☀️ Solar Savings Calculator

Understanding peak sun hours and system losses

Peak sun hours vary substantially by region and season. The system-loss derate factor is a standard modeling convention, not a fixed physical constant, and installers may quote a different figure based on your specific equipment and site.

• This is a simple, illustrative estimate. It excludes panel degradation over the system's lifetime (typically ~0.5%/year), electricity price changes, financing costs, tax credits, and net-metering policy, all of which affect real-world savings and payback.
• Peak sun hours should reflect your specific roof orientation, tilt and shading where possible — a professional solar assessment or satellite-based tool gives a more precise, site-specific figure than a regional average.

What does a solar savings calculator do?

This calculator estimates annual solar energy production using the 'peak sun hours' method: a system's DC nameplate capacity (in kW) is multiplied by the site's average daily peak sun hours and by 365 days, then reduced by a system-loss derate factor that accounts for inverter inefficiency, wiring losses, soiling, temperature effects and other real-world losses between DC panel output and usable AC electricity.

'Peak sun hours' is not the same as daylight hours — it is the number of hours per day during which solar irradiance would need to average 1,000 W/m² to deliver the same total daily energy as the actual variable sunlight received. Sunnier regions have more peak sun hours per day (e.g., desert climates around 6, cloudier temperate regions around 3–4). The default system-loss value of 14% matches the default total system losses used in the National Renewable Energy Laboratory's (NREL) PVWatts Calculator, a widely used solar production estimation tool.

How to use this solar savings calculator

1. Enter the solar system's DC nameplate size in kilowatts (kW), found on the panel/inverter spec sheet or quote.
2. Enter your location's average peak sun hours per day — available from solar irradiance maps or your installer's production estimate.
3. Enter the system losses percentage — 14% is the NREL PVWatts default derate; adjust if your installer provides a site-specific figure.
4. Enter your electricity rate per kWh from your utility bill.
5. Optionally enter the total system cost to see the estimated simple payback period in years.
6. Read the estimated annual generation (kWh), annual and monthly savings, and payback period.

The formula behind solar savings

Annual energy generation equals system size (kW) multiplied by peak sun hours per day, multiplied by 365 days, multiplied by (1 minus the system loss fraction). Annual savings equals that generation multiplied by the electricity rate. Simple payback divides the system cost by annual savings — it does not account for financing costs, electricity price inflation, panel degradation over time, or incentives/rebates.

Worked example: a 5 kW system at 4.5 peak sun hours/day with 14% system losses generates 5 × 4.5 × 365 × 0.86 ≈ 7,063 kWh per year. At $0.25/kWh, that is about $1,766 in annual savings, or about $147.14 per month. At a system cost of $10,000, simple payback is 10,000 ÷ 1,766 ≈ 5.7 years.

Common mistakes

• Confusing peak sun hours with total daylight hours, which overstates expected generation.
• Using a system-loss figure that is too low, ignoring real-world inverter, wiring, soiling and temperature losses.
• Treating the simple payback estimate as inclusive of financing costs, incentives, or electricity price inflation — it is not.
• Entering the AC-rated inverter size instead of the DC panel nameplate size, which can misstate generation depending on the intended convention.

Câu hỏi thường gặp

Tài liệu tham khảo

1. National Renewable Energy Laboratory (NREL) — PVWatts Calculator documentation, including the default 14% total system losses derate factor.
2. U.S. Department of Energy, Solar Energy Technologies Office — overview of solar PV system performance factors.