Solar Panel Size Calculator
Start From What You Need, Not From What a Salesperson Suggests
Rather than guessing at a system size, the more reliable starting point is the household's actual daily electricity consumption. Working backward from that number — through the site's peak sun hours and the system's expected efficiency — produces a system size in kilowatts and a panel count that's grounded in real usage rather than a rule of thumb.
The Formula
Panels Needed = System Size (W) / Panel Wattage, rounded up
Where This Is Useful
- Pre-quote planning — walk into a conversation with an installer already knowing roughly what system size and panel count the home needs.
- Roof space feasibility — a panel count estimate makes it easy to check early whether available roof area can physically fit the system.
- Budgeting — system size in kilowatts is the number installers use to generate a cost estimate, so having your own figure in hand helps sanity-check a quote.
Worked Example
A household using 20 kWh per day, with 5 peak sun hours available and 80% system efficiency, using standard 400-watt panels:
| Daily energy needed | Sun hours/day | System efficiency | System size | Panels needed (400W) |
|---|---|---|---|---|
| 20 kWh | 5 h | 80% | 5.00 kW | 13 panels |
Panel count is always rounded up, since a fractional panel isn't installable — the resulting system will slightly exceed the calculated minimum size.
How to Use This Calculator
- Enter the Daily Energy Needed in kWh, from a utility bill or usage estimate.
- Enter the Peak Sun Hours per Day for the site.
- Optionally enter Panel Wattage — defaults to 400 W if left blank.
- Optionally enter System Efficiency % — defaults to 80% if left blank.
- Select Calculate to get the required system size and panel count.
Related Calculations
Confirm the resulting system's real output with the Solar Panel Output Calculator, or estimate the return on the investment with the Solar Payback Period Calculator.