Size your solar system. This free solar panel calculator turns your electricity use and peak sun hours into system kW, the number of panels, daily generation, roof area and an optional cost.
Enter the electricity you want to cover and your local peak sun hours (roughly 3–4 in cloudier regions, 5–6 in sunny ones). Performance ratio (~75–85%) accounts for real-world losses.
System size (kW) = annual kWh ÷ (peak sun hours × 365 × performance ratio). Panel count = system watts ÷ panel wattage, rounded up. Roof area assumes about 1.9 m² per panel. These are planning estimates — a local installer will refine for roof, shading and orientation.
How the Solar Panel Calculator Works
This tool sizes a rooftop solar system from your electricity usage and local sun exposure, converting the result into a panel count, expected daily generation, approximate roof area, and an optional installed cost.
Formula: System size (kW) = Annual kWh ÷ (Peak Sun Hours × 365 × Performance Ratio). Panels needed = ROUND UP (System size × 1000 ÷ Panel wattage). Daily generation (kWh) = System size × Peak Sun Hours × Performance Ratio. Roof area ≈ Panels × 1.9 m². Estimated cost = System size × 1000 × Cost per Watt.
- Electricity to cover — your target usage, entered per year or per month
- Peak sun hours / day — average daily hours of full-intensity sunlight for your location (roughly 3-4 in cloudier regions, 5-6 in sunny ones)
- Performance ratio — accounts for real-world losses from inverters, wiring, temperature, and soiling (default 80%)
- Panel wattage — the rated output of a single panel, in watts (default 400W)
- Install cost (optional) — an estimated dollar cost per watt to translate system size into a rough budget
Example Scenarios
| Annual Usage (kWh/yr) | Sun Hours/day | Performance Ratio | Panel Wattage | System Size (Panels) |
|---|---|---|---|---|
| 10,000 | 4.5 | 80% | 400 W | 7.61 kW (20 panels) |
| 6,000 | 5.0 | 75% | 350 W | 4.38 kW (13 panels) |
| 6,000 (500/mo) | 4.0 | 80% | 400 W | 5.14 kW (13 panels) |
| 15,000 | 6.0 | 85% | 450 W | 8.06 kW (18 panels) |
| 8,000 | 3.5 | 80% | 400 W | 7.83 kW (20 panels) |
Better Inputs for the Solar Panel Calculator
A solar panel calculator gives a first-pass system size. Its accuracy depends on two numbers: your usage and your sun hours.
- Use a full year of bills. Summer and winter usage can differ widely, so average 12 months of kWh rather than one bill.
- Get local peak sun hours from a solar resource map, not a national average.
- Plan for change. An electric car, heat pump or new appliance raises usage, so add it before running the solar panel calculator.
- Check the roof. Shade, orientation and tilt cut output, and a solar panel calculator assumes an unshaded roof.
Treat the cost figure as a rough guide. Installers add wiring, inverters, permits and labour, and incentives vary by country and year, so get written quotes. Compare quotes against the solar panel calculator size to spot oversized systems.
Tilt, orientation, shading and inverter losses all shift real-world output, so treat the solar panel calculator as a sizing starting point and read up on how photovoltaic systems are designed before requesting installer quotes.
Solar Panel Calculator FAQ
What are “peak sun hours” and how do I find mine?
Peak sun hours are the equivalent number of hours per day the sun delivers full-intensity solar radiation (1000 W/m²), not the same as daylight hours. It varies by latitude and climate — sunnier regions average 5-6 hours, cloudier ones 3-4 — and is typically looked up from a solar irradiance map for your area.Why is my calculated system size different from what a roof can physically hold?
This calculator sizes the system purely from your electricity usage and sun hours — it doesn’t know your roof’s usable area, pitch, or shading. A proper site assessment may reduce the achievable system size below what this estimate suggests, or confirm it fits comfortably.What performance ratio should I use?
80% is a reasonable default covering typical losses from inverter efficiency, wiring, temperature derating, and dust. Systems in hotter climates or with older equipment may perform closer to 75%, while well-maintained modern systems can reach 85%.How many panels will actually fit on my roof?
The roof area estimate here assumes about 1.9 m² per panel with no gaps for spacing, obstructions, or setbacks, so it’s a rough floor, not a guarantee of fit. An installer measuring your actual roof plane will give the real usable panel count.Does this calculator account for battery storage or net metering?
No — it estimates the solar array size needed to generate a given amount of energy, independent of how that energy is stored or credited by your utility. Battery sizing and net metering economics require separate calculations based on your local rate structure.How accurate is the estimated installation cost?
The cost figure is a simple multiplication of system size by your entered cost-per-watt, so it’s only as accurate as that input. Get quotes from local installers for a real number, since pricing varies by region, equipment brand, and installation complexity.Related Calculators
Before sizing a system, check the electricity cost calculator to pin down your actual usage and spend. If you’re also budgeting the roof work itself, the roofing cost calculator and square footage calculator can help estimate available roof space and project cost. Remember this is a planning estimate only — a local installer’s site assessment is needed for a real quote.