Solar Output Calculator

How is Solar Output Estimated?

Solar energy output depends on three key factors: panel capacity (kW), solar irradiance at the location (peak sun hours, PSH), and system efficiency (accounting for inverter, wiring, soiling, temperature, and shading losses).

Peak Sun Hours (PSH) represents the equivalent hours of full 1000 W/m² sunlight per day. A location receiving 1800 kWh/m²/year has 4.93 PSH daily average. PSH varies from 2-3 in northern Europe to 6-7 in desert regions. Tilt angle optimization (typically equal to latitude) can increase annual yield by 10-25%.

Real-world system efficiency is typically 75-85% of nameplate capacity due to: inverter losses (3-5%), DC wiring losses (2-3%), soiling (2-5%), temperature derating (5-15% in hot climates, panels lose ~0.4%/°C above 25°C), shading (0-15%), and degradation (0.5-0.8% per year).

Azimuth offset (deviation from true south in the northern hemisphere, true north in the southern) reduces annual output. The model integrates Liu-Jordan isotropic transposition over the year, so the derate follows the geometry rather than a fixed factor: at tilt = latitude, expect roughly 4% loss at a 45° offset, 16% at 90° (east/west), and about 45% facing due north. A flat (0°) array keeps about 88%. Bifacial panels can partially offset orientation losses by capturing reflected light on the rear side.

Formula: Daily Output = System Size (kW) × PSH × System Efficiency × Tilt/Azimuth Factor Monthly Output = Daily × 30 Annual Output = Daily × 365 Capacity Factor = Annual Output / (Size × 8760)

Example Calculation

10 panels × 400W = 4.0 kW system, PSH = 4.5, system efficiency 80%. Daily = 4.0 × 4.5 × 0.80 = 14.4 kWh. Monthly = 432 kWh. Annual = 5,256 kWh. Capacity factor = 5256/(4.0×8760) = 15.0%.

When to Use This Calculator

• Building owners evaluating rooftop solar feasibility by estimating annual generation against current electricity consumption
• Solar installers providing quick preliminary output estimates to customers before detailed site surveys
• Energy consultants comparing solar output at different tilt angles and orientations to optimize system design
• Financial analysts calculating expected generation for solar investment models, PPA pricing, or feed-in tariff revenue projections

Common Mistakes to Avoid

• Using nameplate panel wattage without derating for temperature — panels lose 0.3-0.5% per °C above 25°C STC, reducing output by 10-15% in hot climates
• Ignoring shading from nearby buildings, trees, or rooftop equipment — even partial shading on one panel can reduce string output by 30-80% without optimizers
• Assuming peak sun hours are the same as daylight hours — PSH represents equivalent hours of 1000 W/m² irradiance, which is typically 3-6 hours even with 12+ hours of daylight
• Forgetting annual degradation — crystalline silicon panels degrade 0.5-0.8% per year; a 25-year cash flow model should use declining output

How to Interpret Results

• Capacity factor of 13-20% is normal for fixed-tilt PV — it equals daily output divided by 24 hours at rated power, so PSH 4.5 at 80% system efficiency gives 15%; values above ~25% usually mean the peak sun hours entry is too high
• Tilt efficiency of 100% means tilt equals latitude with zero azimuth offset; the displayed percentage is the combined orientation derate the model applies on top of system efficiency
• Compare annual output to your annual consumption — a system that covers 100% of annual kWh still imports at night and exports at midday unless storage or net metering absorbs the mismatch
• Daily output is an annual average at the entered PSH — actual summer and winter daily production can differ 2-4× at mid latitudes, so do not size battery autonomy from this number
• The orientation model is simplified — for tilt or azimuth offsets beyond ~45° from optimal, confirm with PVWatts or PVsyst before committing a design

Related Standards & References

• NREL PVWatts Technical Reference — simplified PV production estimation methodology this calculator follows
• IEC 61215 — Terrestrial photovoltaic modules — Design qualification and type approval (STC ratings)
• Duffie & Beckman, Solar Engineering of Thermal Processes (Wiley) — reference for irradiance and tilt geometry

Frequently Asked Questions

What is the optimal tilt angle for solar panels?

As a rule of thumb, set tilt equal to latitude for maximum annual yield. Steeper tilt (latitude + 15°) favors winter generation, shallower (latitude - 15°) favors summer. For grid-connected systems maximizing annual output, latitude angle is optimal. Flat roofs often use 10-15° tilt for self-cleaning and wind load reduction.

How much roof space do I need for solar?

Modern 400W panels are approximately 1.7m × 1.0m (1.7 m²). For a 4 kW system (10 panels): ~17 m² of unshaded, south-facing roof. In practice, add 20-30% for spacing, setbacks, and obstructions. Flat roofs need more space due to tilt rack row spacing.