How do I calculate how many solar panels I need?

Step-by-step formula: 1. Daily usage (kWh) = Monthly bill kWh / 30 2. System size (kW) = Daily usage / (Peak sun hours × System efficiency) 3. Panels needed = System size (W) / Panel wattage, rounded up Example — 900 kWh/month, 4.5 peak sun hours, 80% efficiency, 400W panels: • Daily usage: 900 / 30 = 30 kWh/day • System size: 30 / (4.5 × 0.80) = 8.33 kW • Panels: 8,333W / 400W = 20.8 → 21 panels • Actual system: 21 × 400W = 8.4 kW This gives you a system that covers approximately 100% of your usage. For 50% coverage, halve the daily usage figure.

What are peak sun hours and how are they different from daylight hours?

Peak sun hours (PSH) is the number of hours per day with equivalent solar irradiance of 1,000 W/m² (full direct sunlight). Daylight hours vs peak sun hours: • London gets about 16 hours of daylight in summer — but only 2.5–3.5 PSH on average across the year • Phoenix gets 14 hours of daylight in summer — and 5.5–6.5 PSH annually Why PSH matters: a 1 kW panel array generates 1 kWh per PSH. In London (3 PSH), it generates 3 kWh/day. In Phoenix (6 PSH), it generates 6 kWh/day — twice as much from the same panels. For precise PSH for your exact location and roof tilt/orientation: • US: NREL PVWatts Calculator (pvwatts.nrel.gov) • EU/Global: PVGIS (re.jrc.ec.europa.eu/pvg_tools)

What is a good system efficiency to use?

System efficiency (also called performance ratio or derate factor) accounts for all losses between the panel's rated output and actual AC power delivered: • Inverter efficiency: 96–98% for quality string inverters • Temperature derating: panels lose ~0.4%/°C above 25°C • Wiring and connection losses: ~2% • Soiling/dust: 2–5% (cleaned regularly: lower) • Shading: 0% (no shade) to 20%+ (significant shade) • Mismatch and other: ~2% Total system efficiency: • 85%: Premium system, microinverters/optimisers, no shade, well-maintained • 80%: Standard quality system, string inverter, minimal shade — good default • 75%: Budget system, some shading, older technology • 70%: Poor installation, significant shading issues

How much roof space do I need for solar panels?

A standard 400W solar panel is approximately 1.0m × 1.7m = 1.7 m² (about 18 sq ft). Roof area by system size: • 3 kW system (7–8 panels): ~12–14 m² (130–150 sq ft) • 5 kW system (12–13 panels): ~20–22 m² (215–240 sq ft) • 8 kW system (20 panels): ~34 m² (365 sq ft) • 10 kW system (25 panels): ~42 m² (450 sq ft) Practical notes: • South-facing roofs (in the Northern Hemisphere) produce 15–25% more than east/west-facing roofs • Avoid panels in shade from chimneys, trees, or other roof features • Flat roofs need mounting frames (adds cost) but allow optimal angle • A usable roof area of 20–30 m² is enough for most residential installations

What is the payback period for solar panels?

Payback period = Total system cost / Annual electricity savings Typical payback by country (2025): • USA (sunny states): 6–10 years • USA (cloudy states): 10–14 years • UK: 8–12 years (high electricity rates offset low sun) • Australia: 4–7 years (high sun + high electricity rates) • Germany: 8–12 years • India: 3–6 years (very high sun, incentives) Factors that shorten payback: • Higher electricity rates (saves more per kWh) • More sun hours • Government incentives (US 30% ITC, UK 0% VAT) • Net metering / feed-in tariffs • Battery storage (controversial — often extends payback) After payback, solar panels typically operate for 20–30 years, generating free electricity for 10–20+ years.

Does solar make sense with a battery storage system?

Adding a home battery (e.g. Tesla Powerwall, Sonnen, LG RESU) to a solar system: Pros: • Use solar power at night • Backup power during grid outages • Avoid peak-rate grid electricity • Increases self-consumption rate from ~30% to ~70–80% Cons: • Adds $8,000–$15,000 to system cost • Extends payback by 3–6 years typically • Battery degradation: expect 10–15% capacity loss over 10 years • Not cost-effective if your utility offers good net metering/feed-in tariffs Best case for battery: locations with time-of-use electricity pricing (pay 3× at peak), areas with frequent outages, or places with no net metering. For most homes with good net metering, solar-only without battery is the better financial decision.

Solar Panel Size Calculator — System Size, Panels & Payback

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..

YOUR ENERGY USAGE

MONTHLY ELECTRICITY USAGE (kWh)

kWh

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

LOCATION / SUN ZONE

Medium Sun — SE USA, S Europe, India — 4.5 peak sun hours/day

PANEL WATTAGE (W)

Common modern panels: 400–500W. Higher wattage = fewer panels needed.

SYSTEM EFFICIENCY (%)

Accounts for inverter losses, wiring, temperature derating, and shading.

INSTALL COST ($/W)

US avg: $2.50–$4.00/W installed

ELECTRICITY RATE ($/kWh)

US avg: $0.12–$0.17/kWh · UK: ~$0.30

RESULTS

SYSTEM SIZE

8.40

PANELS NEEDED

× 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

Daily usage30.00 kWh/day

Peak sun hours4.5 hrs/day

System efficiency80%

System size needed8.33 kW

System size (actual)8.40 kW (21 panels)

Roof area35.7 m² (384 sq ft)

Annual production11,038 kWh/yr

Annual savings$1,656/yr

System cost$25,200

Payback period15.2 years

CO₂ offset2.57 tonnes/yr

PEAK SUN HOURS BY REGION

Phoenix AZ / UAE / W Australia5.5–6.5 hrs

Los Angeles / Spain / India4.5–5.5 hrs

New York / France / Japan4.0–4.5 hrs

Seattle / Germany / New Zealand3.0–4.0 hrs

UK / Scandinavia / Canada (N)2.5–3.5 hrs

For exact values: use NREL PVWatts (US) or PVGIS (EU/global)

Created with❤️byeaglecalculator.com

HOW TO USE

1
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.
2
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.
3
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.
4
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.
5
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.

WORKED EXAMPLE

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

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Last updated: April 29, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.