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Calculation Machine

Solar Panel Calculator

System size, panel count and payback period.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
System size needed
8.21 kW
Number of panels
21
Estimated installed cost
$22996
Annual bill offset
$1728
Simple payback
13.3 years

Quick answer

System size in kW equals daily kWh use divided by peak sun hours divided by system efficiency, usually around 0.8.

The formula

kW = daily kWh ÷ sun hours ÷ 0.8

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Monthly electricity use900 kWh
  • Peak sun hours per day4.5 hrs
  • Panel rating400 W
  • Installed cost per watt2.80 $
  • Electricity rate0.16 $/kWh

Result

  • System size needed8.21 kW
  • Number of panels21
  • Estimated installed cost$22996
  • Annual bill offset$1728
  • Simple payback13.3 years

How to use the Solar Panel Calculator

  1. 1Take monthly kWh from your utility bill.
  2. 2Look up peak sun hours for your area.
  3. 3Read the system size and payback period.

Frequently asked questions

What are peak sun hours?

The equivalent hours of full 1,000 W/m² sunlight per day — typically 3.5 in the northern US and 5.5–6 in the southwest.

Does payback include incentives?

No. The 30% US federal tax credit and state rebates typically cut the payback period by around a third.

About this calculator