Inputs
Provide your best estimates. Use roof area to validate space.
Example Data
| Daily Load (kWh) | Sun Hours | Panel (W) | Coverage | PV Size (kW) | Panels | Roof (m²) |
|---|---|---|---|---|---|---|
| 20 | 5.0 | 550 | 100% | 4.7 | 9 | 23.4 |
| 30 | 5.5 | 550 | 100% | 6.0 | 11 | 28.6 |
| 45 | 4.8 | 450 | 80% | 8.9 | 20 | 36.0 |
Formula Used
1) Daily target offset
TargetDaily = DailyLoad × (Coverage% / 100)
2) Performance ratio
PR = (1 − Losses%/100) × (Orientation%/100) × (InverterEff%/100)
3) Required PV size (DC)
PV(kW) = TargetDaily ÷ (PeakSunHours × PR)
4) Panel count and roof area
Panels = ceil(PV(kW) × 1000 ÷ PanelW)
RoofArea = Panels × PanelArea
5) Battery (optional)
UsableEnergy = DailyLoad × Critical% × (AutonomyHours/24)
NominalBattery = UsableEnergy ÷ (DoD × Efficiency)
How to Use
- Enter daily kWh, or enter monthly kWh and leave daily blank.
- Set peak sun hours for your project location.
- Adjust losses, orientation factor, and inverter efficiency.
- Choose a panel wattage and panel area that matches your module.
- Add usable roof area to check if the system fits.
- Fill tariff and cost to estimate savings and payback.
- Enable battery sizing for backup autonomy planning.
- Press Calculate, then export CSV or PDF if needed.
Energy Baseline and Coverage Target
The calculator converts monthly use to an average daily load using 30.4 days, then applies your coverage target. For example, 900 kWh/month becomes 29.6 kWh/day; at 80% coverage, the target offset is 23.7 kWh/day. If you know weekday patterns, size for the average and confirm peak daytime loads are within inverter limits. This approach supports quick early-stage planning.
Solar Resource and Derate Assumptions
Production depends on peak sun hours and a performance ratio (PR). PR combines losses, orientation factor, and inverter efficiency. If losses are 14%, orientation is 95%, and inverter efficiency is 96%, PR equals 0.783. Peak sun hours should reflect the project location; use a conservative annual average first, then refine with monthly values. Document soiling, shading, and temperature impacts.
Array, Inverter, and Roof Planning
Required PV power is TargetDaily ÷ (PSH × PR). Panel count is rounded up to whole modules and roof area is estimated from module footprint. Enter usable roof area to flag fit risk early, and consider adding 10–15% extra space for walkways, setbacks, and maintenance access. Many rooftop arrays add roughly 12–20 kg/m² dead load, so coordinate structural checks and waterproofing details.
Financial and Carbon Outputs
Annual savings are estimated from self-consumed energy multiplied by tariff; exported energy is not assumed unless consumption is lower than production. Installed cost uses a cost-per-kW input for quick budgeting, and you can add contingency for permitting, scaffolding, and site logistics. For planning, consider module degradation near 0.5%/year. CO₂ avoided uses the grid emission factor for reporting. Include meter upgrade costs where net metering is planned.
Battery Backup Scenarios
When battery sizing is enabled, the tool sizes usable energy for critical load over the selected autonomy window. Nominal battery capacity accounts for depth of discharge and efficiency. Add margin for inverter standby losses and future load growth, and confirm battery power rating (kW) can handle motor starts. Use this for preliminaries, then validate protection settings, earthing, and local code requirements during design.
FAQs
1) What peak sun hours should I use?
Start with a conservative annual average for your location (often 4–6 h/day), then refine with monthly values if winter performance or seasonal loads are important.
2) Why does the roof check say it may exceed usable area?
The estimate uses module footprints only. Real layouts need setbacks, walkways, spacing, and clearance around obstructions. Reserve 10–15% extra area and confirm the actual usable roof zone.
3) Can I size above 100% coverage?
Yes, up to 150% to model export or future load growth. If export is limited or not compensated, oversizing may reduce effective savings because extra production is curtailed or under-valued.
4) What does DC/AC ratio mean?
It is PV DC nameplate divided by inverter AC rating. Higher ratios improve shoulder-hour production but can clip at midday. Many commercial rooftop systems use about 1.1–1.4 depending on constraints.
5) Is the payback result accurate for procurement?
It is a screening estimate based on tariff and self-consumption. It excludes financing, O&M, degradation, demand charges, incentives, and export credits. Use it to compare options, then validate with a full model.
6) How is battery size calculated?
The tool sizes usable energy for the chosen critical-load share and autonomy hours, then converts to nominal capacity using depth of discharge and battery efficiency. Verify battery kW rating and surge currents separately.