How to Estimate Daily kWh from Solar Panels

Published: 20 Oct 2026

The daily solar panel energy output is one of the most important metrics when designing or analyzing a solar power system. It tells you how much electricity (in kilowatt-hours per day) your panels will generate under your local sunlight conditions. This estimate depends mainly on four key factors: panel watt rating (Wp), number of panels, peak sun hours (PSH), and system derate or losses.

Understanding the Basic Formula
The calculator above uses the formula: Daily kWh = (Panel Wp × Number of Panels × Peak Sun Hours × (1 − Derate)) ÷ 1000. Each variable represents a physical aspect of your solar array’s performance: Panel Wp: The rated power of a single solar panel under standard test conditions (usually 1000 W/m² at 25°C). Number of Panels: Total count of panels in your system. Peak Sun Hours (PSH): The average number of full-sun equivalent hours per day your location receives. Derate: System efficiency losses caused by inverters, cabling, temperature, shading, and other real-world effects.

Understanding the Basic Formula

The calculator above uses the formula: Daily kWh = (Panel Wp × Number of Panels × Peak Sun Hours × (1 − Derate)) ÷ 1000. Each variable represents a physical aspect of your solar array’s performance:

Panel Wp: The rated power of a single solar panel under standard test conditions (usually 1000 W/m² at 25°C).
Number of Panels: Total count of panels in your system.
Peak Sun Hours (PSH): The average number of full-sun equivalent hours per day your location receives.
Derate: System efficiency losses caused by inverters, cabling, temperature, shading, and other real-world effects.

What Are Peak Sun Hours?

Peak Sun Hours measure how much solar energy hits one square meter of your area daily, expressed in “hours of full sun.” For example, if your area gets 5 PSH, that means your panels receive the equivalent of 5 hours per day at 1000 W/m² of sunlight intensity. This number varies by location, time of year, and tilt angle. You can find typical PSH values.

The Role of System Derate and Efficiency Losses
Even high-quality solar systems don’t operate at 100% of their theoretical capacity. Losses occur due to inverter inefficiency (about 5–10%), wire resistance, soiling, temperature effects, and panel mismatch. A typical system derate factor is 15–25%, meaning your system performs at around 75–85% efficiency overall. You can enter your own derate value or use presets like “Typical (20%)” for realistic results.

The Role of System Derate and Efficiency Losses

Even high-quality solar systems don’t operate at 100% of their theoretical capacity. Losses occur due to inverter inefficiency (about 5–10%), wire resistance, soiling, temperature effects, and panel mismatch. A typical system derate factor is 15–25%, meaning your system performs at around 75–85% efficiency overall. You can enter your own derate value or use presets like “Typical (20%)” for realistic results.

Example Calculation
Suppose you have 10 panels rated at 400 Wp each and your site receives 4.5 Peak Sun Hours daily. Assuming a 20% derate: Daily kWh = (400 × 10 × 4.5 × 0.8) ÷ 1000 = 14.4 kWh/day This means your array could produce approximately 14.4 kWh of usable energy per day. Over a 30-day month, that’s around 432 kWh per month, depending on seasonal changes.

Example Calculation

Suppose you have 10 panels rated at 400 Wp each and your site receives 4.5 Peak Sun Hours daily. Assuming a 20% derate:

Daily kWh = (400 × 10 × 4.5 × 0.8) ÷ 1000 = 14.4 kWh/day

This means your array could produce approximately 14.4 kWh of usable energy per day. Over a 30-day month, that’s around 432 kWh per month, depending on seasonal changes.

Factors That Affect Real-World Output

Orientation & Tilt: Panels facing true south (in the Northern Hemisphere) or true north (in the Southern Hemisphere) yield higher output.
Shading: Even partial shading on one panel can reduce the array’s total performance.
Temperature: High temperatures slightly reduce panel voltage, lowering output.
Soiling & Dust: Dirt accumulation can reduce efficiency by 5–10% if panels are not cleaned regularly.
System Age: Panels degrade about 0.5%–1% per year, slowly reducing daily output over time.

Improving Your System Efficiency

You can boost your daily solar kWh by optimizing tilt angles, using high-efficiency inverters, minimizing cable losses, and scheduling periodic cleaning. Adding an MPPT charge controller or microinverters can further enhance conversion efficiency in partially shaded systems.