How is solar output calculated?

Energy per day equals the array size in kW times the peak sun hours per day times the performance ratio. For example a 7.2 kW array at 5 peak sun hours and 0.8 performance ratio makes 7.2 times 5 times 0.8 equals 28.8 kWh per day.

What is the performance ratio?

The performance ratio is the fraction of the panels' ideal output that the system actually delivers after real-world losses — inverter conversion, wiring resistance, high panel temperature, soiling, and mismatch. Typical fixed rooftop systems land around 0.75 to 0.85.

Why is it different from the panel nameplate?

Panels are rated at standard test conditions (25 C cell, 1000 W/m2). In the field, panels run hotter, light is weaker, and conversion has losses, so real output is lower. The performance ratio bridges nameplate to delivered energy.

How do I get peak sun hours for my area?

Use a solar resource map or the companion sizing tool's bundled city table. Peak sun hours are the equivalent hours of full 1000 W/m2 sunlight per day, typically 3.5 to 6.5 across the continental US.

Does output vary by season?

Yes, significantly. Summer peak sun hours can be roughly double winter values, and high summer temperatures slightly reduce panel efficiency. This tool uses your single peak-sun-hours figure, so enter an annual average for yearly totals or a monthly value for that month.

Solar Panel Daily/Monthly Output Calculator

Once you know an array’s size, the next question is how much energy it will make. This calculator estimates daily, monthly, and annual output for a fixed PV array from the panel rating, count, local sun hours, and a real-world performance ratio.

How it works

The core formula is simple:

daily kWh = array kW x peak sun hours x performance ratio

where:

array kW = panel watts × number of panels ÷ 1000,
peak sun hours = equivalent hours of full 1000 W/m² sunlight per day,
performance ratio = the fraction of ideal output actually delivered after inverter, wiring, temperature, and soiling losses (typically 0.75–0.85).

Monthly and annual figures scale the daily result by 30.4 and 365 days.

Worked example

An 18-panel array of 400 W modules at 5.0 peak sun hours and a 0.8 performance ratio:

Array size: 18 × 400 / 1000 = 7.2 kW DC.
Daily: 7.2 × 5.0 × 0.8 = 28.8 kWh.
Monthly: 28.8 × 30.4 ≈ 876 kWh.
Annual: 28.8 × 365 ≈ 10,512 kWh.

That comfortably offsets a home using about 900 kWh per month.

Notes and tips

For a yearly estimate, use the location’s annual-average peak sun hours; for a single month, plug in that month’s value.
The performance ratio is the biggest lever on accuracy. Hot climates and shaded roofs justify 0.75; cool, clean, unshaded installs reach 0.85.
Panel output degrades slowly over time — about 0.5% per year — so multiply by a degradation factor for long-term financial models.

Every calculation runs locally in your browser. No data is sent to any server.