PowerCalcLab

Solar calculator

Solar Panel Size Calculator

Turn a daily energy target and local peak-sun resource into array watts, then round up to whole panels without hiding system losses.

Advanced assumptions

Results update as you type. The calculator runs entirely in your browser — nothing you enter is sent anywhere.

Required solar array1,597 W

Calculation breakdown

Whole panels4 × 450 W
Installed array1,800 W
Estimated daily energy6,480 Wh/day
Array W = daily Wh × margin ÷ peak sun hours ÷ efficiency
= 5000 × 1.15 ÷ 4.5 ÷ 0.8
= 1,597 W
  • Peak sun hours must come from a location- and season-appropriate solar resource; they are not daylight hours.

Method

How the number is reached

Peak sun hours convert variable irradiance into equivalent hours at rated array power. They must be chosen for the location, tilt and design season.

System efficiency combines real losses not already included in the load target. NREL PVWatts uses explicit system-loss inputs because weather, temperature, wiring, soiling and mismatch matter.

Array W = daily Wh × (1 + margin) ÷ peak sun hours ÷ efficiency

Symbols

PSH
Equivalent full-sun hours per day

Worked examples

The same maths, applied

Example

5 kWh/day with 4.5 peak sun hours

An 80% efficient system with 15% design margin uses 450 W modules.

Array = 5,000 × 1.15 ÷ 4.5 ÷ 0.80 = 1,597 W
Panels = ceil(1,597 ÷ 450) = 4

Result: 1,597 W required; install four 450 W panels (1,800 W).

Limits of the model

What it assumes, and where it stops

Assumptions

Peak sun hours represent the design season and array plane.

Not covered

This is an energy-balance estimate, not a site-specific irradiance simulation.

Sources

Where these figures come from

  • Modeling Resources for Photovoltaic System OwnersU.S. Department of EnergyGovernment guidance explaining that PV energy models combine site meteorological inputs with system specifications, orientation, topology and shading conditions.
  • How to Size a Solar System for Your RVVictron EnergyManufacturer sizing walkthrough connecting daily energy, days of autonomy, system voltage and chemistry-dependent usable capacity, with an explicit reminder to include inverter losses.

Next steps

Related calculators

  • Solar Charge Controller Size Calculator

    A controller must pass two different limits: battery-side charging current and the PV array's highest possible open-circuit voltage. This tool checks both.

  • Solar Battery Charging Time Calculator

    Estimate how many design-solar days an array needs to move a battery from its starting SOC to the target, with array and charging losses shown separately.

  • Solar Battery Bank Calculator

    Calculate the nominal kWh and amp-hours needed to carry your loads through the chosen number of low-solar days, with every derating visible.

All solar calculators →

Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.