TryBuildCalcSolar Panel Size Calculator (Array Size, Panel Count & Roof Area)
Calculate solar panel size instantly.
🕒 Last updated: August 28, 2026
Inputs
ℹ️From your utility bill — average several recent months for a representative figure.
ℹ️Your location's average daily peak sun hours — varies significantly by climate. See the reference table below for typical ranges.
ℹ️100% covers your current usage. Use a higher target for a known future load addition, or lower for a partial/budget system.
ℹ️Combined real-world loss factor — inverter, wiring, soiling, mismatch. 77% is a standard estimate; use a lower figure for a shadier or less-than-ideal installation.
ℹ️The specific panel model's rated wattage — common residential panels range roughly 300-550W.
ℹ️Used to estimate roof area needed — check your specific panel's datasheet, typically 15-23% for common panels.
Cost
Recommended Solar Array: 8.00 kW (20 panels)
Covers 104% of your 29.6 kWh/day usage
Sizing Calculation
Daily energy usage: 29.6 kWh (from 900 kWh/month)
Target offset: 100% of usage
Required array size: 7.68 kW
Rounded up to: 20 × 400W panels = 8.00 kW
Estimated Production
Daily production: 30.8 kWh
Annual production: 11,250 kWh
Actual offset achieved: 104% of your usage
Roof Area Needed
40.0 m² (431 sq ft)
Based on 20% panel efficiency — confirm against your specific panel model's actual dimensions.
Assumptions Used
Uses a single system-efficiency factor (default 77%) covering inverter losses, wiring, soiling, and mismatch combined — not a detailed shading/tilt/orientation model. Peak sun hours must reflect your specific location. This is a reference estimate — confirm with a licensed solar installer before purchase.
Sizing a battery bank to store this array's energy? UPS Battery Capacity Calculator →
Checking your panel/service has capacity for this array? Panel Size Calculator →
What Is a Solar Panel Size Calculator?
A solar panel size calculator estimates the solar array size (kW), number of panels, and roof area needed to offset a target percentage of your electricity usage — using your average monthly usage, your location's peak sun hours, a real-world system efficiency factor, and the specific panel wattage you're considering.
This calculator sizes the solar array only — for battery/backup storage sizing, use the UPS / Inverter Battery Capacity Calculator; for checking your electrical service has capacity for the array, use the Electrical Load / Panel Size Calculator.
Why getting array size right matters:
- An undersized array leaves you buying more grid electricity than planned, undermining the system's value
- An oversized array costs more upfront than necessary, especially where excess export isn't compensated well
- Peak sun hours vary hugely by location — using a generic average instead of your specific location's figure is the single biggest source of a wrong estimate
- Roof area is a hard physical constraint — knowing it early avoids designing a system your roof can't actually fit
Solar Panel Size Formula: How Is It Calculated?
The standard formula converts your energy need into a required array capacity, accounting for real-world system losses.
Step 1 — Daily Energy Target
Daily Usage (kWh) = Monthly Usage ÷ 30.4375
Target Daily Energy = Daily Usage × Offset Target %
Converts your monthly bill figure to a daily target, scaled by how much of it you want the solar array to cover.
Step 2 — Required Array Size
Required Array (kW) = Target Daily Energy ÷ (Peak Sun Hours × System Efficiency)
Dividing by peak sun hours and system efficiency accounts for both your location's solar resource and real-world losses between the panel's rated output and usable electricity.
Step 3 — Panel Count and Roof Area
Panel Count = ROUND UP(Required Array × 1,000 ÷ Panel Wattage)
Roof Area = Panel Count × (Panel Wattage ÷ (Panel Efficiency × 1,000))
Panel count always rounds up to a whole panel; roof area is derived from panel efficiency rather than a fixed lookup, so it scales correctly for any wattage/efficiency combination.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Monthly Usage | 900 kWh | Sets the daily energy target |
| Peak Sun Hours | 5.0 hrs/day | Sets required array size for your location |
| Panel | 400W, 20% efficient | Sets panel count and roof area |
Step 1 — Daily Energy Target
| Step | Calculation | Result |
|---|---|---|
| Daily usage | 900 ÷ 30.4375 | 29.6 kWh |
| Target daily energy | 29.6 × 100% | 29.6 kWh |
Step 2 — Required Array Size
| Required array | 29.6 ÷ (5.0 × 77%) | 7.68 kW |
Step 3 — Panel Count and Roof Area
| Panel count | ROUND UP(7.68 × 1000 ÷ 400) | 20 panels |
| Actual array size | 20 × 400W | 8.00 kW |
| Roof area | 20 × (400 ÷ (20% × 1000)) | 40.0 m² (431 sq ft) |
Therefore, to cover 100% of your 29.6 kWh/day usage at 5.0 peak sun hours, you need 20 × 400W panels (8.00 kW), needing about 431 sq ft of roof area.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Energy Usage Input Accuracy+-
- Actual average monthly usage confirmed from utility bills, not estimated
✓Peak Sun Hours & Location Accuracy+-
- Peak sun hours sourced for your specific location, not a generic or national average
- Peak sun hours figure reflects an annual average, not a single best-case summer day
✓System Sizing & Panel Selection+-
- Panel wattage matches the actual product being considered, not a placeholder value
✓Roof, Installation & Code Compliance+-
- Actual usable, unobstructed roof area confirmed sufficient for the calculated panel count — not just the calculated area in the abstract
- Roof structural capacity for panel weight, wind load, and snow load confirmed by a qualified professional
- Local permitting, utility interconnection, and net-metering rules checked separately
- Licensed solar installer site assessment obtained before purchase or installation
Full QC Checklist+−
Verification checklist for solar array sizing — covering energy usage input accuracy, peak sun hours/location accuracy, system sizing/panel selection, and roof/installation/code compliance. Use the Essential Checklist for critical checks before purchase; expand to Full QC Checklist for complete verification.
✓Energy Usage Input Accuracy+-
- Actual average monthly usage confirmed from utility bills, not estimated
- A representative average month used, not an unusually high or low outlier month
- Reasonably foreseeable future loads (EV charger, pool, home addition) considered before finalizing the offset target
- Seasonal usage variation considered, not assumed flat across the year
- Usage figure confirmed to match the specific property/meter the system is being sized for
✓Peak Sun Hours & Location Accuracy+-
- Peak sun hours sourced for your specific location, not a generic or national average
- Peak sun hours figure reflects an annual average, not a single best-case summer day
- Shading from trees, buildings, or other obstructions considered separately from the system efficiency factor
- Roof orientation and tilt confirmed reasonably close to optimal, or a lower efficiency factor used if not
- Local climate/weather pattern (persistent cloud cover, monsoon season, heavy haze) reflected in the peak sun hours figure used
✓System Sizing & Panel Selection+-
- System efficiency/derate factor confirmed appropriate for your installation, not left at the generic default for an unusual setup
- Panel wattage matches the actual product being considered, not a placeholder value
- Panel efficiency used for the roof area calculation matches the actual panel technology's spec sheet
- Solar offset target genuinely matches your actual goal — full offset, partial offset, or oversized for future growth
- Understood that panel count rounds up to a whole number, so the actual array typically exceeds the bare minimum requirement
✓Roof, Installation & Code Compliance+-
- Actual usable, unobstructed roof area confirmed sufficient for the calculated panel count — not just the calculated area in the abstract
- Roof structural capacity for panel weight, wind load, and snow load confirmed by a qualified professional
- Local permitting, utility interconnection, and net-metering rules checked separately
- Electrical integration (inverter capacity, service panel capacity) cross-checked separately
- Battery/backup storage, if desired, sized separately — not modeled by this calculator
- Licensed solar installer site assessment obtained before purchase or installation
Typical Peak Sun Hours by Climate
General reference ranges by broad climate type — always look up your specific location's actual solar resource data for a real figure, since it varies significantly even within one climate category.
| Climate Type | Typical Peak Sun Hours/Day |
|---|---|
| Desert / Arid (high sun, low cloud cover) | 6.0-7.5 |
| Tropical / Subtropical (sunny with a wet season) | 4.5-6.0 |
| Temperate / Mid-latitude (moderate seasonal variation) | 3.5-5.0 |
| Northern / Frequently Overcast (high latitude or persistent cloud) | 2.5-4.0 |
Panel Technology Reference
Typical wattage and efficiency ranges by common panel technology.
| Panel Technology | Typical Wattage | Typical Efficiency |
|---|---|---|
| Monocrystalline (most common residential) | 350-550 W | 19-23% |
| Polycrystalline | 300-400 W | 15-18% |
| Thin-Film (flexible/lightweight applications) | 100-200 W | 10-13% |
System Efficiency Factor Breakdown
What the default 77% system efficiency estimate is made of — adjust individual factors if you have more specific data.
| Loss Source | Typical Loss |
|---|---|
| Inverter conversion | ~3% |
| Wiring resistance | ~2% |
| Soiling/dust on panels | ~2% |
| Module mismatch/tolerance | ~2% |
| Other (temperature, degradation, downtime) | remaining balance |
When should you use this solar panel size calculator?
- Getting an early planning estimate of array size, panel count, and roof area before contacting an installer.
- Comparing how different panel wattages or efficiencies change your panel count and roof area needs.
- Checking whether your roof likely has enough space for your target offset percentage.
- Sizing a system with headroom for a known future load addition, like an EV charger.
- Sanity-checking an installer's proposed system size against an independent estimate.
Quick Solar Sizing Tips
- Average multiple months of actual utility bills for your usage figure, not a single month.
- Look up your specific location's peak sun hours rather than assuming a generic average — it's the biggest swing factor in the whole calculation.
- If you're planning a future EV or major appliance addition, size above 100% offset now rather than expanding the system later.
- Compare a couple of panel wattages to see the roof-area trade-off before committing to one product.
- Use a more conservative system efficiency figure if your roof has any real shading or a less-than-ideal orientation.
- Cross-check your electrical panel's available capacity for solar interconnection using the Panel Size Calculator.
- Get at least one professional site assessment before finalizing a system size — this calculator is a planning estimate, not a substitute.
Common Mistakes
- Using a generic national/global average peak sun hours instead of your specific location's actual figure.
- Sizing from a single unusually high or low usage month instead of a representative average.
- Ignoring shading, roof orientation, and tilt, then being surprised when real-world production falls short of the estimate.
- Assuming the calculated roof area is exactly what's usable, without accounting for obstructions and setbacks.
- Skipping a structural roof assessment for an older roof or a region with heavy snow load.
- Forgetting to plan for a known future load (like an EV) and having to expand the system later at extra cost.
- Treating this calculator's estimate as a final design instead of a starting point for a professional site assessment.
Limitations
- Uses one combined system efficiency factor — does not individually model shading, tilt, orientation, or temperature losses.
- Does not include battery/backup storage sizing — use the UPS / Inverter Battery Capacity Calculator separately.
- Does not model local permitting, utility interconnection, or net-metering/export compensation rules, which vary by location and change over time.
- Does not perform a structural roof assessment for panel weight, wind, or snow load.
- Peak sun hours must be sourced by you for your specific location — this calculator does not look them up automatically.
- This is a planning reference estimate, not a substitute for a licensed solar installer's site assessment.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.