Derate factors explained (15–25% losses)
Wire, controller, temperature and tilt all reduce panel output. Here’s how to pick a realistic derate.
Quick take
Cables, controllers and heat reduce output. A 15–25% derate fits most rooftop setups.
How to use the calculator
- Open the calculator linked below.
- Enter the numbers that match your setup.
- Use our rules of thumb to sanity-check the result.
Rules of thumb
- Start with daily kWh ÷ peak sun hours.
- Apply a 15–25% derate for losses.
- Round up to the next standard panel size.