Everyday Life · Practical Life · Environment
Solar Panel Output Calculator
Estimates the daily and annual electricity output of a solar panel system based on panel wattage, quantity, peak sun hours, and system efficiency.
Calculator
Formula
E_{daily} is the daily energy output in watt-hours (Wh); N is the number of solar panels; P_{panel} is the rated power of each panel in watts (W); H_{peak} is the average peak sun hours per day for your location (h/day); \eta_{system} (eta) is the overall system efficiency as a decimal, accounting for inverter losses, wiring losses, temperature derating, and soiling — typically 0.75 to 0.85 for a well-designed residential system. Divide the result by 1000 to convert Wh to kWh. Annual output is simply the daily output multiplied by 365.
Source: National Renewable Energy Laboratory (NREL) — PVWatts Methodology; IEC 61724 Photovoltaic System Performance Monitoring.
How it works
Solar panels are rated under Standard Test Conditions (STC) — 1000 W/m² of irradiance and a cell temperature of 25 °C. In the real world, panels rarely operate at their rated wattage continuously. The key metric that bridges the gap between rated capacity and actual production is peak sun hours — the number of hours per day when solar irradiance averages 1000 W/m². This is a location-dependent figure: a rooftop in Phoenix, Arizona receives around 6.0 peak sun hours per day, while a site in the UK might average only 2.5–3.0.
The core formula multiplies the number of panels (N) by each panel's rated wattage (P), the daily peak sun hours (H), and the system efficiency factor (η). The result in watt-hours is divided by 1000 to yield kilowatt-hours (kWh) — the unit used on electricity bills. System efficiency accounts for inverter conversion losses (typically 3–5%), DC and AC wiring losses (2–3%), temperature derating (solar cells lose roughly 0.3–0.5% output per degree Celsius above 25 °C), soiling from dust and bird droppings, and shading. A combined derate factor of 75–85% is standard for a residential grid-tied system. Multiplying daily output by 365 gives a useful annual production estimate.
This estimate is the foundation for calculating payback periods, return on investment, and the size of battery storage needed for off-grid or hybrid systems. It also helps verify whether a proposed system will cover a household's actual electricity consumption, which can be read directly from utility bills in kWh.
Worked example
Consider a homeowner in central Spain installing a rooftop system with the following specification:
- 10 panels at 400 W each → system capacity = 4.0 kW
- Peak sun hours: 5.2 h/day (typical for Madrid)
- System efficiency: 80% (0.80 derate factor)
Step 1 — Daily output in Wh:
10 × 400 W × 5.2 h × 0.80 = 16,640 Wh/day
Step 2 — Convert to kWh:
16,640 ÷ 1,000 = 16.64 kWh/day
Step 3 — Annual output:
16.64 × 365 = 6,074 kWh/year
The average Spanish household consumes around 3,500–4,500 kWh per year, so this 4 kW system would comfortably cover annual consumption and likely export surplus to the grid. At a self-consumption value of €0.15/kWh, the system saves approximately €911 per year in electricity costs before considering any feed-in tariff income.
Limitations & notes
This calculator produces an estimate based on average annual conditions and does not account for seasonal variation — winter months will generate significantly less energy than summer months, which is critical for battery sizing in off-grid applications. Peak sun hours are averages; your actual figure depends on roof pitch, azimuth angle, local shading from trees or buildings, and regional cloud cover — use NREL's PVWatts or the EU's PVGIS tool to find an accurate local figure. The system efficiency derate is a single blended factor; a detailed engineering model would separate inverter efficiency curves, temperature profiles, and soiling schedules. Panel degradation (typically 0.5% per year) means annual output will decline slightly over the system's 25–30 year lifespan. This calculator does not estimate financial savings, payback periods, or battery storage requirements — see our related calculators for those outputs.
Frequently asked questions
What are peak sun hours and how do I find mine?
Peak sun hours represent the number of hours per day when solar irradiance averages 1,000 W/m² — the standard used to rate panel output. They are not the same as daylight hours. You can find accurate peak sun hours for your location using free tools such as NREL's PVWatts (US), the EU's PVGIS database (Europe and Africa), or the Global Solar Atlas. Typical values range from 2.5 h/day in northern Europe to 6.5 h/day in desert regions like the Middle East or US Southwest.
What system efficiency percentage should I use?
For a well-designed residential grid-tied system, 75–82% is a realistic range. Use 80% as a good default. Losses come from the inverter (3–5%), DC wiring resistance (1–3%), AC wiring (1–2%), soiling and dust (2–5%), and temperature derating (2–8% depending on climate). If your panels are frequently shaded or located in a very hot climate, use 70–75%. High-quality microinverter or DC optimizer systems can push efficiency closer to 85%.
How does panel wattage affect output?
Panel wattage is the rated power under STC — essentially the 'nameplate' value. Modern residential panels range from 300 W to 600 W per panel, with 400–450 W being the most common in 2024–2025. Higher wattage panels produce more electricity per panel, which means fewer panels are needed to reach the same system capacity. For a fixed roof area, higher-efficiency panels (measured in W/m²) allow a larger total system capacity.
How much electricity does a typical solar panel produce per day?
A single 400 W panel in a location with 4.5 peak sun hours and 80% system efficiency produces approximately 1.44 kWh per day (400 × 4.5 × 0.80 ÷ 1000). Over a year, that's around 526 kWh — roughly equivalent to running a refrigerator for a year. A 10-panel, 4 kW system would produce about 14.4 kWh/day or 5,256 kWh/year under the same conditions.
Can I use this calculator for off-grid system sizing?
Yes, but with caution. For off-grid systems, you must size for the worst month — typically December in the northern hemisphere — rather than using annual averages. Find the minimum monthly peak sun hours for your location and use that figure to ensure your panels can still meet your load even in winter. You'll also need to account for battery storage capacity and charge controller efficiency, which are separate sizing steps not covered by this calculator.
Last updated: 2025-01-15 · Formula verified against primary sources.