Calculator Inputs
Use the fields below to estimate solar generation, system size, bill reduction, roof space, and long-term value.
Example Data Table
| Scenario | Usage (kWh/mo) | Sun Hours | Offset Goal | System Size (kW) | Panels | Annual Savings |
|---|---|---|---|---|---|---|
| Small apartment | 450 | 5.2 | 80% | 2.93 | 7 | $562 |
| Average family home | 900 | 5.5 | 90% | 6.30 | 14 | $1,491 |
| Large household | 1400 | 5.0 | 95% | 10.35 | 23 | $2,485 |
| High-usage villa | 2200 | 5.8 | 100% | 13.95 | 31 | $4,070 |
Formula Used
1) Monthly usage estimate from bill: Monthly Usage = Monthly Bill ÷ Tariff per kWh.
2) Target monthly solar output: Target Solar = Monthly Usage × Target Offset.
3) Required system size: System kW = Target Monthly Solar ÷ (Sun Hours × 30.4375 × Performance Ratio).
4) Panel count: Panel Count = Ceiling[(System kW × 1000) ÷ Panel Wattage].
5) Energy generation: Daily Solar = Installed kW × Sun Hours × Performance Ratio. Monthly and annual output scale from daily production.
6) Savings: Annual Savings = Minimum(Annual Solar, Annual Usage) × Tariff.
7) Payback: Payback = System Cost ÷ Annual Savings.
8) Roof space: Roof Needed = Panel Count × Panel Area.
9) Lifetime value: The tool applies annual panel degradation and tariff escalation each year to estimate long-term savings and net benefit.
How to Use This Calculator
- Choose whether you want to estimate from monthly usage or from a monthly bill.
- Enter your tariff, average sun hours, performance ratio, and target offset.
- Add panel wattage, panel footprint, roof area, and installed cost assumptions.
- Include degradation, tariff escalation, analysis years, and emissions factor for deeper long-term results.
- Click Estimate Solar System to see the result summary above the form.
- Use the CSV and PDF buttons to export your finished estimate.
FAQs
1. What does peak sun hours mean?
Peak sun hours represent average daily solar energy intensity. They convert changing sunlight into an equivalent number of full-power sunlight hours for production estimates.
2. Why is performance ratio important?
Performance ratio accounts for real system losses from heat, inverter conversion, dust, cable losses, mismatch, and shading. It makes estimates more realistic than panel wattage alone.
3. Can I estimate from my bill instead of usage?
Yes. Enter the bill amount and your tariff per kWh. The calculator converts that bill into estimated energy use, then sizes the solar system from it.
4. Why does the panel count round up?
Panels are installed as whole units. Rounding up ensures the design can actually deliver the target system size instead of falling slightly short.
5. Does this include battery sizing?
No. This tool estimates solar generation, roof space, savings, and long-term value. Battery backup needs separate load, autonomy, and depth-of-discharge calculations.
6. Why might my real savings differ?
Actual savings depend on net metering rules, time-of-use rates, seasonal weather, shading, maintenance, and whether your usage pattern matches daytime solar production.
7. Is roof area alone enough to confirm installation?
No. Roof shape, setbacks, vents, tilt, orientation, access paths, and structural constraints matter too. The area check is only an early screening estimate.
8. What is net lifetime benefit?
Net lifetime benefit equals projected lifetime bill savings minus initial system cost. It helps compare long-term financial value under your chosen degradation and tariff growth assumptions.