TryBuildCalcOff-Grid Solar Panel Calculator (Partial Offset / Off-Grid Sizing)
Size an off-grid solar array.
🕒 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: 3.50 kW (10 panels)
Covers 124% of your 9.9 kWh/day usage
Sizing Calculation
Daily energy usage: 9.9 kWh (from 300 kWh/month)
Target offset: 120% of usage
Required array size: 3.38 kW
Rounded up to: 10 × 350W panels = 3.50 kW
Estimated Production
Daily production: 12.3 kWh
Annual production: 4,474 kWh
Actual offset achieved: 124% of your usage
Roof Area Needed
18.4 m² (198 sq ft)
Based on 19% 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.
Off-grid solar array sizing
An off-grid system is typically sized above 100% offset (this page defaults to 120%) to build in a margin for cloudy days and battery charging losses, with a more conservative system efficiency assumption.
For battery sizing to store this array's energy for use after dark, use the UPS / Inverter Battery Capacity Calculator separately — this tool sizes the panels only.
- Off-grid systems commonly target 120-150% offset to build in a margin, unlike a grid-tied system that can rely on the grid for shortfalls.
- A more conservative system efficiency (below the 77% grid-tied default) is common for off-grid systems given additional battery-charging losses.
- Battery bank sizing is a separate calculation — use the UPS / Inverter Battery Capacity Calculator for that.
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 | 300 kWh | Sets the daily energy target |
| Peak Sun Hours | 5.0 hrs/day | Sets required array size for your location |
| Panel | 350W, 19% efficient | Sets panel count and roof area |
Step 1 — Daily Energy Target
| Step | Calculation | Result |
|---|---|---|
| Daily usage | 300 ÷ 30.4375 | 9.9 kWh |
| Target daily energy | 9.9 × 120% | 11.8 kWh |
Step 2 — Required Array Size
| Required array | 11.8 ÷ (5.0 × 70%) | 3.38 kW |
Step 3 — Panel Count and Roof Area
| Panel count | ROUND UP(3.38 × 1000 ÷ 350) | 10 panels |
| Actual array size | 10 × 350W | 3.50 kW |
| Roof area | 10 × (350 ÷ (19% × 1000)) | 18.4 m² (198 sq ft) |
Therefore, to cover 120% of your 9.9 kWh/day usage at 5.0 peak sun hours, you need 10 × 350W panels (3.50 kW), needing about 198 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.