Pick two panels and compare real-world energy outputs. Adjust sun hours, losses, and pricing easily. Get clearer sizing decisions for rooftops, batteries, and bills.
| Scenario | Target kW | Sun Hours | Loss% | Rate | Panel W | Panels | Daily kWh | Payback |
|---|---|---|---|---|---|---|---|---|
| Urban roof | 5.0 | 5.0 | 15 | 0.15 | 550 | 10 | 23.23 | 6.9y* |
| High-rate utility | 6.0 | 5.5 | 12 | 0.25 | 450 | 14 | 28.03 | 4.8y* |
| Lower sun | 4.0 | 3.8 | 18 | 0.14 | 400 | 10 | 12.43 | 8.7y* |
Higher watt panels reduce the number of modules needed to reach a target system size. In this calculator, panels needed equals the ceiling of target watts divided by panel wattage, so a 5.0 kW target typically needs 10×550 W panels but 12×450 W panels. Fewer panels can simplify racking, wiring, and installation time.
Daily energy is estimated as DC size × sun hours × performance ratio. Performance ratio combines system losses and inverter efficiency, so 14% losses and 97% inverter efficiency produce a 83.42% ratio. With 5.5 sun hours, a 5.5 kW DC array delivers about 25.2 kWh/day under those assumptions.
Panel price alone can mislead. The calculator separates per-panel price from balance-of-system and installation costs, then reports system cost per watt. If BOS plus installation is a fixed 4,000, a higher watt option often spreads fixed costs across more output, lowering system cost per watt and improving payback sensitivity. Tracking cost per watt helps compare quotes across brands, warranties, and efficiencies, while keeping assumptions consistent across options for a clear procurement decision.
Annual savings equals annual kWh × electricity rate × net credit. At 0.14 per kWh and 100% credit, 9,000 kWh/year offsets about 1,260 yearly. Reducing net credit to 70% cuts savings proportionally, stretching payback even if production stays constant. Use your local tariff and export credit terms.
To compare long-term value, the tool estimates lifetime energy using yearly degradation, such as 0.6% per year. Over 25 years, that small decline meaningfully reduces cumulative kWh, which affects the lifetime savings and the LCOE proxy. When two options have similar payback, higher lifetime kWh can be the tie-breaker.
What does panel wattage change most?
It mainly changes how many panels you need for the same target kW. Fewer panels can reduce roof space, wiring complexity, and some labor, while keeping output similar at equal DC size.
Why is my payback showing N/A?
Payback becomes unavailable when annual savings are zero or negative. Check electricity rate, net credit, and ensure system costs are not blank. Also confirm sun hours and losses are reasonable.
Should I trust the lifetime savings number?
Treat it as a planning estimate. It includes simple degradation but excludes maintenance, financing, incentives, and discounting. Use it to compare options consistently, then validate with your installer’s proposal.
How do losses affect the result?
Losses reduce the performance ratio applied to sun hours. Increasing losses from 14% to 20% can noticeably cut daily and annual kWh, which lowers savings and lengthens payback for every panel option.
What roof area value should I enter?
Enter the usable area for modules only, excluding setbacks, skylights, and obstructions. If you do not know it, leave it blank and compare energy and costs without the fit check.
Can I compare panels with different efficiencies?
Yes. Efficiency and area provide context for space-constrained roofs, but sizing and energy are driven by DC kW, sun hours, and losses. Use efficiency to judge how much power you get per square meter.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.