Numeros
Categories Financial Calculators Health Calculators Math Calculators Date & Time Construction Engineering Physics Business Education Lifestyle Converters Generators
About

Solar Panel Size Calculator

By

Enter your daily energy usage and local sun hours to estimate the solar system size and number of panels needed.

Solar Panel Size Calculator

Live
Panels needed
19
System size needed
7.5 kW
"Peak sun hours" is not the same as daylight hours - it's the equivalent hours of full-intensity sunlight your location averages.

The formula

Required system size (W) = (Daily usage in Wh) / (Sun hours x System efficiency) Panels needed = Required system size / Panel wattage
Example

30 kWh daily usage, 5 peak sun hours, 80% system efficiency, 400W panels: required size = 30,000 / (5 x 0.80) = 7,500W, so 19 panels (rounding up from 18.75).

Step-by-step guide

  1. Find your daily energy usage - check a recent utility bill and divide the monthly kWh by roughly 30, or look for a daily average if your provider shows one.
  2. Find your area's peak sun hours - this varies significantly by location and season; solar mapping tools and installer resources publish regional averages.
  3. Leave system efficiency around 75-85% unless you have a specific reason to adjust it - this accounts for inverter losses, wiring, panel degradation, and non-ideal conditions.
  4. Read the estimated system size and panel count as a starting point for further research or an installer quote.

Why "sun hours" isn't the same as daylight hours

A location might have 14 hours of daylight but only 5-6 "peak sun hours" - the metric represents the equivalent number of hours at maximum solar intensity (1,000 W/m²) that would deliver the same total energy as the actual, varying sunlight throughout the day. Morning and evening sun, cloud cover, and season all reduce real-world output well below what raw daylight hours might suggest.

Note: This is a planning estimate. A solar installer will perform a detailed site assessment - accounting for roof orientation, shading, local weather data, and your utility's specific rate structure - before finalizing an actual system design.

Common mistakes

Using annual average sun hours without considering seasonal variation - a system sized for the yearly average may underperform significantly in winter months in many climates.
Ignoring roof shading or orientation - a roof section with partial shade or a poor sun-facing angle produces meaningfully less energy than this idealized calculation assumes.

Frequently asked questions

Where do I find my daily energy usage?

Your utility bill typically shows monthly kWh usage - divide by the number of days in that billing period for a daily average, ideally using a few months to account for seasonal variation.

Where do I find peak sun hours for my location?

Government solar resource maps and most solar installer websites publish regional peak sun hour averages, often broken down by month for more precise seasonal planning.

Why does system efficiency matter so much?

Real solar systems never convert 100% of theoretical sunlight into usable electricity - the inverter, wiring, panel temperature effects, and dust or dirt on panels all reduce actual output below the panels' rated capacity.

Related calculators

See the full list of Engineering calculators, or try: