Solar Power Generation Estimator

Estimate the energy a solar array produces from its wattage, peak sun hours, a system derate, and the number of days.

Result

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How to use this calculator

Enter your array size in watts (total rated panel wattage).
Enter peak sun hours per day for your location (US average ~4–5).
Set the system derate (80% conservative, 86% PVWatts default).
Choose the number of days (365 for annual) and read the energy generated.

About this calculator

A solar array’s energy output over time is governed by its size, how much usable sun your location gets, and real-world system losses. This estimator multiplies the array’s rated kilowatts by the peak sun hours per day — the equivalent hours of full-strength (1000 W/m²) sunlight — and by a derate factor that accounts for inverter inefficiency, wiring and soiling losses, and heat. The result is daily energy in kilowatt-hours, which it scales to monthly, annual, or any number of days you choose. The derate matters: NREL’s PVWatts model uses roughly 14% total losses (an 86% factor), while 80% is a more conservative planning number. The tool also reports the effective annual yield per kilowatt installed, a handy figure (often 1,200–1,600 kWh/kW/year in sunny regions) for comparing locations or sizing a system against your electricity usage.

How it works — the formula

Daily kWh = (Watts ÷ 1000) × Peak sun hours × Derate
Total = Daily × Days
Yield = Annual kWh ÷ kW installed

Rated power times usable sun gives ideal output; the derate trims it to realistic generation, scaled across the chosen period.

Source: NREL — PVWatts Calculator

Worked examples

Example 1

5000 W, 5 sun h, 80%, 365 d

Inputs:: watts=5000, sunHours=5, derate=80, days=365
Output:: 20 kWh/day → 7,300 kWh/year

Example 2

400 W, 4.5 sun h, 80%, 30 d

Inputs:: watts=400, sunHours=4.5, derate=80, days=30
Output:: 1.44 kWh/day → 43.2 kWh/month

Example 3

10 kW, 6 sun h, 86%, 365 d

Inputs:: watts=10000, sunHours=6, derate=86, days=365
Output:: 51.6 kWh/day → ~18,834 kWh/year

Limitations

Uses average peak sun hours and a flat derate — no seasonal modeling.
Ignores tilt, orientation, and temperature specifics.
For precise output use NREL PVWatts for your coordinates.

Planning estimate; validate with PVWatts and your utility data.

Frequently asked

How much energy will my solar panels produce?+

Multiply your array size in kilowatts by your peak sun hours per day and a derate factor (~0.8). A 5 kW array at 5 sun hours and 80% derate makes about 20 kWh per day, or roughly 7,300 kWh per year.

What are peak sun hours?+

Peak sun hours are the equivalent number of hours per day that solar irradiance averages 1,000 W/m². A location with 5 peak sun hours receives the same daily energy as five hours of perfect midday sun, spread across the whole day.

Why apply a derate factor?+

Panels never deliver their full rated output in service. Inverter losses, wiring resistance, dust, shading, panel mismatch, and high temperatures all reduce production. A derate of about 80% (or PVWatts’ ~86%) converts nameplate watts into realistic energy.

What is kWh per kW per year?+

It is the annual energy a system produces for each kilowatt of panels — a normalized yield that lets you compare locations. Sunny areas might see 1,400–1,600 kWh/kW/year, cloudier ones 1,000–1,200. This tool reports it from your inputs.

How accurate is this estimate?+

It is a good first approximation using averages. It does not capture seasonal swings, your exact roof tilt and orientation, or local weather. For a precise, location-specific projection, run your design through NREL’s free PVWatts calculator.

How do I size an array to my usage?+

Divide your annual electricity use (kWh, from your bills) by the expected yield per kW (kWh/kW/year) for your area. If you use 9,000 kWh a year and expect 1,500 kWh/kW, you would need about a 6 kW array.