Solar Panel Calculator

Solar photovoltaic (PV) panels convert sunlight into electricity, reducing reliance on grid power and lowering electricity bills. As panel costs have fallen by more than 90% over the past decade, rooftop solar has become one of the most accessible forms of renewable energy for homeowners and businesses alike. Understanding how to estimate a system's output and financial returns is essential before making an installation decision.

How Solar Panel Output Is Calculated

The fundamental formula for daily energy production is: Daily kWh = System Size (kW) × Peak Sun Hours × System Efficiency Factor. System size is the total installed capacity in kilowatts. Peak sun hours represent the number of hours per day during which sunlight intensity averages 1,000 watts per square meter — a standard measure of solar irradiance. The system efficiency factor (commonly 0.75–0.85) accounts for real-world losses including inverter conversion losses, wiring resistance, dust on panels, shading, and temperature-related derating.

For example, a 6 kW system in a location with 4.5 peak sun hours using an 80% efficiency factor would produce approximately 6 × 4.5 × 0.80 = 21.6 kWh per day, or roughly 7,884 kWh per year. Multiplying by the local electricity rate gives the annual savings estimate.

Peak Sun Hours and Why They Matter

Peak sun hours vary significantly by location and season. Regions near the equator or with dry, clear climates — such as the southwestern United States, the Middle East, and parts of Australia and Japan's southern islands — typically receive 5–7 peak sun hours daily. Temperate regions such as central Europe, the UK, and the northeastern United States average 3–4.5 peak sun hours. Nordic countries and parts of Canada may see fewer than 3 peak sun hours in winter.

Most solar panel manufacturers and installers use annual average figures when estimating system output. Solar resource databases such as NASA POWER, the European Commission's PVGIS, and NREL's PVWatts provide location-specific data for more precise estimates.

Panel Efficiency Degradation

Solar panels slowly lose efficiency over time due to UV exposure, thermal cycling, and material aging. The industry standard degradation rate is approximately 0.5% per year, meaning a panel producing 100% in year one will produce about 87.8% in year 25 (0.995^25 ≈ 0.878). Most tier-one manufacturers offer a 25-year linear performance warranty guaranteeing at least 80% of rated output at end of warranty period.

When estimating 25-year cumulative savings, this degradation compounds annually. A system with a 0.5%/year degradation rate will generate roughly 12–15% less total energy over its lifetime compared to a hypothetical system with no degradation. This calculator applies the degradation rate year by year when computing the 25-year total savings figure.

Simple Payback Period

The simple payback period divides total system cost by first-year annual savings. It gives a quick indication of how many years it takes to recover the upfront investment purely through electricity bill reductions. A typical residential rooftop solar installation achieves payback in 6–12 years depending on system size, local electricity rates, available incentives, and sunlight availability.

Simple payback does not account for time value of money, changing electricity prices, maintenance costs, or tax incentives. For a complete financial picture, homeowners should also consider net present value (NPV), internal rate of return (IRR), and any government rebates or feed-in tariff schemes available in their jurisdiction.

Factors That Affect Real-World Performance

Several factors can cause actual system output to differ from estimates. Roof orientation and tilt angle are critical: a south-facing roof (in the northern hemisphere) at an angle close to the local latitude maximises annual production. East- or west-facing roofs typically produce 15–20% less than an optimally oriented south-facing installation. North-facing roofs in the northern hemisphere may produce 30% or less of optimal output.

Shading from trees, chimneys, neighboring buildings, or rooftop equipment can significantly reduce output, especially if the system uses string inverters. Microinverters or DC power optimisers can mitigate partial shading losses.

Temperature also plays a role. Although panels need sunlight, high temperatures reduce their efficiency. Most crystalline silicon panels have a temperature coefficient of about −0.3% to −0.5% per degree Celsius above their rated test condition of 25°C. Hot climates therefore see somewhat lower efficiency than the peak-condition ratings suggest.

Using This Calculator

Enter your planned or existing system size in kilowatts. If you are still planning, typical residential rooftop systems range from 3 kW to 15 kW. Input the average peak sun hours for your location — your local solar installer, utility, or online solar resource tool can provide this figure. Enter your current electricity rate per kilowatt-hour; check your utility bill for the exact rate. Provide the total installed system cost including panels, inverters, mounting hardware, and installation labour. The degradation rate defaults to 0.5% per year, which is a standard industry figure; you can adjust it based on your panel manufacturer's warranty.

The results show estimated daily, monthly, and yearly energy production, along with monthly and yearly savings, a simple payback period, and cumulative 25-year savings accounting for panel degradation. These are estimates based on a standard 80% system performance ratio and the inputs you provide. Actual results will vary.