Solar Panel Output Calculator
Estimates the daily kilowatt-hour output of a solar panel system based on wattage, peak sun hours, and system efficiency losses. Use it when sizing a new system or auditing an existing installation's expected production.
About this calculator
A solar panel's daily energy output depends on three factors: its rated wattage, the number of peak sun hours at your location, and real-world system efficiency (accounting for wiring losses, inverter losses, and heat). The formula is: Output (kWh) = (panelWattage × sunHours × (efficiency / 100)) / 1000. Panel wattage is the nameplate DC rating (e.g., 400 W). Peak sun hours represent the equivalent hours per day of full 1,000 W/m² irradiance — not simply daylight hours. System efficiency typically ranges from 75–85% to account for inverter conversion losses, shading, and temperature derating. Dividing by 1,000 converts watt-hours to kilowatt-hours, the unit your utility bill uses. Understanding this calculation helps you match panel count to your household's daily consumption.
How to use
Suppose you have a 400 W panel, your location receives 5 peak sun hours per day, and your system efficiency is 80%. Step 1 — Enter Panel Wattage: 400 W. Step 2 — Enter Peak Sun Hours: 5 hours. Step 3 — Enter System Efficiency: 80%. Step 4 — The calculator computes: (400 × 5 × (80 / 100)) / 1000 = (400 × 5 × 0.80) / 1000 = 1,600 / 1,000 = 1.6 kWh per day. A household of ten such panels would produce roughly 16 kWh/day, enough to cover the average U.S. home's daily usage.
Frequently asked questions
What are peak sun hours and how do they affect solar panel output?
Peak sun hours are not the same as daylight hours. They represent the number of hours per day when solar irradiance averages 1,000 W/m² — the standard test condition for panel ratings. A location might have 10 hours of daylight but only 4.5 peak sun hours. Higher peak sun hours directly increase output: doubling peak sun hours from 3 to 6 doubles your daily kWh production. You can look up your location's peak sun hours using NREL's PVWatts tool or a solar irradiance map.
Why does system efficiency reduce my solar panel's rated output?
A panel's nameplate wattage is measured under ideal lab conditions. In a real installation, power is lost at several stages: the inverter converting DC to AC typically loses 4–8%, high temperatures derate panels by roughly 0.4% per °C above 25°C, wiring resistance causes further losses, and even light soiling can reduce output by 2–5%. Combining these losses, a well-designed residential system typically operates at 75–85% overall efficiency. Entering a realistic efficiency value gives you a more accurate production estimate than using 100%.
How many solar panels do I need to cover my monthly electricity bill?
First, find your average daily consumption by dividing your monthly kWh usage by 30 (e.g., 900 kWh ÷ 30 = 30 kWh/day). Then use this calculator to find the daily output of a single panel at your location and efficiency. Divide your daily consumption by the single-panel output to get the number of panels needed (e.g., 30 kWh ÷ 1.6 kWh = 19 panels). Always round up and factor in roof constraints or shading before finalising your design. A qualified installer can refine this estimate using on-site assessment.