TryBuildCalcSolar System Size Calculator (kW) (Required Array Capacity in kW)
Calculate your solar system size in kW.
🕒 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: 9.60 kW (24 panels)
Covers 103% of your 39.4 kWh/day usage
Sizing Calculation
Daily energy usage: 39.4 kWh (from 1,200 kWh/month)
Target offset: 100% of usage
Required array size: 9.31 kW
Rounded up to: 24 × 400W panels = 9.60 kW
Estimated Production
Daily production: 40.7 kWh
Annual production: 14,850 kWh
Actual offset achieved: 103% of your usage
Roof Area Needed
48.0 m² (517 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 →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Solar Panel Size Calculator.
Solar system size in kW
System size in kW is the core sizing figure most quotes and comparisons are based around — this page is pre-set to a larger household usage scenario with good sun hours.
Edit the usage and peak sun hours above to match your own home and location.
- System size (kW) is your array's total DC capacity at standard test conditions, not its real-world average output.
- Higher peak sun hours (sunnier climates) need a smaller kW system for the same usage offset.
- Compare this figure against installer quotes, which are typically also expressed in kW.
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 | 1,200 kWh | Sets the daily energy target |
| Peak Sun Hours | 5.5 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 | 1,200 ÷ 30.4375 | 39.4 kWh |
| Target daily energy | 39.4 × 100% | 39.4 kWh |
Step 2 — Required Array Size
| Required array | 39.4 ÷ (5.5 × 77%) | 9.31 kW |
Step 3 — Panel Count and Roof Area
| Panel count | ROUND UP(9.31 × 1000 ÷ 400) | 24 panels |
| Actual array size | 24 × 400W | 9.60 kW |
| Roof area | 24 × (400 ÷ (20% × 1000)) | 48.0 m² (517 sq ft) |
Therefore, to cover 100% of your 39.4 kWh/day usage at 5.5 peak sun hours, you need 24 × 400W panels (9.60 kW), needing about 517 sq ft of roof area.
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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.