Solar Panel Calculator

About the Solar Panel Calculator

This solar panel calculator estimates the photovoltaic capacity required to offset daily energy use, the number of panels, battery storage for the selected backup period, and a preliminary system cost. It is useful for early planning and comparing installer proposals.

Formula and assumptions

Required solar capacity = daily kWh ÷ (peak sun hours × efficiency). Convert kW to watts, then divide by panel wattage and round up. Battery capacity = daily kWh × backup days. Estimated solar cost = system watts × price per watt + installation and permits.

Worked example

For 30 kWh per day, 4.5 peak sun hours, and 80% efficiency, the required system is 8.33 kW. With 400 W panels, round up to 21 panels. One day of battery backup is about 30 kWh. At $1.20 per watt plus $1,500 installation and permits, the preliminary total is about $11,501.

Measurement checklist

• Use a full year of utility bills when possible.
• Check roof orientation, pitch, shade, and usable area.
• Confirm local solar resource and weather conditions.
• Include inverter, racking, wiring, batteries, permits, and labor in the final quote.
• Ask the installer about interconnection and utility export rules.

Manual calculation steps

1. Find average daily electricity consumption.
2. Multiply peak sun hours by expected efficiency.
3. Divide daily energy by that usable production factor.
4. Convert capacity to watts and divide by panel wattage.
5. Round panel count upward and check roof space.
6. Add batteries, equipment, labor, permits, and incentives separately.

Common mistakes

• Using daylight hours instead of peak sun hours.
• Ignoring seasonal production and shade.
• Rounding panel count down.
• Assuming battery capacity equals usable delivered capacity.

Frequently asked questions