Solar Panel Wattage Calculator

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Walk-through

How to Use This Calculator

3 steps

1

Enter your panel area and irradiance

Enter the total collector area of your array (not your roof area) and pick m² or ft². Leave irradiance at the 1,000 W/m² default unless you have a site-specific measurement — that default is Standard Test Conditions (STC), the industry benchmark used on every panel spec sheet.

2

Set efficiency and peak sun hours

Enter your panel's rated efficiency (most residential panels run 18–22%) and your location's average peak sun hours per day. The calculator updates the array wattage instantly as you type.

3

Read wattage, daily, and annual output

The hero result is your array's rated wattage. Switch to the Daily Output and Annual Output tabs to see estimated energy production per day and per year, including a system derate factor for real-world losses.

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Reference

Formula & Methodology

2 formulas

Array wattage

Watts = Area (m²) × Irradiance (W/m²) × Efficiency (%)

The array's rated power output is the product of its collector area, the sunlight irradiance it receives, and its panel efficiency (the fraction of incoming sunlight converted to electricity). Irradiance defaults to 1,000 W/m² — Standard Test Conditions (STC), the condition manufacturers use to rate every panel's nameplate wattage.

Daily and annual output

Daily kWh = Watts × Sun Hours ÷ 1000; Annual kWh = Daily kWh × 365 × Derate

Daily energy output multiplies the array's rated wattage by your location's peak sun hours (the number of hours of 1,000 W/m² equivalent sunlight per day), then converts watt-hours to kilowatt-hours. Annual output extends that across 365 days and applies a system derate factor — typically ~80% — to account for wiring losses, heat de-rating, soiling, and inverter conversion losses that the nameplate rating doesn't capture.

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Glossary

Key Terms Explained

7 terms

Irradiance The power of sunlight hitting a surface, measured in watts per square meter (W/m²). Peak midday sun delivers roughly 1,000 W/m² under clear skies — the same value used as the Standard Test Conditions benchmark.

Panel efficiency The percentage of incoming sunlight a solar panel converts into usable electricity. Most modern residential panels fall between 18% and 22%; premium panels can exceed 22%.

Peak sun hours The number of hours per day during which sunlight would need to shine at 1,000 W/m² to deliver the same total energy as the actual, variable sunlight received. It compresses a full day's variable sunlight into an equivalent count of 'full sun' hours and varies by location and season.

STC (Standard Test Conditions) The lab conditions used to rate every solar panel's nameplate wattage: 1,000 W/m² irradiance, 25°C cell temperature, and an AM1.5 solar spectrum. Real-world output is almost always lower than the STC rating because outdoor conditions vary.

kWp (kilowatt-peak) The rated power output of a solar array under Standard Test Conditions, expressed in kilowatts. A 20 m² array with 20% efficiency at STC irradiance is rated at 4 kWp — the same number this calculator's hero result shows in watts.

Solar array A group of interconnected solar panels wired together to produce a combined power output larger than any single panel could deliver on its own.

Derate factor A percentage applied to a solar system's rated (STC) output to estimate real-world annual production, accounting for wiring resistance, panel heating, dust/soiling, and inverter conversion losses. A commonly used default is ~80%.

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Scenarios

Real-World Examples

3 worked examples

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Rooftop homeowner

20 m² array at 20% efficiency

Area 20 m² Efficiency 20%

At STC irradiance (1,000 W/m²), a 20 m² array at 20% efficiency is rated at 4,000 W (4 kW). With 5 peak sun hours a day, that produces about 20 kWh/day — roughly the daily usage of an efficient home.

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Daily-output check

Same 4 kW array, 5 peak sun hours

Watts 4,000 W Sun hours 5

Daily output = 4,000 W × 5 hours ÷ 1000 = 20 kWh/day. Peak sun hours vary by location and season, so this is a location-dependent estimate, not a fixed number.

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Annual estimate with derate

20 kWh/day at an 80% system derate

Daily kWh 20 Derate 80%

Annual output = 20 kWh/day × 365 days × 0.80 derate ≈ 5,840 kWh/year. The 80% derate rolls in wiring losses, panel heating, soiling, and inverter conversion — the gap between a lab-rated STC number and what the system actually delivers over a year.

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Deep Dive

Understanding the Solar Panel Wattage Calculator

3 chapters · 4 min read

This calculator converts three panel specs — area, irradiance, and efficiency — into the number that matters when sizing a system: rated array wattage. From there it estimates daily and annual energy output so you can compare a proposed array against your actual electricity usage.

How the Solar Panel Wattage Calculator works

The core formula is Watts = Area × Irradiance × Efficiency. Area is the total collector surface of the array, not the roof footprint (racking, spacing, and unusable roof sections mean the two are rarely equal). Irradiance defaults to 1,000 W/m² — Standard Test Conditions (STC), the condition every panel manufacturer uses to publish a nameplate wattage rating. Efficiency is the fraction of that incoming sunlight the panel actually converts to electricity, typically 18–22% for residential panels today.

Daily and annual output extend the wattage figure using peak sun hours (a location-specific measure of how much usable sunlight a site receives per day) and a system derate factor that accounts for the losses a lab rating doesn't — wiring resistance, heat, dust, and inverter conversion.

Inputs and what they mean

Area is the biggest lever on the result — doubling the array area doubles the wattage. Irradiance and efficiency are usually left at their defaults (1,000 W/m² and 20%) unless you have manufacturer spec-sheet numbers to plug in. Peak sun hours is the input most likely to surprise newcomers: it is not the number of daylight hours, but the equivalent hours of full-strength (1,000 W/m²) sun a site receives — commonly 3–4 in cloudier northern climates and 5–6+ in the sunny Southwest.

Limits and edge cases

This is a sizing-and-output estimate, not a substitute for a professional site survey. It doesn't account for shading, panel orientation/tilt, temperature de-rating throughout the day, or seasonal irradiance swings — a qualified installer's production estimate (typically from tools like PVWatts) will be more precise for a real installation. A panel area of zero or less returns no result, since wattage scales directly from area.

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Questions

Frequently Asked Questions

6 questions

What is the formula for solar panel wattage?+

Watts = Area (m²) × Irradiance (W/m²) × Efficiency (%). With the default 1,000 W/m² STC irradiance and a stated efficiency, this gives the array's rated power output — the same number printed on a panel's nameplate, scaled up to your total collector area.

What are Standard Test Conditions (STC)?+

STC is the lab benchmark used to rate every solar panel: 1,000 W/m² irradiance, 25°C cell temperature, and an AM1.5 solar spectrum. It's the industry-standard condition manufacturers use to publish a panel's nameplate wattage, which is why this calculator defaults irradiance to 1,000 W/m².

What are peak sun hours and where do I find mine?+

Peak sun hours compress a full day's variable sunlight into an equivalent number of hours at 1,000 W/m². They're location-dependent — sunny southwestern US locations often see 5–6+ peak sun hours a day, while cloudier northern regions see closer to 3–4. Solar-resource maps from NREL or your installer's site assessment are good sources for a location-specific figure.

What's a typical solar panel efficiency?+

Most residential solar panels sold today are 18–22% efficient. Premium monocrystalline panels can exceed 22%, while older or budget panels may sit closer to 15–17%. Efficiency is the fraction of incoming sunlight a panel converts into usable electricity.

How is annual output calculated?+

Annual output = Daily kWh × 365 × Derate. Daily kWh comes from the array's wattage multiplied by peak sun hours, and the derate factor scales that ideal number down to account for real-world losses that don't show up in a lab rating.

What is a derate factor and why is it around 80%?+

A derate factor rolls together the real-world losses a nameplate (STC) rating doesn't capture — resistance in wiring, panels running hotter than the 25°C test condition, dust and debris on the panel surface, and inverter conversion losses. ~80% is a commonly cited starting estimate for a reasonably well-sited residential system; well-maintained systems in cool climates can do better, while dusty or hot-climate systems may derate further.

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Solar Panel Wattage Calculator

Array Inputs

Array Wattage

How to Use This Calculator

Enter your panel area and irradiance

Set efficiency and peak sun hours

Read wattage, daily, and annual output

Formula & Methodology

Key Terms Explained

Real-World Examples

Rooftop homeowner

Daily-output check

Annual estimate with derate

Understanding the Solar Panel Wattage Calculator

How the Solar Panel Wattage Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

Solar Panel Wattage Calculator

Array Inputs

Array Wattage

Output breakdown

How to Use This Calculator

Enter your panel area and irradiance

Set efficiency and peak sun hours

Read wattage, daily, and annual output

Formula & Methodology

Key Terms Explained

Real-World Examples

Understanding the Solar Panel Wattage Calculator

How the Solar Panel Wattage Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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