# Solar Panel Output Calculator

Estimate solar panel energy output per day, month, and year. Enter panel wattage, sun hours, and system efficiency to calculate real-world kWh production and savings.

## What this calculates

A 400W solar panel does not actually produce 400 watts all day. Real output depends on how many hours of strong sunlight you get, plus losses from heat, inverter conversion, wiring, and dirt. This calculator uses all those factors to give you a realistic estimate of how much energy your panels will actually generate and how much money that saves.

## Inputs

- **Panel Wattage (STC Rating)** (W) — min 0 — Standard Test Conditions rating from the panel spec sheet
- **Number of Panels** — min 1
- **Peak Sun Hours per Day** (hours) — min 0, max 12 — US average: 4-6 hours. Check PVWatts for your location.
- **System Efficiency Factor** (%) — min 10, max 100 — Accounts for inverter, wiring, soiling, shading, temperature

## Outputs

- **Daily Output** (Wh) — Estimated daily energy production
- **Daily Output** (kWh) — Daily production in kilowatt-hours
- **Monthly Output** (kWh) — Estimated monthly energy production (30 days)
- **Yearly Output** (kWh) — Estimated yearly energy production
- **Estimated Yearly Savings** — formatted as currency — At $0.12/kWh average US rate

## Details

## How Solar Output Is Calculated

**Daily Output = Panel Wattage x Peak Sun Hours x System Efficiency**

A 400W panel with 5 peak sun hours and 80% system efficiency produces: 400 x 5 x 0.80 = 1,600 Wh (1.6 kWh) per day.

### What Are Peak Sun Hours?

Peak sun hours are not the same as hours of daylight. One peak sun hour equals 1,000 W/m squared of solar irradiance for one hour. A location with 8 hours of daylight might only get 4-5 peak sun hours because morning and evening sun is weaker.

| Region | Approx. Peak Sun Hours |
|--------|----------------------|
| Arizona / Southern California | 6-7 |
| Texas / Florida | 5-6 |
| Midwest / Northeast US | 4-5 |
| Pacific Northwest / UK | 3-4 |
| Germany / Canada | 3-4 |

### System Efficiency Breakdown

The 75-85% efficiency factor accounts for multiple losses:

- **Inverter conversion:** 3-5% loss
- **Wiring and connections:** 1-3% loss
- **Temperature derating:** 5-15% loss (panels lose efficiency in heat)
- **Soiling (dust/dirt):** 2-5% loss
- **Shading:** 0-20% loss depending on surroundings
- **Age degradation:** 0.5% per year

### Quick Sizing Guide

The average US household uses about 30 kWh per day. With 5 peak sun hours and 80% efficiency, you would need about 7,500W (19 panels at 400W each) to cover that entirely with solar.

## Frequently Asked Questions

**Q: What does STC rating mean on a solar panel?**

A: STC stands for Standard Test Conditions: 1,000 W/m squared irradiance, 25 degrees C cell temperature, and air mass 1.5. A 400W panel produces exactly 400W under these lab conditions. Real-world output is almost always lower because rooftop temperatures exceed 25 degrees C and sunlight intensity varies throughout the day.

**Q: How many peak sun hours does my location get?**

A: Use the NREL PVWatts tool (pvwatts.nrel.gov) to find your exact location's solar resource. In the US, the southwest averages 6-7 peak sun hours, the southeast 5-6, the midwest 4-5, and the northwest 3-4. These are annual averages; summer produces more than winter.

**Q: Why is 80% used as the default efficiency?**

A: The 80% system efficiency factor is a conservative but realistic estimate for a typical residential installation. It includes inverter losses (96-97% efficient), wiring losses, soiling, temperature effects, and minor shading. Well-optimized systems can reach 85%, while poorly oriented or partially shaded systems might drop to 70-75%.

**Q: Do solar panels produce anything on cloudy days?**

A: Yes, but much less. Panels still generate power from diffuse light on overcast days, typically 10-25% of their rated output. Heavy cloud cover drops that to 5-10%. The peak sun hours metric already accounts for this by averaging across sunny and cloudy days in your area.

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Source: https://vastcalc.com/calculators/physics/solar-panel-output
Category: Physics
Last updated: 2026-04-08