Calculator
Enter what you know. Use advanced losses to auto-calculate performance ratio, or set performance ratio directly.
Example Data Table
| Monthly Use (kWh) | Peak Sun Hours | PR | Panel (W) | Est. Size (kW DC) | Panels |
|---|---|---|---|---|---|
| 600 | 4.5 | 0.80 | 450 | 4.00 | 9 |
| 900 | 5.0 | 0.78 | 550 | 5.60 | 11 |
| 1400 | 5.5 | 0.75 | 600 | 9.30 | 16 |
| 2200 | 4.0 | 0.80 | 500 | 13.80 | 28 |
Formula Used
- Daily use = Monthly kWh ÷ 30.4 (if daily not provided)
- Target daily = Daily use × (Coverage % ÷ 100)
- Required DC kW = Target daily ÷ (Peak Sun Hours × PR)
- Reserve margin = Required DC kW × (1 + Margin %)
- Panels = ceil(Required DC kW × 1000 ÷ Panel W)
- Actual DC kW = Panels × Panel W ÷ 1000
- Annual kWh = Actual DC kW × Peak Sun Hours × PR × 365
- Inverter kW AC ≈ Actual DC kW ÷ DC/AC ratio
- Roof area = Panels × Area per panel
- Cost = Actual DC kW × 1000 × Cost per watt
How to Use This Calculator
- Enter your monthly kWh from the utility bill, or your daily kWh if known.
- Set target coverage, peak sun hours, panel wattage, and reserve margin.
- Choose a performance ratio directly, or enable auto PR and enter losses.
- Press Calculate to see system size and panel count above.
- Use Download CSV or Download PDF for reports.
Translate Usage Into DC Power
This calculator converts your average energy demand into a DC array size. It derives daily use from either daily kWh or monthly kWh divided by 30.4. Next, it applies your target coverage percentage, so a 120% goal oversizes for future load or seasonal mismatch. Required DC power equals target daily kWh divided by peak sun hours and performance ratio.
Peak Sun Hours Drive Output
Peak sun hours represent equivalent full‑sun hours per day for your location and tilt. Typical values range from about 3 to 6, but microclimates and winter shading can push lower. Because sizing scales inversely with sun hours, moving from 5.5 to 4.5 increases required capacity by roughly 22%. Use annual average PSH for base sizing, then test a conservative low value to see panel count changes.
Performance Ratio Captures Losses
Performance ratio (PR) rolls up losses such as temperature, wiring, inverter efficiency, soiling, mismatch, and shading. Many residential systems fall near 0.75–0.85. The auto‑PR option multiplies inverter efficiency by (1 − loss) terms, producing a clear loss budget you can tune. A 0.80 PR versus 0.75 PR changes DC size by about 6.7%, often one to two panels.
Panel Count, Space, And Layout
Once DC kW is known, the tool converts it into whole panels using your selected module wattage. Higher‑watt modules reduce panel count, but roof geometry, setbacks, and racking may still limit placement. Area estimates use a per‑panel footprint, commonly 1.8–2.2 m² for modern modules. Compare computed roof area to usable space and account for obstructions.
Cost, Offset, And Sensitivity Checks
Cost is estimated using installed cost per watt multiplied by total DC watts, so it scales linearly with system size. Use this with your coverage target to compare scenarios, such as 80% versus 100% offset. Suggested inverter size uses a DC/AC ratio, often 1.1–1.3, helping prevent chronic clipping. For budgeting, run several panel wattages and PR values to understand price, space, and offset tradeoffs clearly. Validate the annual kWh estimate against bills and local production tools before final design.
FAQs
Use an annual average value from a solar map, proposal, or nearby system data. If unsure, run a conservative case (for example 4–5) and compare the panel count and annual energy.
For many rooftop systems, 0.75–0.85 is a practical planning range. Choose the lower end for hotter climates, dust, and shading. Use auto‑PR when you can estimate losses.
The inverter estimate applies your chosen DC/AC ratio. A higher ratio suggests a smaller inverter and may increase clipping on perfect days, while a lower ratio suggests a larger inverter and higher cost.
No. It focuses on solar array sizing and panel count. If you plan storage, size the array for both daytime loads and battery charging, and check backup runtime separately.
Tilt and azimuth mainly change peak sun hours. Use a PSH value that already reflects your roof orientation, or reduce PSH slightly for less‑than‑ideal directions to avoid under‑sizing.
Real output varies with weather, temperature, clipping, system downtime, shading, and soiling. Compare the annual kWh to utility bills and local production tools, then refine PSH and PR for your site.