2.50–$4.00/W installed
US avg: $0.12–$0.17/kWh · UK: ~$0.30
RESULTS
SYSTEM SIZE
8.40
kW
PANELS NEEDED
21
× 400W panels
ROOF AREA
35.7 m²
384 sq ft
ANNUAL OUTPUT
11,038
kWh/year
SYSTEM COST
$25,200
ANNUAL SAVINGS
$1,656
PAYBACK
15.2 yrs
CO₂ OFFSET
2.57 t/yr
PANEL ARRAY — 21 panels · 8.40 kW system
25-YEAR SAVINGS PROJECTION
FULL SUMMARY
PEAK SUN HOURS BY REGION
For exact values: use NREL PVWatts (US) or PVGIS (EU/global)
Enter your electricity usage from your bill — either as monthly kWh (easiest, found directly on your electricity statement) or as a daily kWh average. The US average home uses about 900 kWh/month, the UK average is around 300 kWh/month, and India averages around 200 kWh/month. If you want to size for a specific portion of your bill, enter that percentage of your total.
Select your sun zone or enter a custom peak sun hours value. Peak sun hours (PSH) is the number of hours per day your location receives equivalent full-sun irradiance of 1,000 W/m². It is NOT the number of daylight hours. Use NREL PVWatts (US) or PVGIS (EU/global) for a precise figure for your exact location and roof orientation.
Choose your panel wattage — modern residential panels range from 350W to 550W. Higher wattage means fewer panels for the same system size. Set system efficiency: 80% is a good default for a well-designed system with a quality inverter; use 75% for budget systems or partial shading, 85% for premium microinverter or DC optimiser systems.
Enter your installed cost per watt (US average is $2.50–$4.00/W all-in) and your electricity rate per kWh from your bill. These two figures determine the payback period calculation. Note that many US homeowners also qualify for a 30% federal tax credit (ITC) which significantly reduces the net cost.
Read the results: system size in kW, number of panels, roof area needed, annual energy production, annual savings, total system cost, payback period, and annual CO2 offset. The roof visualiser shows your panels laid out on a house, and the 25-year savings chart shows your cumulative return over time with the payback breakeven point marked.
Sizing a solar system for a US home using 900 kWh/month: Inputs: • Monthly usage: 900 kWh → daily: 30 kWh/day • Location: Dallas TX → 5.0 peak sun hours/day • System efficiency: 80% • Panel size: 400W • Install cost: $3.00/W • Electricity rate: $0.14/kWh Step 1: Required system size System kW = 30 kWh/day / (5.0 hrs × 0.80) = 30 / 4.0 = 7.5 kW Step 2: Panels needed 7,500W / 400W = 18.75 → round up to 19 panels Actual system = 19 × 400W = 7.6 kW Step 3: Roof area 19 panels × 1.7 m² = 32.3 m² (348 sq ft) Step 4: Annual production 7.6 kW × 5.0 hrs × 365 days × 0.80 = 11,096 kWh/year Step 5: Annual savings 11,096 kWh × $0.14/kWh = $1,553/year Step 6: System cost 7,600W × $3.00/W = $22,800 Step 7: Payback period $22,800 / $1,553 = 14.7 years After 30% US federal tax credit: $22,800 × 0.70 = $15,960 → payback: 10.3 years Step 8: CO2 offset 11,096 kWh × 0.233 kg/kWh = 2,585 kg = 2.59 tonnes/year
Last updated: April 29, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.