Quick answer
Insolation is solar radiant energy received per unit area over a stated period, commonly expressed in kWh/m². It is irradiance integrated over time. Specify the surface orientation, period and dataset before comparing values. Insolation is a solar-resource input; it is not the electrical energy a PV array delivers after conversion and system losses.
Insolation and irradiance measure different quantities
Sandia’s PV Performance Modeling Collaborative distinguishes irradiance as power per area from insolation as energy per area. Irradiance is commonly expressed in W/m²; integrating it over time gives energy in Wh/m² or kWh/m².
A daily total and an annual total cannot be compared without their periods. A daily value can also mean one observed day or a long-term daily average. Preserve that distinction in a report.
Example with explicit units
Suppose irradiance on a specified surface were constant at an illustrative 800 W/m² for five hours. The integrated energy would be:
800 W/m² × 5 h = 4,000 Wh/m² = 4 kWh/m².
The constant input is hypothetical. Actual irradiance varies, so a dataset calculation should integrate the values over their represented intervals rather than treating one instantaneous reading as a daily resource estimate.
Check the surface and weather basis
Global horizontal resource describes a horizontal surface. A tilted array needs the corresponding plane-of-array input or a documented conversion. See the GHI definition for the horizontal quantity and plane-of-array irradiance for the array-plane concept.
Record location, surface orientation, observation or modeled period, temporal resolution and dataset version. A typical meteorological year and an individual measured year answer different questions. Do not use an unsourced regional daily range as a guaranteed resource for the customer’s site.
Insolation does not equal delivered electricity
A PV energy model also needs equipment, temperature, optical and electrical behavior, shading and other loss assumptions. A percentage change in resource does not automatically establish the same percentage change in customer savings or lifetime financial return.
Keep resource, modeled generation and delivered or exported electricity distinct. Use specific yield when normalizing electrical output by the stated DC capacity, rather than comparing it directly with resource energy per square metre.
Practical next step
Ask for the weather source and the surface/time basis behind a proposal’s production estimate. Compare alternative designs using consistent inputs, and record any changed assumptions. Evaluate how those inputs appear in the solar simulation workflow; resource data alone is not a production guarantee.
Reviewed September 29, 2026 using the linked Sandia terminology. Worked arithmetic is illustrative; no first-hand measurement campaign or regional production survey is claimed.
Related Glossary Terms
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