Example Data Table
| Scenario | Energy (kWh/day) | PSH | Losses | Panel (W) | Voltage | Panels | Array (kW) |
|---|---|---|---|---|---|---|---|
| Home backup | 12 | 5.0 | 20% | 550 | 48V | 6 | 3.30 |
| Small shop | 20 | 4.5 | 22% | 450 | 48V | 12 | 5.40 |
| Farm pump | 35 | 5.5 | 18% | 550 | 72V | 14 | 7.70 |
Formula Used
This calculator sizes an array from energy needs, sunlight, and derating.
How to Use This Calculator
- Enter daily or monthly energy from your bills or meter.
- Set peak sun hours using local solar resource data.
- Keep losses realistic; 15–25% is common.
- Choose panel wattage and system voltage from your design.
- Enable battery backup if you need nighttime autonomy.
- Press Calculate to view results above the form.
- Download CSV or PDF to share with suppliers or installers.
Inputs That Drive Panel Quantity
Your panel count is primarily driven by daily energy use, local peak sun hours, and total system losses. This tool converts monthly bills into kWh/day, then adjusts for inverter and battery efficiencies when those components are selected. Adding an oversize margin helps cover cloudy weeks, seasonal dips, and panel aging. For quick checks, keep losses between 15–25% and start with a 5–15% oversize factor.
Understanding Peak Sun Hours and Seasonal Risk
Peak sun hours represent the equivalent hours of full sunlight per day, not clock hours. A site with 4.0 PSH will need a larger array than a site with 5.5 PSH for the same load. If your location has strong summer sun but weak winter sun, size closer to the worst month or increase oversize. When you plan for net‑metering limits, reduce the offset factor to reflect export restrictions.
Losses, Derating, and Real-World Output
Derating compresses real conditions into one factor: 1 − losses%. Losses typically include temperature rise, dust, shading, module mismatch, wiring, and conversion inefficiencies. A small shade strip can reduce production far more than expected, so treat partial shading as a design issue, not just a percentage. Cleaning schedules, correct tilt, and short cable runs can reduce losses and improve delivered energy.
Roof Area, Electrical Current, and Safety Margins
Roof space is a hard constraint. Multiply panel area by count, then add extra clearance for walkways, tilt frames, and wind loading. The current estimate is array watts divided by system voltage, and the 125% figure provides a planning margin for conductors and protection devices. If current becomes high, moving from 24V to 48V often improves efficiency and reduces cable size.
Cost, Savings, and Payback Interpretation
Economic results are directional, not contractual. Total cost combines panels, inverter, optional batteries, and other balance‑of‑system items. Annual savings depend on delivered energy, your tariff, and the offset factor that accounts for self‑use, curtailment, and downtime. A long payback can still be acceptable when reliability matters. Compare scenarios by changing panel price, battery choice, and electricity rate. Document your assumptions so installers can validate sizing on site quickly.
FAQs
1) What should I use for peak sun hours (PSH)?
Use an annual average from a trusted solar resource, then sanity‑check worst months. If winter PSH is much lower, size for the winter value or add oversize margin to avoid shortfalls.
2) Why does enabling battery backup change the sizing?
Batteries add round‑trip losses and autonomy energy, so the array must generate more PV‑side kWh to deliver the same usable kWh. Higher autonomy days, lower DoD, and lower battery efficiency all increase required capacity.
3) How do I pick a system voltage?
Choose a higher voltage when power levels rise. Higher voltage reduces current, wiring loss, and conductor size. Many medium residential systems use 48V; smaller DC systems may use 12V or 24V depending on load and equipment.
4) What is a realistic system losses percentage?
Start with 15–25% for clean, mostly unshaded arrays with good wiring practices. Use higher values if you expect heat, dust, long cable runs, or frequent shading. Improving layout and maintenance can reduce losses.
5) Does the calculator replace an installer’s design?
No. It provides planning numbers based on assumptions. Final design should confirm shading, roof structure, tilt, wiring limits, protection devices, local codes, and equipment datasheets before purchasing components.
6) How should I interpret the payback estimate?
Payback is a rough ratio of total cost to annual savings. It depends heavily on electricity price, self‑consumption, and system uptime. Use it to compare scenarios, not as a guaranteed financial outcome.