Solar Panel Calculator
Estimate solar-panel count, DC array size, annual energy, roof area and equipment cost from daily electricity use and peak sun hours.
Default project preview
18 panels
Required array
7.32 kW
Roof area
387 ft²
Annual energy
11,583 kWh
Daily output
31.7 kWh
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Required DC watts = daily kWh × 1,000 ÷ (peak sun hours × usable-system factor). Panel count = round up (required watts ÷ panel watts).
FAQ
Solar Panel Calculator questions
What are peak sun hours?+
Peak sun hours express daily solar energy as equivalent hours at 1,000 watts per square metre. They are not the same as daylight hours.
Why include system losses?+
Wiring, inverter conversion, temperature, dirt, mismatch and other effects reduce delivered energy.
Does this replace a solar proposal?+
No. Final production requires site-specific shade, orientation, equipment and utility modelling.
Project guide
Use this calculator with confidence
Last reviewed: July 19, 2026
The solar-panel calculator estimates array power, panel count, roof area and annual energy from electricity use, solar resource and system-loss assumptions. It is designed for early feasibility, not system design. Shading, orientation, local weather, inverter limits, structural capacity, utility rules and hourly load shape all affect a real installation, so compare the estimate with an established solar model and qualified site assessment.
How to use it
- 1Enter annual electricity use from complete utility bills.
- 2Use a location-appropriate solar resource value.
- 3Enter the selected panel rating and realistic system losses.
- 4Check roof area, orientation and shading before treating the panel count as feasible.
Worked example
A household using 10,000 kWh per year with 1,500 equivalent full-sun hours and 15% system losses needs roughly 7.84 kW DC before rounding to whole panels. A 400 W module would make the planning count about 20 panels.
What the defaults mean
Panel wattage
Replace with the selected module nameplate rating.
System losses
Represents wiring, conversion, temperature, mismatch and other effects.
Solar resource
Must be location- and orientation-specific.
| Input | What it represents | Best source |
|---|---|---|
| Annual use | site electricity demand | 12 months of bills |
| Solar resource | available energy | location model |
| Panel rating | module DC power | product data sheet |
| System losses | real-world reductions | design/model assumptions |
Common measurement mistakes
- Using a single month of electricity as annual demand.
- Treating daylight hours as equivalent full-sun hours.
- Ignoring shading and roof obstructions.
- Assuming annual production equals self-consumed energy.
Limits and safety
- Not a structural, electrical or utility-interconnection design.
- Annual estimates do not show hourly production or storage behavior.
- Use a site-specific model and qualified installer for final design.
Continue the project
calculate solar array roof area — Check module footprint and spacing.
compare solar-panel tilt — Review orientation geometry.
estimate solar battery capacity — Model storage separately from annual production.
estimate solar self-consumption — Compare production with on-site use.
estimate solar payback — Add costs, savings and escalation assumptions.