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Solar Panel Output Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Formula based on NREL PVWatts methodology. Last verified 14 June 2026.

This solar panel output calculator estimates how much electricity your solar array will generate per day, per month, and per year. The calculation uses the NREL PVWatts methodology: multiply total panel wattage by daily peak sun hours and system efficiency to get daily kilowatt-hours. Peak sun hours vary significantly by location, typically 3.5 hours per day in the Pacific Northwest to over 6 hours per day in the Southwest. For precise location data, use NREL PVWatts at pvwatts.nrel.gov. System efficiency (default 0.80) accounts for inverter conversion losses, wiring resistance, cell temperature derating, soiling, and module mismatch. Real-world systems typically achieve 75% to 85% overall efficiency. Enter your panel's nameplate wattage (found on the back of the panel or in the datasheet), the number of panels in your array, your location's average peak sun hours, and your estimated system efficiency. The calculator gives daily, monthly (30-day), and annual output. Use these figures to estimate electricity bill savings by multiplying kWh by your local utility rate.

Nameplate rating per panel (e.g. 350, 400, 450)
Total panels in your array
Varies by location (3.5 to 6+). Use NREL PVWatts for your address.
Accounts for inverter, wiring, and temperature losses (0.80 typical)
System Size--
Daily Output--
Monthly Output (30 days)--
Annual Output (365 days)--

Solar output formula

Daily Output (kWh) = Panel Wattage (W) x Number of Panels x Peak Sun Hours x Efficiency / 1,000
Monthly Output (kWh) = Daily Output x 30
Annual Output (kWh) = Daily Output x 365

Worked example

20 panels at 400 W each, 4.5 peak sun hours per day, 80% system efficiency:

  1. System size = 20 x 400 = 8,000 W (8 kW)
  2. Daily output = 8,000 x 4.5 x 0.80 / 1,000 = 28.80 kWh/day
  3. Monthly output = 28.80 x 30 = 864.00 kWh/month
  4. Annual output = 28.80 x 365 = 10,512.00 kWh/year

At a rate of $0.15/kWh, this system would generate roughly $1,577 worth of electricity per year. Find your peak sun hours at NREL PVWatts.

Frequently asked questions

What are peak sun hours?

Peak sun hours are not the same as hours of daylight. One peak sun hour represents an hour of sunlight at an intensity of 1,000 W per square metre (the standard test condition for solar panels). A location may have 10 hours of daylight but only 4 to 5 peak sun hours because the sun is at a lower intensity early in the morning and late in the afternoon. Use NREL's PVWatts tool (pvwatts.nrel.gov) to find accurate peak sun hours for your specific location.

Why is actual output less than the panel's rated wattage?

Solar panel wattage ratings are measured under Standard Test Conditions (STC): 25 degrees Celsius cell temperature, 1,000 W/m2 irradiance, and an air mass of 1.5. Real-world conditions rarely match STC. High temperatures reduce panel output (typically 0.3% to 0.5% per degree above 25 C). Add inverter losses (3% to 5%), wiring resistance, dust and soiling, shading, and module mismatch, and a realistic combined system efficiency is 75% to 85%. A default of 80% is conservative but representative.

What does a typical residential solar system look like?

A typical US residential solar installation is 6 kW to 10 kW, made up of 15 to 25 panels rated at 350 W to 400 W each. With 4 to 5 peak sun hours per day and 80% system efficiency, a 6 kW system (say, 17 panels x 350 W) generates roughly 19 to 24 kWh per day, or 7,000 to 8,700 kWh per year. The US average household uses about 10,500 kWh per year (EIA, 2023), so a well-sized system can cover most of that.

How do I estimate my electricity bill savings?

Multiply your annual kWh output by your electricity rate in dollars per kWh. For example, 8,000 kWh per year at $0.15/kWh saves $1,200 per year before any feed-in tariff credits for exported power. Use your latest utility bill to find your exact rate. Rates vary widely by state and utility. The U.S. Energy Information Administration (EIA) publishes average retail electricity prices by state at eia.gov.

What is the difference between a panel's rated output and its actual output?

Rated output (in watts) is measured under lab conditions. Actual output depends on location, season, shading, temperature, panel age, and system efficiency. On a clear summer day at noon your panels may reach or briefly exceed rated output. On cloudy days output may fall to 10% to 25% of rated. Annual average output, divided by rated capacity, is the capacity factor. For residential rooftop solar in the US, the capacity factor is typically 15% to 22%, meaning a 400 W panel averages 60 W to 88 W over the full year.

Sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Estimates only. Use NREL PVWatts for site-specific modelling.