Solar panel annual yield, in kWh.

Formula

Annual kWh = system kW DC × peak sun hours per day × 365 × (1 − system losses) × inverter efficiency.

Worked example

A 6 kW system at 4.0 peak sun hours: 6 × 4.0 × 365 = 8,760 kWh before losses. At 14% losses that is 8,760 × 0.86 = 7,533.6 kWh; at 96% inverter efficiency, 7,533.6 × 0.96 = about 7,232 kWh per year, or roughly 603 kWh per month.

Where the defaults come from

The 14% total system loss is the NREL PVWatts default, built from soiling (2%), shading (3%), snow (0%), mismatch (2%), wiring (2%), connections (0.5%), light-induced degradation (1.5%), nameplate rating (1%), age (0%) and availability (3%). Individual losses are combined multiplicatively, not added. NREL notes that 1 − 14/100 = 0.86 and that inverter efficiency is a separate input (nominal 96%) not included in the system loss. Peak sun hours equal average daily plane-of-array irradiation in kWh/m²/day, which PVWatts reports by month. Sources checked 2026-09-30 (PVWatts V5 manual, September 2014; the current PVWatts calculator is version 8).

Limits: this is an annual-average shortcut. It ignores seasonal and hourly variation, temperature, tilt and orientation (already folded into the sun-hours figure you enter), clipping and year-over-year degradation. Use PVWatts or an installer's simulation before a purchase decision.

Source: NREL PVWatts Version 5 Manual · PVWatts V1 technical reference (0.77 derate) · PVWatts calculator

FAQ

What are peak sun hours?

The number of hours per day at 1 kW/m² that would deliver the same energy as the day's real sunlight. It is the same number as daily irradiation in kWh/m²/day.

What system losses should I use?

Start with PVWatts' 14%. Raise it for heavy shading or snow and lower it only if your installer documents why.

Is this the same as a PVWatts result?

No. PVWatts simulates hourly weather for your location. This tool multiplies annual averages, so use it as a quick check on a quote.