What is the unit of measurment for heliograph?

A heliograph is typically used to measure solar radiation and is often calibrated in units of watts per square meter (W/m²). This unit quantifies the intensity of sunlight received on a given surface area over time. In some contexts, it may also be related to solar energy metrics such as kilowatt-hours per square meter (kWh/m²) for accumulated solar energy over a specific period.