Before running a full simulation, solar installers need a fast, credible production number to put in front of a customer. This estimator multiplies the system size by bundled monthly irradiance for the chosen climate zone and applies a performance ratio to give monthly and annual energy yield, plus an optional bill-offset figure.
How it works
The peak-sun-hours method is the industry-standard quick estimate. The daily irradiance in kilowatt-hours per square metre is numerically equal to the equivalent peak sun hours, because the module rating is defined at one kilowatt per square metre. So each month:
E_month = kWp x H_daily x days_in_month x PRwhere H_daily is the average daily irradiance for that month and PR is the performance ratio that derates for real-world losses. Summing the twelve months gives annual yield, and dividing by the system size gives specific yield in kilowatt-hours per kilowatt peak per year.
Accuracy and next steps
The bundled irradiance values are representative climatology for each broad zone at roughly latitude tilt. They are ideal for the first proposal conversation and a rough payback estimate, but they are not site-specific. For a binding quote, move to NREL PVWatts or SAM with the site’s typical meteorological year data, the actual tilt and azimuth, and a real shading analysis. The performance ratio is where most of the uncertainty lives, so choose it honestly based on climate, mounting, and equipment age.